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Role of Super-Cooling in the Process of Quick Freezing
Bardakh, E.
https://publications-cnrc.canada.ca/fra/droits
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https://nrc-publications.canada.ca/eng/view/object/?id=fec4a09d-64a6-439b-8fe3-99b9d5ca3ace https://publications-cnrc.canada.ca/fra/voir/objet/?id=fec4a09d-64a6-439b-8fe3-99b9d5ca3ace
NATIONAL RESEARCH COUNCIL OF CANADA
3
bl
Technical Translation T T - r t
Title:
The role of super-cooling in the process
of quick freezing,
By
". E ,Ja,
Bardakh,
Reference:
Kholodilnaja Promyshlennost 1 5 (5)
: 19-26,
1937,
Recent experience has e s t a b l i s h e d t h a t d u r i n g quick f r e e z i n g s m a l l e r c r y s t a l s a r e formed i n t h e t i s s u e s of food products t h a n d u r - i n g r e l a t i v e l y slow f r e e z i n g . For a t h e o r e t i c a l e x p l a n a t i o n of t h i s phenomenon one u s u a l l y c i t e s Tamman's t h e o r y of c r y s t a l l i z a t i o n , How- e v e r , when t h e t h e o r y of c r y s t a l l i z a t i o n i s a p p l i e d t o t h e m a n i f e s t a - t i o n s o f f r e e z i n g i n t i s s u e s , e r r o r s occur, According t o TammanPs t h e o r y t h e b a s i c f a c t o r determining t h e s i z e of c r y s t a l s
i s
t h e de- g r e e of super-cooling d u r i n g which t h e c r y s t a l s a r e formed, Meanwhile, i n l i t e r a t u r e concerning r e f r i g e r a t i o n , t h e formation of c r y s t a l s o f v a r i o u s s i z e i s u s u a l l y explained by t h e speed of f r e e z i n g w i t h o u t any r e l a t i o n t o super-cooling, The t h e o r y i n d i c a t e s t h a t i f d u r i n g t h e f r e e z i n g of water i n t i s s u e s t h e r e were no super-cooling t h e n l a r g e c r y s t a l s would be formed r e g a r d l e s s of t h e speed of f r e e z i n g ;small c r y s t a l s could o n l y be formed i n t h e casg o f f r e e z i n g under con-
I
d i t i o n s of super-cooling, Indeed t h e speed of f r e e e i n g i s but one o f
a number of s u b s t a n t i a l f a c t o r s t h a t c o n t r i b u t e t o super-cooling of
f r e e z a b l e food products and t h e o n l y f a c t o r t h a t contemporary r e f r i g - e r a t i o n methods u t i l i z e i n o r d e r t o b r i n g about super-cooling, Other t h a n t h e speed of f r e e z i n g a whole s e r i e s of f a c t o r s can have a n i n -
f l u e n c e on t h e r e a l i z a t i o n of super-cooling d u r i n g f r e e z i n g , i , e , , f a c t o r s t h a t can favour t h e formation of f i n e c r y s t a l s ,
I0
I n o r d e r t o e l u c i d a t e t h e connection between t h e degree of super- cooling and t h e s i z e of r e s u l t a n t c r y s t a l s , l e t us f i r s t d w e l l b r i e f l y on t h e b a s i c a s p e c t s of t h e t h e o r y of c r y s t a l l i z a t i o n ,
A s a c o n d i t i o n of c r y s t a l growth t h e r e must be a t l e a s t a small
degree of super-cooling of t h e s o l u t i o n o r melt i n which c r y s t a l l i z a - t i o n t a k e s p l a c e , s i n c e i n a c o n t r a r y case t h e medium w i l l be i n a s t a t e
of e q u i l i b r i u m wfth t h e c r y s t a l and t h e r a t e o f c r y s t a l l i z a t i o n w i l l be
e q u a l t o zero, For t h e formation of c r y s t a l s t h e r e must be, o t h e r t h a n super-cooling, a presence of n u c l e i around which c r y s t a l s begin t o grow, When n u c l e i a r e absent c r y s t a l l i z a t i o n does n o t occur even i f t h e r e were
a s t a t e of super-cooling and a corresponding, determined r a t e of c r y s t a l - l i z a t i o n , I n t h e process of c r y s t a l l i z i n g a substance t h e n u c l e i -
c e n t r e s o f c r y s t a l l i z a t i o n
-
can e i t h e r be i n t r o d u c e d e x t e r n a l l y o r can a r i s e w i t h i n t h e c r y s t a l l i z i n g substance i t s e l f , Therefore, i n o r d e r t h a t c r y s t a l l i z a t i o n can t a k e p l a c eit
i s n e c e s s a r y t o haves 1 ) n u c l e i of c r y s t a l l i z a t i o n (abbreviated a s n , c r , ) and 2) a determined r a t e ofc r y s t a l l i z a t f o n (abbreviated as
r ,
c r , )The l a r g e number-of s t u d i e s c a r r i e d o u t by Tamman on t h e c r y s t a l -
l i z a t i o n a r i s i n g i n a u n i t of time and a u n i t o f volume depends on t h e degree of super-cooling, A r e p r e s e n t a t i o n of t h e n a t u r e of t h i s r e -
l a t i o n s h i p i s given by t h e graph ( ~ i g , 1 )
,
o b t a i n e d by t h e c i t e d a u t h o r f o r p i p e r i n e The m e l t i n g p o i n t of p i p e r i n ei s
12909 but t h e f i r s t n , c r , a r i s e o n l y a t a temperature o f about80°,
i , e , , a t a super-cool-n , c r , i n c r e a s e s , a t a d e t e r m i n ~ d super-cooling
it
reaches a maximum,t h e n w i t h f u r t h e r i n c r e a s e i n degree of super-cooling t h e number of n o w o d e c r e a s e s , r e a c h i n g zero a f t e r more s e v e r e super-cooling, I n t h i s man- n e r , beginning a t a c e r t a i n temperature, a s u b s t a n c e completely l o s e s i t s a b i l i t y f o r spontaneous c r y s t a l l i z a t i o n ,
The r a t e of c r y s t a l l i z a t i o n , as shown by t h e s t u d i e s of Tamman on m e l t s , a l s o d e p ~ n d s on t h e degree of super-cooling ( F i g , 2), A t t h e s o l i d f f i c a t f o n p o i n t c r y s t a l l i z a t i o n t a k e s p l a c e a t a determined i n s f g - n i f i c a n t r a t e , 7 i t h an i n c r e a s e i n degree of super-cooling t h e r a t e of c r y s t a l l i z a t i o n i n c r e a s e s ( a r e a s A & B ) and a%itains a maximum v a l u e , I n
t h e c a s e of substances having a c r y s t a l l i z a t i o n r a t e higher t h a n 5 mm/min, (water i s i n c l u d e d i n t h i s c a t e g o r y , having a maximum c r y s t a l l i z a t i o n
r a t e i n t h e o r d e r of 1 0 mm/rnin), c r y s t a l l i z a t i o n t a k e s p l a c e a t t h e maxi-
mum r a t e i n t h e e n t i r e a r e a c , which i n c l u d e s a t t i m e s 30-80°, During
more s e v e r e super-cooling ( a r e a s D & E ) t h e r a t e of c r y s t a l l i z a t i o n de- c r e a s e s and approaches z e r o ,
The s t u d i e s o f Tamman showed t h a t t h e a d d i t i o n of s u b s t a n c e s t h a t a r e s o l u b l e o r i n s o l u b l e i n t h e c r y s t a l l i z i n g s u b s t a n c e e f f e c t s a t times a s i g n i f i c a n t i n f l u e n c e on i t s a b i l i t y f o r spontaneous c r y s t a l -
l i z a t i o n , i , e , , on t h e r , c r . The a d d i t i o n of c e r t a i n s u b s t a n c e s evokes a s i g n i f i c a n t i n c r e a s e i n t h e number of n o w , ; t h e a d d i t i o n of o t h e r substances can, t o a s i g n i f i c a n t degree, d e c r e a s e t h e number of n , c r , o r even reduce it t o z e r o , and f i n a l l y , a t h i r d substance can have no
i n f l u e n c e on t h e emergence of n , c r , Apparently c e r t a i n c o l l o i d s promote spontaneous c r y s t a l l i z a t i o n , Admixtures always l e a d t o a d e c r e a s e i n
t h e r a t e of c r y s t a l l i z a t i o n compared t o t h a t of a p u r e s u b s t a n c e , B e s i d e s i n a l l c a s e s t h e n a t u r e o f curves showing t h e r e l a t i o n s h i p of n , c r , , and a l s o r , c r , , t o t h e d e g r e e of s u p e r - c o o l i n g i s m a i n t a i n e d , When f r e e z i n g w a t e r i n a n i m a l and v e g e t a b l e t i s s u e s we a r e n o t d e a l i n g w i t h t h e c r y s t a l l i z a t i o n o f a m e l t , i , e , , t h e l i q u i d p h a s e of some p u r e s u b s t a n c e , e , g , w a t e r , b u t w i t h t h e c r y s t a l l i z a t i o n o f a weak s o l u t i o n of c e r t a i n s a l t s which c o n t a i n s a l s o r e v e r s i b l e and n o n r e v e r s i b l e c o l l o i d s , Laws concerning t h e c r y s t a l l i z a t i o n of such
s o l u t i o n s have, up t o t h i s t i m e , n o t been s t u d i e d , However, on t h e b a s i s of y e a r s of meticulous work on t h e p a r t of P r o f e s s o r
BOA,
Bochvara on a l l o y s of two components, one can surmf s e t h e f o l l o w i n g s f ) I n t h e c r y s t a l l i z a t i o n o f l i q u i d s found i n a n i m a l t f s s u e s t h e dependence d i s c u s s e d above i s maintained, i , e , , r , c r , and t h e number of n , c r , t ot h e d e g r e e of super-cooling; i f ) when f r e e z i n g w a t e r i n food p r o d u c t s a f t e r i n c r e a s i n g t h e c o n c e n t r a t i o n o f t h e l i q u i d i n t h e t i s s u e , t h e r a t e of c r y s t a l l i z a t i o n d e c r e a s e s ,
The laws concerning c r y s t a l l i z a t i o n d i s c u s s e d above e n a b l e one t o approach t h e e l u c i d a t i o n of t h e most i m p o r t a n t problem i n t h e t h e o r y of q u i c k f r e e z i n g , namely, what d o c s t h e s i z e o f i c e c r y s t a l s formed i n food p r o d u c t s d u r i n g f r e e z i n g , depend on, Obviously t h e s i z e of c r y s t a l s s t a n d i n r e l a t i o n t o t h e r , c r , and t h e number of n , c r , , f o e , , i n t h e f i n a l c o n s i d e r a t f o n t o t h e d e g r e e of s u p e r - c o o l i n g d u r i n g which c r y s t a l s a r e formed,
During i n s i g n i f i c a n t s u p e r - c o o l i n g t h e r , c r , i s small, a t t h e same time o r d i n a r i l y t h e number of n , c r , i s e i t h e r s m a l l o r even non-
e x i s t e n t , So, f o r example, i n t h e c a s e of p i p e r i n e t h e n , c r , begin t o form only a f t e r a super-cooling of 50°, a t lower super-coolfng c r y s t a l - l i z a t i o n can be i n i t i a t e d o n l y by i n t r o d u c i n g n , c r o e x t e r n a l l y , Under t h e s e c o n d i t i o n s l a r g e c r y s t a l s form, During an i n c r e a s e i n super-cool- i n g a change i n t h e s i z e of c r y s t a l s depends on t h e r e l a t i v e i n c r e a s e i n r , c r , and t h e number of n o w o i s g r e a t e r t h a n t h e r e l a t i v e i n c r e a s e
i n r , c r , , does an i n c r e a s e i n super-cooling l e a d t o a d e c r e a s e i n cry- s t a l s i z e , So, i f ( F i g ,
3)
t h e temperature of c o o l i n g is lowered fromtl
t ot 2
t h e n t h e c r y s t a l l i n e s t r u c t u r e w i l l become f i n e r s i n c e , i n lowering t h c temperature of super-cooling fromtl
t o t 2 , t h e r a t e of c r y s t a l l i z a t i o n i s i n c r e a s e d by two times w h i l e t h e number of n , c r ,i n c r e a s e s t o a s i g n i f i c a n t l y g r e a t e r d e g r e e , I f d u r i n g an i n c r e a s e i n t h e degree of super-cooling t h e number of n , c r , and t h e r o c r , i n c r e a s e t o t h e same degree t h e n t h e s i z e of c r y s t a l s remains unchanged, I f , however, w i t h an i n c r e a s e i n degree of super-cooling t h e r , c r , i n - c r e a s e s r e l a t i v e l y more q u i c k l y t h a n t h e number of n , c r , , t h e n l a r g e r c r y s t a l s w i l l begin t o form,
These t h e o r e t i c a l conclusions a r e confirmed by t h e experiments of Baker, Baker discovered t h a t i n t h e c a s e of bismuth an i n c r e a s e i n t h e degree of quenching brought about
a
d e c r e a s e i n c r y s t a l s i z e , while a t t h e same t i n e i f antimony were s u b j e c t e d t o t h e same condi- t i o n s t h e s i z e of c r y s t a l s i n c r e a s e d ,I n t h i s manner t h e formation of i c e c r y s t a l s of d i f f e r e n t s i z e a t v a r y i n g r a t e s i n t h e f r e e z i n g of food p r o d u c t s i s ~ x p l a i n e d by
when t h e d e g r e e of s u p e r - c o o l i n g i s i n c r e a s e d t h e r , c r , o f w a t e r i n
t h e l i q u i d s found i n t i s s u e s i n c r e a s e s s l o w e r t h a n t h e number o f n u c l e i ,
There remains t o be e x p l a i n e d t h e f a c t o r s on which t h e o n s e t of s u p e r -
c o o l i n g depends,
The p o s s i b i l i t y of s u p e r - c o o l i n g and i t s e x t e n t i s s t i p u l a t e d by
a small number of n o c r o ahd a small r , c r , I t i s c l e a r t h a t i f n , c r ,
s t i l l do n o t a r i s e a f t e r c o o l i n g below t h e m e l t i n g t e m p e r a t u r e , t h e n
s u c h a s u b s t a n c e e a s i l y becomes super-cooled,
So, f o r example, i f one g e t s a r e l a t i o n s h i p r e p r e s e n t e d by c u r v e
I n , c r , ( F i g , 3 ) , t h e n t h e s u b s t a n c e can e a s i l y be super-cooled t o tem- p e r a t u r e
tl,
It would be d i f f i c u l t t o s u p e r - c o o l t h e s u b s t a n c e t ot 2
o r below s i n c e a lowering o f t e m p e r a t u r e t o t 2 w i l l r e s u l t i n a v e r y
r a p i d i n c r e a s e i n b o t h t h e number of n , c r , and r , c r ,
The r e l a t i o n s h i p shown by curve I1 n . c r o a l l o w s one t o r e a c h e a s i l y
a v e r y e x t e n s i v e s u p e r - c o o l i n g s i n c e t h e n , c r o begin t o a p p e a r o n l y when
t h e r , c r , i s c l o s e t o z e r o ; i n s u c h c a s e s it i s p o s s i b l e t o o b t a i n a
g l a s s - l i k e s u b s t a n c e , i o e , , i n a s t a t e where a h a r d n o n - c r y s t a l l i n e
s u b s t a n c e o c c u r s , f o r i n s t a n c e , g l a s s , A s u b s t a n c e i n a g l a s s - l i k e state
i s a h i g h l y super-cooled l i q u i d having a v i s c o s i t y c l o s e t o t h a t o f a
s o l i d and can even be b r i t t l e , I n super-cooled s u b s t a n c e s where t h e r , c r ,
i s v e r y l a r g e , w a t e r f o r example, t h e a p p e a r a n c e of t h e f i r s t n o w , results
i n a r a p i d c r y s t a l l i z a t i o n o f t h e e n t i r e s u b s t a n c e ,
The dependence of t h e number of n o c r , and t h e r , c r , on t h e d e g r e e
of s u p e r - c o o l i n g p r o v i d e s t h e p o s s i b i l i t y o f e x p l a i n i n g why t h e r e i s a n
-8-
a determined temperature d r o p between t h e c o o l i n g medium and t h e body t o be f r o z e n , with a g i v e n t h e r m a l c o n d u c t i v i t y and, i n p a r t i c u l a r , g i v e n s i z e s o f bodies and a determined c o e f f i c i e n t of t h e r m a l conduct- i v i t y , e t c , ,
it
i s p o s s i b l e t o remove a c e r t a i n l i m i t e d number of c a l - o r i e s from t h e body t o be f r o z e n , I f t h e r a t e of c r y s t a l l i z a t i o n and t h e number of n u c l e i a r e l a r g e t h e n much h e a t w i l l be g i v e n o f f d w -i n g c r y s t a l l i z a t i o n and t h e amount of s u b s t a n c e being c r y s t a l l i z e d w i l l
be l i m i t e d by c o n d i t i o n s of t h e r m a l c o n d u c t i v i t y , I f by v i r t u e of a
small r , c r , o r t h e l i m i t i n g o f n o c r o
it
is p o s s i b l e t o e x t r a c t more h e a t t h a n i s given o f f d u r i n g c r y s t a l l i z a t i o n t h e n t h e s u b s t a n c e w f l lbecome super-cooled, Temperature jumps which a r e o f t e n observed d u r - i n g t h e f r e e z i n g of food p r o d u c t s a r e e x p l a i n e d by t h e f a c t t h a t c r y - s t a l l i z a t i o n beginning i n t h e super-cooled t i s s u e s g e n e r a t e s more h e a t t h a n
it
i s p o s s i b l e t o e l i m i n a t e , I f t h e amount of h e a t e l i m i n - a t e d i s g r e a t e r t h a n t h a t g e n e r a t e d d u r i n g t h e f r e e z i n g of w a t e r t h e n t h e r e i s a n o n s e t of deep super-cooling,The n e x t f a c t o r t h a t a i d s t h e p r o c e s s of super-cooling f s t h e
d i v i s i o n of s u b s t a n c e s i n t o minute c e l l s , According t o TammanPs t h e o r y , f o r e v e r y d e g r e e of super-cooling t h e r e i s a corresponding emergence,
i n a u n i t of volume, o f ' a determined number of n,cr,-no I f one were t o d i v i d e t h e given u n i t o f volume i n t o n d i f f e r e n t p a r t s , t h e n t h e sum of n u c l e i emerging i n a u n i t of time i n a l l s e c t i o n s w i l l s t i l l be
e q u a l t o n o But, a c c o r d i n g t o t h e t h e o r y of p r o b a b i l f t y , i n some s e c - t i o n s t h e r e w i l l be s e v e r a l n o c r o p and
it
f o l l o w s t h a t i n o t h e r s e c - t i o n s t h e r e w i l l be no n , c r o , and i n t h e l a t t e r , super-coolingw i l l be more s e v e r e , I t i s a p p a r e n t t h a t a d i v i s i o n i n t o more t h a n n
s e c t i o n s w i l l l e a d t o a n w e n g r e a t e r s u p e r - c o o l i n g , The f a c t o r under d i s c u s s i o n f s p a r t i c u l a r l y w e l l expressed when r e l a t e d t o s u b s t a n c e s l i k e w a t e r which i s c h a r a c t e r i z e d by a high r . c r , and t h e emergence of
ar f n s f @ f f i c a n t number of n . c r , I t f o l l o w s t h a t t h e small volume of
v e g e t a b l e and animal c e l l s should f a c i l i t a t e s u p e r - c o o l i n g ,
F i n a l l y onc should c o n s i d e r s u p e r - c o o l i n g i n c a p i l l a r f e s , The l i q u i d i n c a p i l l a r i e s i s e a s i l y super-cooled and t h e s m a l l e r t h e d i a - meter of capillaries t h e g r e a t e r t h e s u p e r - c o o l i n g t h a t can be a t t a i n e d ,
So, f o r example, Chambers and Gail observed prolonged s u p e r - c o o l i n g of fpog muscle f i b e r s a t a t e m p e r a t u r e of -15O w h i l e t h e f r e e z i n g temper- a t u r e of t h e l i q u i d e x t r a c t e d from t h e f i b e r s was -004500 I n a l l t h e c i t e d c a s e s , s u p e r - c o o l i n g i s enhanced i f t h e o b j e c t t o be cooled i s
i n a s t a t e of r e s t ,
B e f o r e making a d e t a i l p d s t u d y of t h e phenomena o c c u r r i n g i n food p r o a u c t s d u r i n g f r e e z i n g ,
it
would be n a t u r a l t o examine t h e c r y s t a l - l i z a t i o n p r o p e r t i e s of w a t e r and water s o l u t i o n s , However, i s it n o ts t r a n g e t h a t water in t h i s r e s p e c t has h a r d l y been s t u d f e d ? Only t h e l a t e s t works
st am man
and Bukhner, 1935) shed any l i g h t on t h i s q u e s t i o n ,On t h e b a s i s of a v a i l a b l e s t u d i e s one can c o n s i d e r t h a t w a t e r un-
d e r c o n s i d e r a b l y high super-cooling has l i t t l e p r o p e n s i t y f o r spontan- eous c r y s t a l l i z a t i o n , i O e o g d u r i n g such s u p e r - c o o l i n g few n , c r , emerge, For example, 0,4 c c , of d i s t i l l e d w a t e r a t a t e m p e r a t u r e of -12O r e - mained i n a super-cooled s t a t e f o r o v e r
16
min, The same experimentsshowed t h a t a i r d i s s o l v e d i n w a t e r a i d s i n s u p e r - c o o l i n g ; t h e same i n - f l u ~ n c e was o b t a i n e d by immersing an i r o n w i r e i n t o t h e w a t e r ,
The r a t e of c r y s t a l l i z a t i o n of i c e i n w a t e r and i n w a t e r s o l u t i o n s h a s been s t u d i e d i n much g r e a t e r d e t a i l , A s a r e s u l t i t h a s been poss-
i b l e t o ~ s t a b l f s h t h e r e l a t i o n o f t h e r o c r , of p u r e w a t e r t o s u p e r - c o o l - i n g up t o
-13$,
A t - 1 3 O r, c r , was6
rnlmin, and t h e maximum r , c r , was1 0 m/min During s t u d i e s on t h e i n f l u e n c e o f d i s s o l v e d s u b s t a n c e s
it
was found t h a t t h e a d d i t i o n of sodium c h l o r i d e o r s u c r o s e s u b s t a n t i a l l y d e c r e a s e s t h e r a t e of c r y s t a l l i z a t i o n , Such a n i n f l u e n c e e x e r t e d by sodium c h l o r i d e and p a r t i c u l a r l y on s u c r o s e i s v e r y i n t e r e s t i n g and fm- p o r t a n t s i n c e a d e c r e a s e i n r , c r , i n f l u e n c e s t h e s i z e of c r y s t a l s and
it
i s c l e a r t h a t t h e c o n c e n t r a t i o n o f s y r u p i n which f r u f t are f r o z e n c a n n o t be viewed w i t h i n d i f f e r e n c e ,There a r e p r a c t i c a l l y no o b s e r v a t i o n s made on t h e r a t e o f c r y s t a l - l i z a t i o n of i c c i n food p r o d u c t s , One c a n o n l y c o n s i d e r t h e r , c r o o f i c e on t h e b a s i s o f o b s e r v a t i o n made by C h a m b ~ r s and G a f l on f r o g muscle f i b e r s : a t a t e m p e r a t u r e o f -1 go r , c r , v a r f e d between 12-18 mmlmin;; a t t e m p e r a t u r e s down t o - 3 O t h e r a t c i n c r e a s e d c o n s i d e r a b l y , I n t h e s e e x p e ~ i m e n t s t h e c e l l s were surrounded by a t h f n l a y e r of w a t e r and t h e
r a t e of c r y s t a l l i z a t i o n i n t h e s u r r o u n d i n g w a t e r was always g r e a t e r t h a n t h e r , c r , i n t h e c e l l s o
It i s f m p c r a t i v e i n t h e n e a r f u t u r e t o c a r r y o u t r e s e a r c h on t h e
r , c r , and t h e number of n , c r , f o r food p r o d u c t s , These s t u d i e s are
The b a s i c s t a t e o f t h e c r y s t a l l i z a t i o n t h e o r y t h a t w e have d i s -
c u s s e d e n a b l e s us t o approach t h e m a n i f e s t a t i o n s t h a t o c c u r d u r i n g
t h e f r e e z i n g o f f o o d p r o d u c t s , Repeated e x p e r i m e n t s show t h a t i f f r u i t , v e g e t a b l e s , e g g s , meat, f i s h , a r e p l a c e d i n a medium having a tempera- t u r e below 0°, t h e n a t f i r s t one o b s e r v e s a s u p e r - c o o l i n g o f t h e t i s -
s u e s and o n l y t h e n d o e s t h e f r e e z i n g b e g i n , I n d e t e r m i n i n g t h e f r e e z - i n g t e m p e r a t u r e s o f f r u i t P r o n i n found t h a t i n t h e c a s e o f "French plumsn t h e r e i s f i r s t a n o n s e t o f s u p e r - c o o l i n g t o -8,z0 w h i l e t h e
f r e e z i n g t e m p e r a t u r e i s o n l y - l O 8 O ; i n t h e c a s e o f n r o s h c h i n n s t r a w - b e r r i e s t h e c o r r e s p o n d i n g t e m p e r a t u r e s were -5,8O and -1033/ e t c o Ac- c o r d i n g t o t h e o b s e r v a t i o n s o f K a r r i k , a p p l e $ k e p t a t a t e m p e r a t u r e o f
0,68O below f r e e z i n g remained i n a s u p e r - c o o l e d s t a t e f o r
1-3
d a y s , Kolabakhov and N i k o l u s k i j observed s u p e r - c o o l i n g t o- 4 , 8 O
d u r i n g slow f r e e z i n g o f l i v e c r u c f o n s and c a r p i n a i r , F i n a l l yM,V,
Tukhshnaidshowed e x p e r i m e n t a l l y t h a t eggs c a n be e a s i l y super-cooled t o - 6 O and
c a n be k e p t
i n
t h e super-cooled s t a t e ,The e a s e w i t h which s u p e r - c o o l i n g i s a t t a f n e d i n v e g e t a b l e and a n i m a l t i s s u e s c a n be e x p l a i n e d by c e r t a i n s p e c i f i c p e c u l i a r i t i e s i n t h e s t r u c t u p e of t i s s u e . One of t h e b a s i c f a c t o r s i s t h a t t h e w a t e r s o l u t i o n i n v e g e t a b l e and a n i m a l t l s s u e i s d i s t r i b u t e d among e x t r e m e l y t h i n c a p i l l a r i e s , f o e , , among c e l l s and t h e i n t e r c e l l s p a c e s , I n t u r n some c e l l s a r e t h e m s e l v e s d i v i d e d i n t o even s m a l l e r c a p i l l a r i e s : e g o , muscle f i b e r s c o n s i s t o f t h e t h i n n e s t f i b r i l s which i n c l u d e a
somewhat l i q u i d protoplasm,
The second important f a c t o r t h a t c o n t r i b u t e s t o s u p e r - c o o l i n g i n c e l l s i s concerned w i t h t h e f a c t t h a t t h e c e l l w a l l i s a p r o t e c -
t i v e b a r r i e r n o t a l l o w i n g t h e p e n e t r a t i o n of i c e c r y s t a l s
-
p r i m e r s-
i n t o t h e c e l l , The i m p e n e t r a b i l i t y o f t h e c e l l wall t o i c e c r y s t a l s was s t u d i ~ d i n d e t a i l by Chambers and Gail by u s i n g l i v e muscle t i s -s u e s of f r o g s P amoebae and t h e l i v i n g t i s s u e of onion, The c e l l s , s u r - rounded by a t h i n l a y e r of w a t e r , were examinrd u n d ~ r a microscope, A f t e r t h e f i b e r and t h e s u r r o u n d i n g l a y e r of w a t e r a t t a i n e d t h e de- s i r e d t e m p e r a t u r e of s u p e r - c o o l i n g , t h e i c e c r y s t a l s found on
the
end of t h e m i c r o - p i p e t t e come i n c o n t a c t w i t h t h e o u t s i d e of t h e c e l l w a l l , Immediately t h e r e i s a f o r m a t i o n of small s c a l e s of i c e whfch s u r -rounded t h e c e l l from a l l s i d e s , Formation of i c e i n s i d e t h e c e l l was n o t observed d u r i n g t h e s e e x p ~ r i m e n t s , I n o r d e r t h a t i c e forma- t i o n should b e g i n i n s i d e t h e c e l l
it
was n e c e s s a r y t o i n t r o d u c e a primer d i r e c t l y i n t o t h e c e l l , u s i n g a m i c r o - p i p e t t e , Analogous r e - s u l t s were o b t a i n e d w i t h onion and amoeba c e l l s , It i s r e g r e t t a b l et h a t t h e q u e s t i o n of whether o r n o t t h e l o s s o f i m p e n e t r a b i l i t y on t h e p a r t of c e l l walls was evoked by v e r y low
temperatures
w a s n o t s u b j e c t e d t o i n v e s t i g a t i o n ,On t h e b a s i s of what has been w r i t t e n we came t o t h e c o n c l u s i o n t h a t t h e p r o c e s s e s of f r e e z i n g i n t h e i n t e r - c e l l space and i n t h e c e l l s d i f f e r s h a r p l y , I n t h e i n t e r - c e l l s p a c e , t h e moment i c e c r y s t a l s ap- p e a r t h e s t a t e of s u p e r - c o o l i n g cannot be r e t a i n e d because t h e r e a r e
no boundaries i n t h e i n t e r - c e l l space t h a t can s e r v e a s impediments f o r t h e propagation of i c e formation, A t t h e same time, because of t h e p r o t e c t i v e a c t i o n of t h e c e l l walls, c r y s t a l l i z a t i o n w i t h i n t h e
c e l l w a l l may not y e t begin; even i f i c e c r y s t a l s a r e formed i n s e p a r - a t e c e l l s , i n o t h e r c e l l s t h e s t a t e of super-cooling i s r e t a i n e d , A h i g h degree o f super-cooling i n c e l l s i s promoted by t h e i r being min- u t e and being i s o l a t e d one from t h e o t h e r , The c o n t e n t s of c e l l s , having a weak p r o p e n s i t y f o r spontaneous c r y s t a l l i z a t i o n , may form n o c r , i n c e r t a i n c e l l s a f t e r a prolonged t i m e , i . e , , when t h e d e g r e e of super-cooling becomes v e r y g r e a t , Furthermore, i n most c a s e s of food r e f r i g e r a t i o n t h e r e a r e no shocks o r v i b r a t i o n , a f a c t o r which a l s o c o n t r i b u t e s t o super-cooling,
The t h e o r y we have d e s c r i b e d concerning t h e f r e e z f n g t h a t o c c u r s i n c e l l s and i n t h e i n t e r - c e l l space a l l o w s one t o understand t h e n a t - u r e of t h e processes t a k i n g p l a c e d u r i n g t h e f r e e z i n g of t i s s u e and t o e x p l a i n t h e connection between super-cooling and d e h y d r a t i o n of c e l l s , I n o r d e r t o d e s c r i b e t h e s e processes
it
i s convenient t o u t i l -i z e a diagram o f t h e t y p e of F i g ,
4 ,
The curve OP i s t h e curve o f s p o r a t i o n of i c e from a s o l u t i o n ,A s i s known, t h e concentration of l i q u i d s i n t h e i n t e r - c e l l
space i s l e s s than t h a t of t h e l i q u i d s i n t h e c e l l s , Therefore, t h e water i n t h e i n t e r - c e l l space attempts t o pass i n t o t h e c e l l s , a f a c - t o r t h a t explains t h e t u r g o r of c e l l s . Let us s i g n i f y t h e cryoscopfc temperature, i.e., t h e temperature a t which i c e begins t o form i n
11
c e l l s , a s
t$
and i n t h e i n t e r - c e l l space a stk
Under t h e s e circum- s t a n c e s , of course,t&l>tio
The p o i n t s A and L c h a r a c t e r i z e t h e s t a t e of t h e l i q u i d s i n c e l l s and i n t h e i n t e r - c e l l space a t a c e r t a i n i n i - t i a l temperaturet o
During f r e e z i n g , when t h e temperature reachesti1,
t h e f r e e z i n g of the l i q u i d s i n t h e i n t e r - c e l l space u s u a l l y does n o t y e t begin but super-cooling takes place, During f u r t h e r cooling t h e temperature reachesti
and consequently super-cooling begins i n t h e c e l l s , A t a c e r t a i n temperaturetl,
a very low temperature, t o whicht h e super-cooling had descended ( p o i n t s B and
M)
,
t h e f r e e z i n g of i n t e r - c e l l l i q u i d s begins, a s a r e s u l t of t h e emergence on t h e i n t r o d u c t i o n of n u c l e i of c r y s t a l l i z a t i o n , The quicker t h e f r e e z i n g t h e lower w i l l be t h e temperature of super-cooling,During f r e e z i n g , a s a r e s u l t of t h e generation of heat by c r y s t a l - l i z a t i o n , t h e temperature i n t h e i n t e r - c e l l space increases, I f f r e e z - i n g occurs slowly then when t h e temperature r i s e s it reaches
ti;
i , e o , t h e point of f r e e z i n g of t h e i n t e r - c e l l s o l u t i o n ( l i n eMN),
During quick f r e e z i n g t h e temperature does not have time t o r i s e t o t h e cryoscopic p o i n t , P a r t of t h e h e a t generated during c r y s t a l l i z a t i o n h e a t s up t h e c e l l s and t h e temperature of t h e c e l l r i s e s , However, i n view of t h e f a c t t h a t heat i s generated not within t h e c e l l s buton t h e o u t s i d e , t h e temperature i n t h e c e l l s probably r i s e s t o a somewhat
lower l e v e l t h a n tbll ( ~ o i n t D ) .
Under f u r t h e r lowering of t e m p e r a t u r e t h e s t a t e of t h e l i q u i d s of t h e i n t e r - c e l l s p a c e w i l l be r e p r e s e n t e d by p o i n t s a l o n g t h e e q u i l i b r a t i o n c u r v e
-
t h e curve of i c e formation OP-
and a lowering of t e m p e r a t u r e w i l l be accompanied by s t e a d y f r e e z i n g o f w a t e r and by a n i n c r e a s e i n c o n c e n t r a t i o n ,bweh t h e temperature i n t h e i n t e r - c e l l s p a c e and i n t h e c e l l s a g a i n r e a c h e s
ti ,
t h e n as a r e s u l t of t h e f r e e z i n g of water i n t h e i q t e r c e l l space t h e c o n c e n t r a t i o n bccomes e q u a l t o t h a t found i n t h e c e l l , With f u r t h e r d e c r e a s e i n t e m p e r a t u r e t h e r e i s a renewed o n s e t ofs u p e r c o o l i n g ( l i n e ~1 E ) ; i n t h e i n t e r - c e l l s p a c e s t h e f r e e z i n g of w a t e r
w i l l oontinue, A s a r e s u l t t h e c o n c e n t r a t i o n i n t h e i n t e r - c e l l space be- comes g r e a t e r t h a n i n t h e c e l l s , D i f f u s i o n of w a t e r b e g i n s from t h e c e l l t o t h e i n t e r - c e l l s p a c e and t h e r e t h e w a t e r f r e e z e s , I n t h i s manner t h e f r e e z i n g of water i n t h e i n t e r - c e l l s p a c e and t h e maintenance of a s t a t e of super-cooling i n t h e c e l l s r e s u l t i n a c o n c e n t r a t i o n i n t h e c e l l t h a t
i s l e s s t h a n t h a t found i n t h e i n t e r - c e l l space and, i n t h e f i n a l c o n s i d e r -
a t i o n , r e s u l t s i n d e h y d r a t i o n of t h e c e l l s , The l i n e K'E i s somewhat i n - c l i n e d t o t h e a b s c i s s a i n view o f t h e f a c t t h a t i n t h e c e l l s , s i m u l t a n e o u s l y w i t h a l o w e r i n g of t e m p e r a t u r e , t h e c o n c e n t r a t i o n i n c r e a s e s a t t h e expense of a l o s s of w a t e r , The more i n t e n s e t h e d e h y d r a t i o n t h e more perpendfcu- l a r t h i s l i n e should be, The slower t h e f r e e z i n g t h e l o n g e r w i l l be t h e p r o c e s s of d e h y d r a t i o n of c e l l s and t h e g r e a t e r t h e l o s s of water from t h e c e l l s ,
A t a c t ~ r t a i n t e m p e r a t u r e o f s u p e r - c o o l l n ~
tz
1; i s nott h e same f o r d i f f e r e n t c e l l s ) , f r e e z i n g b e g i c s w i t h i n c e l l s e i t h e r because n , c r , have cmerged o r because t h e c e l l - w a l l has l o s t i t s p r o t e c t i v e pro- p e r t i e s , During t h e f o r m a t i o n of i c r c r y s t a l s i n t h e c c l l s t h e r e i s a n i n c r e a s e i n t e m p e r a t u r e t o t h e c e r t a i n p o l n t
t3,
It i s e n t i r e l y p r o b a b l et h a t t h e t e m p e r a t u r e r i s e a s s o c i a t e d w i t h t h e o r i g f n a l i c e f o r m a t i o n i n t h e c e l l s i s n e g l i g i b l e , T h i s i s a l l t h e more p r o b a b l e because t h e small s i z e of c e l l s f a v o u r a v e r y r a p i d d i s p e r s T o n of h e a t , A t t h e same time t h e c e l l s a r e surrounded by i c e t h a t o c c u r s i n t h e i n t e r - c e l l s p a c e which f s i t s e l f an accumulator of c o l d , The p r o c e s s of p a r t i a l m e l t i n g of t h i s i c e p r o g r e s s e s a l o n g t h e curve o f c q u i l i b r i u m i n t h e d i r e c t i o n from G t o 0 , The r a t e of f r e e z i n g , of c o u r s e , has t h e g r e a t e s t meaning s i n c e t h e r a t e of c x p ~ l l i n g t h e h e a t of c r y s t a l l i z a t i o n depends on i t , I n t h e c e l l , t h e rise i n t e m p e r a t u r e i s f o l l o w e d by a q u i c k d r o p i n t e m p e r a t u r e down t o t h e t e m p e r a t u r e of t h e s u r r o u n d i n g c e l l s , w h i l e s i m u l t a n e o u s l y t h e w a t e r i n t h e c e l l f r e e z e s , The c o n c e n t r a t i o n i n t h e c e l l i n c r e a s e s and r e a c h e s an e q u i l i b r i u m w i t h t h a t o f t h e i n t e r - c e l l s p a c e ( p o i n t D ) , Only a f t e r t h i s p r o c e s s does t h e ( d i f f u s i o n o f w a t e r i n t h e i n t e r - c e l l s p a c e s t o p , Then a l o n g t h e curve o f t h e e q u i l i b r i u m i n t h e d i r e c t i o n o f t h e cryohydrfc p o i n t
P,
t h e t e m p e r a t u r e s i m u l t a n e o u s l y d e c r e a s e s i n t h e c e l l and i n t h e i n t e r - c e l l s p a c e , It seems t h e n t h a t t h e p r o c e s s of f r e e z i n g , a l t h o u g h ~ i m p l e a tIV
A d e t a i l e d e x a m i n a t i o n of t h e f r e e z i n g p r o c e s s on t h e b a s i s o f con-
c l u s i o n s drawn above e n a b l e s one: 1) t o e x p l a i n t h e v a r y i n g n a t u r e of
t h e t e m p e r a t u r e c u r v e s o b t a i n e d from slow and q u i c k f r e e z i n g , and 2) t o g i v e a c o r r e c t , t h e o r e t i c a l l y s u p p o r t e d e x p l a n a t i o n f o r t h e o b t a i n i n g o f
a f i n e c r y s t a l l i n e s t r u c t u r e d u r i n g q u i c k f r e e z i n g ,
A comparison o f t h e c u r v e s o b t a i n e d from slow and q u i c k f r e e z i n g ,
shows t h a t c u r v e s o b t a i n e d d u r i n g q u i c k f r e e z i n g have c h a r a c t e r i s t i c p e c u l i a r i t i r s , F i r s t o f a l l , t h e y show no h o r i z o n t a l s e c t i o n which would have i n d i c a t e d t h e f r e e z i n g o f a l a r g e q u a n t i t y o f w a t e r a t a t e m p e r a t u r e
c l o s e t o t h e c r y o s c o p i c t e m p e r a t u r e o f t i s s u e s o l u t i o n s , A comparison o f t h e c u r v e s t a k e n a t t e m p e r a t u r e s lower t h a n t h e c r y o s c o p f c t e m p e r a t u r e i n d i c a t e s t h a t i n t h e c a s e o f q u i c k f r e e z i n g t h ~ r e i s a c o m p a r a t i v e slow-
i n g down o f t h e t e m p e r a t u r e d r o p , These p e c u l i a r i t i e s o f t h e c u r v e f o l - low c o m p l e t e l y from t h e d e s c r i p t i o n o f t h e f r e e z i n g o f t i s s u e given above,
During slow f r e e z i n g , t h e c r y s t a l l i z a t i o n of l i q u i d s c o n t a i n c d i n t h e i n t e r - c e l l s p a c e s b e g i n s a t t h e c r y o s c o p i c . t e m p e r a t u r e , A t t h e same t e m p e r a t u r e t h e w a t e r t h a t is d i f f u s e d from t h e c e l l s a l s o f r e e z e s , Be- c a u s e d u r i n g slow f r e e z i n g t h e amount o f h e a t given o f f corresponds t o t h e amount of h e a t g e n e r a t e d d u r i n g c r y s t a l l i z a t i o n , o r even l i m i t e d c r y s t a l l i z a t i o n , s u p e r - c o o l i n g i n t h e i n t e r - c e l l s p a c e d o e s n o t o c c u r , By v i r t u e o f t h e slowness o f f r e e z i n g a l a r g e q u a n t i t y o f w a t e r c a n es-
cape from t h e super-cooled c e l l s and t h e b a s h amount o f c o l d i s expended a t a t e m p e r a t u r e n e a r t h e c r y o s c o p i c t e m p e r a t u r e i n t h e f r e e z i n g of water
During q u i c k f r e e z i n g , t h e water found i n t h e i n t c r - c e l l space a l s o f r e e z e s a t t h e c r y o s c o p i c t e m p e r a t u r e , However, because of t h e i n - s i g n i f i c a n t d e h y d r a t i o n of c e l l s , t h e u s u a l amount of w a t e r f r e e z i n g a t t h i s t e m p e r a t u r e is s m a l l and t h e h o r i z o n t a l s e c t i o n o f t h e curve i s a b s e n t ; indeed, t h e q u a n t i t y o f h e a t g i v e n o f f i s g r e a t e r t h a n t h e h e a t g e n e r a t e d d u r i n g c y r s t a l l i z a t i o n , The l a r g e i n c l i n a t i o n of t h e +,cmpera- t u r e c u r v e s c l o s e t o t h e c r y o s c o p i c p o i n t i n d i c a t e s t h a t b a s i c a l l y t h e c o l d i s expended i n t h e s u p e r - c o o l i n g of t h e s o l u t i o n remaining i n t h e c e l l s . The super-cooled l i q u i d of t h e c e l l s f r e e z e s a t c o n s i d e r a b l y ex-
t e n ~ f v e super-cooling which l e a d s t o a r e d u c t i o n i n t h e d e c r e a s e of t e m p e r a t u r e a t low t e m p e r a t u r e s ,
Many r e s e a r c h e r s have had t h e o p p o r t u n i t y t o o b s e r v e t h a t c u r v e s o b t a i n e d from f a s t and slow f r e e z i n g d i f f e r from one a n o t h e r , However,
it
was I , S , Badyl'kes who f i r s t p o i n t e d t o t h e i n t e r r e l a t i o n s h i p betweent h e r a t e of f r e e z i n g and s u p e r - c o o l i n g , B a d y l P k e s d i d n o t s t u d y t h e s i z e of c r y s t a l s o b t a i n e d i n h i s experiments, I n h i s r e s e a r c h conducted d u r i n g 1929 i n i n d u s t r i a l f r e e z i n g of f i s h and f o w l i n a b r i n e , B a d y l q k e s proved c o n v i n c i n g l y t h a t i n such c a s e s super-cooling does o c c u r w h i l e a t
t h e same t i m e d u r i n g slow f r e e z i n g i n a i r t h e r e i s no super-cooling,
B a d y l g k e s a r r i v e d a t t h e c o n c l u s i o n concerning t h e emergence o f s u p e r -
c o o l i n g d u r i n g q u i c k f r e e z i n g on t h e b a s i s of t e m p e r a t u r e c u r v e s o b t a i n e d d u r i n g q u i c k and slow f r e e z i n g and on t h e b a s i s o f t h e c a l o r i m e t r i c de- t e r m i n a t i o n o f t h e e x p e n d i t u r e of c o l d , C a l o r i m e t r i c r e s e a r c h showed t h a t d u r i n g slow f r e e z i n g , i n o r d e r t o c o o l f i s h t o a g i v e n t e m p e r a t u r e i n t h e r a n g e o f -2 t o -15O, a n e x p e n d i t u r e of more c o l d f o r f r e e z i n g i n b r i n e i s
r e q u i r e d , f o e , i n t h e l a t t e r c a s e , super-cooling t a k e s p l a c e , F i n a l experiments showed t h a t t h e completion of f r e e z i n g t a k e s p l a c e i n t h e s t o r a g e chambers, It i s c l e a r t h a t t h e f r e e z i n g of t h e super-cooled
water i s one of t h e reasons f o r t h e i n c r e a s e i n t h e s i z e o f i c e c r y s t a l s
d u r i n g t h e f i r s t days of s t o r a g e ,
On t h e b a s i s of t h e above d i s c u s s e d problems concerning t h e t h e o r y of c r y s t a l l i z a t i o n and s t u d i e s concerning t h e s i g n i f i c a n c e of super- c o o l i n g d u r i n g f r e e z i n g , we were a b l e t o s u b s t a n t i a t e s c i e n t i f i c a l l y t h e reasons f o r t h e formation of a f i n e c r y s t a l l i n e s t r u c t u r e d u r i n g quick-freezing, There are s e v e r a l works on t h e problem of t h e r e l a t i o n between t h e s i z e of i c e c r y s t a l s and t h e r a t e of f r e e z i n g , However a l l of t h e s e works s u f f e r from one p r i n c i p a l deficiency8 t h e s i z e of i c e c r y s t a l s i s connected n o t with t h e degree of super-cooling d u r i n g which
t h e c r y s t a l s a r e formed, but with t h e speed of f r e e z i n g ,
The
degree of super-cooling i s a c t u a l l y t h e b a s i c f a c t o r t h a t determines t h e s i z e of c r y s t a l s ,I n examination of t h e problem concerning t h e s i z e of c r y s t a l s , l e t us f i r s t d w e l l i n some d e t a i l on t h e process of t h e f r e e z i n g of w a t e r i n t h e i n t e r - c e l l spaces, I n t h e i n t e r - c e l l s p a c e s c r y s t a l l i z a - t i o n can propagate i t s e l f unhindered because t h e r e a r e no o b s t r u c t i o n s impenetrable f o r i c e t h a t are analogous t o t h e c e l l w a l l s , Therefore,
i n t h e beginning, i f a more o r l e s s e x t e n s i v e super-cooling of t h e i n t e r - c e l l l i q u i d t a k e s p l a c e , t h e n t h e e n t i r e i n t e r - c e l l s p a c e can be fm- mediately c r y s t a l l i z e d , This phenomenon i s r e f l e c t e d i n t h e temperature
jump t h a t accompanies t h e beginning of f r e e z i n g , I n t h i s c a s e t h e water f r e e z e s i n t h e i n t e r - c e l l space without any g r e a t propagation from t h e
p l a c e where it a p p e a r s , A q u i c k p r o p a g a t i o n of i c e f o r m a t i o n i s poss-
i b l e because even a t super-cooling of s e v e r a l d e g r e e s t h e r a t e of c r y s -
t a l l i z a t i o n i n t h e l i q u i d s of t i s s u e i s g r e a t , as shown by t h e obser-
v a t i o n s of Chambers and G a i l , Here a doubt a r i s e s concerning t h e poss- i b i l i t y of t h e t w i s t e d form of t h e i n t e r - c e l l space forming a n o b s t a c l e t o r a p i d p r o p a g a t i o n of c r y s t a l l i z a t i o n , However, a number of e x p e r i - mental works, f o r example Borne, show t h a t i c e c r y s t a l s having a r a y - l i k e s t r u c t u r e overcome w i t h e a s e any o b s t r u c t i o n s p l a c e d i n t h e i r p a t h because o f t h e growth of r a y s o f t h e second and h i g h e r o r d e r s , I f w a t e r
i s super-cooled by 6 O t h e n an o b s t a c l e such as f f n e sand does n o t de-
c r e a s e t h e r a t e of c r y s t a l l i z a t i o n ,
I f super-cooling d i d n o t supercede c r y s t a l l i z a t i o n , as happens
i n c e r t a i n foods d u r i n g slow f r e e z i n g , t h e n a small number of l a r g e
c r y s t a l s form i n t h e i n t e r - c e l l s p a c e and t h e r e i s a c o n s i d e r a b l e over- d i s p e r s i o n of w a t e r , The p r o c e s s o c c u r s because d u r i n g slow f r e e z i n g t h e r a t e of g i v i n g o f f h e a t i s small and, by v i r t u e o f t h i s f a c t o r ,
d u r i n g t h e f r e e z i n g of c e r t a i n f o o d s s u p e r - c o o l i n g i s a t t i m e s com-
p l e t e l y meaningless, But a c c o r d i n g t o t h e t h e o r y of c r y s t a l l i z a t i o n
a t s l i g h t super-cooling a small number of n o c r , a r i s e and t h e r o c r , i s
slow and a small number of l a r g e r c r y s t a l s form; s i n c e t h e s e c r y s t a l s a r e s c a t t e r e d o v e r r a t h e r l a r g e d i s t a n c e s t h e w a t e r i s a t t r a c t e d t o them
from somewhat d i s t a n t p l a c e s , About t h e s e n u c l e i a l a r g e q u a n t i t y of w a t e r d i f f u s e d from c e l l s i s c r y s t a l l i z e d , According t o t h e measure o f i n c r e a s i n g t h e r a t e of t h e f r e e z i n g p r o c e s s i n t h e i n t e r - c e l l s p a c e t h e r e i s an o n s e t of somewhat e x t e n s i v e super-cooling and c o r r e s p o n d i n g l y
r a t h e r f i n e c r y s t a l s a r e formed, The d e h y d r a t i o n o f c e l l s promotes t h e same d e c r e a s e i n t h e s i z e of c r y s t a l s ,
P a s s i n g t o t h e problem of c r y s t a l f o r m a t i o n i n c e l l s , we have e v e r y r e a s o n t o suppose t h a t i n view o f t h e l i t t l e p r o p e n s i t y o f w a t e r s o l u t i o n s toward spontaneous c r y s t a l l i z a t i o n and because o f t h e i s o - l a t i o n o f c e l l s , locked-in walls t h a t are impenetrable f o r i c e , c r y s t a l s
o f i c e b e g i n t o a p p e a r i n c e l l s o n l y when t h e r e
i s
an o n s e t o f e x t e n s i v e s u p e r - c o o l i n g , I f s u f f i c i e n t s u p e r - c o o l i n g has n o t been a t t a i n e d i n t h e c e l l s , t h e n even b e f o r e n u c l e i of c r y s t a l l i z a t i o n emergep t h e w a t e r w i l l d i f f u s e o u t of t h e c e l l and f r e e z e i n t h e i n t e r - c e l l s p a c e ,A
d e c r e a s e i n c o n c e n t r a t i o n i n t h e c e l l means a c t u a l l y a d e c r e a s e i n s u p e r -
c o o l i n g , I f , d u r i n g dehydpation of t h e super-cooled c e l l s , t h e concen- t r a t i o n becomcs e q u a l t o t h a t of t h e c u r v e o f t h e e q u i l i b r i u m t h e n s u p e r - c o o l i n g w i l l become n i l , T h i s f a c t o r e x p l a i n s why d u r i n g slow f r e e z i n g t h e r e i s h a r d l y e v e r rany f o r m a t i o n of c r y s t a l s , It i s r e g r e t a b l e t h a t
s i n c e no one, up t o t h e p r e s e n t t i m e , has i n v e s t i g a t e d t h e c o n n e c t i o n between t h e s i z e of c r y s t a l s and t h e d e g r e e o f s u p e r - c o o l i n g , d i r e c t ex- p e p i m e n t a l d a t a c o n c e r n i n g t h i s q u e s t i o n i s l a c k i n g ,
I n o r d e r t o show t h e r e l a t i o n o f t h e s i z e of c r y s t a l s t o t h e d e g r e e o f s u p e r - c o o l i n g one can u t i l i z e t h e work of V e l D d on t h e f r e e z - i n g of f i s h i n b r i n e p l a c e d i n a t i n c o n t a i n e r , I n h i s e x p e r i m e n t s V e l P d d e t e r m i n e d t h e a i z e of i c e c r y s t a l s i n r e l a t i o n t o t h e r a t e o f f r e e z i n g and he o b t a i n e d t h e f o l l o w i n g t e m p e r a t u r e c u r v e s ( ~ i g ,
5),
Curve I , o b t a i n e d from f r e e z i n g i n b r i n e a t a t e m p e r a t u r e of -9.5'; c u r v e I1-
a t -12; I11 a t -18; and I V a t -21,6O, From t h e s e c u r v e s it i s a p p a r e n t
t h a t i n c a s e I and I1 f r e e z i n g o c c u r r e d without e x t e n s i v e super-cool-
i n g , a f a c t o r which i s shown by a n a l m o s t h o r i z o n t a l s e c t i o n i n t h e curve, I n c a s e I , i c e c r y s t a l s 1 7 5 ~ i n d i a m e t e r appeared i n t h e i n t e r - c e l l
s p a c e and t h e c e l l s were damaged, I n c a s e I1 t h c c r y s t a l s had a d i a -
meter of approximately 1 3 0 p and were o u t s i d e t h e c e l l walls; t h e c e l l s t o o k on a half-moon shape and were s h r i v e l l e d , I n c a s e 111
it
i s appar-e n t from t h e c u r v e
-
g r e a t l y i n c l i n e d-
t h a t i c e f o r m a t i o n o c c u r r e d d u r i n g g r e a t super-cooling and i c e c r y s t a l s 100)1
i n d i a m e t e r were formed w i t h i n t h e c e l l s , one c r y s t a l p e r c e l l , I n c a s e I V f r e e z i n g o c c u r r e d a t an even h i g h e r d e g r e e of super-cooling: a n element o f h o r i - z o n t a l e f f e c t o c c u r r e d i n t h e curve a t -6O; i n t h i s c a s e i n each c e l ls e v e r a l c r y s t a l s
63
p
i n d i a m e t e r were formed,I n examining t h e c o n d i t i o n s of c r y s t a l l i z a t i o n i n c e l l s t h e ques- t i o n n a t u r a l l y a r i s e s concernfng t h e r e a s o n why, r e g a r d l e s s of t h e g r e a t r a t e of c r y s t a l l f z a t i o n , s e v e r a l c r y s t a l s can form i n each c e l l . The p o s s i b i l i t y of s e v e r a l n , c r , emerging i n a c e l l i s a p p a r e n t l y + e x - p l a i n e d by t h e f a c t t h a t t h e s t r u c t u r e of c e r t a i n c e l l s h i n d e r s t h e f r e e propagation of a c r y s t a l from one n u c l e u s , So, f o r e x a n p l e , muscle f i b r e s c o n s i s t of extremely f i n e p a r a l l e l f i b r i l s between which t h e r e i s a l i v i n g protoplasm i n which t h e needle c r y s t a l s a r e formed, A f t e r t h e formation of a needle-shaped c r y s t a l , continued f r e e z i n g can t a k e p l a c e o n l y i f super-cooled w a t e r flows t o t h e c r y s t a l from t h e s e p a r a t e p a r t s of t h e f i b e r ,
It
Is c l e a r t h a t c o n s i d e r a b l e timeis r e q u i r e d f o r t h i s d i f f u s i o n p r o c e s s , b u t under s u f f i c i e n t super- e o o l f n g t h e time n e c e s s a r y f o r t h e cnergence of t h e second and f o l l o w -
i n g n , c r , i s l e s s t h a n t h e time r e q u i r e d t o f r e e z e t h e c o n t e n t s of a cell, A t t h e same t i m e , because of t h e absence of a n y o b s t r u c t i o n s , l a r g e c r y s t a l s form even d u r i n g q u i c k f r e e z i n g , as i n t h e c a s e of t h e g l a s s - l i k e s u b s t a n c e of e y e s , e t c , He w i l l n o t e t h a t , in f u l l agreement w i t h t h e examined t h e o r y o f c r y s t a l l i z a t i o n , a t v e r y high r a t e s o f f r e e z i n g
it
i s p o s s i b l e t o g e t food p r o d u c t s i n a g l a s s - l i k e s t a t e , The o b t a i n i n g o f a g l a s s - l i k e s t a t e , r e g a r d l e s s o f a h i g h r a t e of c r y s t a l l i z a t i o n i n t h e t i s s u e li- q u i d s , i s e x p l a i n e d by t h e w a t e r of food p r o d u c t s b e i n g locked i n minute i s o l a t e d c e l l s , F i n a l l y , i n o r d e r t o demonstrate t h e l a c k of c l a r f t y t h a t e x i s t s*
up t o t h e p r e s e n t time i n t h e t h e o r y of q u i c k f r e e z i n g,
we w i l lmention t h e argument t h a t a r o s e between G e i s s and B a d y l v k e s , Badylg%es, i n h i s work, showed t h a t d u r i n g q u i c k f r e e z i n g t h e e x p e n d i t u r e o f cold i n t h e c o o l i n g o f f i s h t o below t h e cryoscopfc t e m p e r a t u r e i s l e s s t h a n d u r i n g slow f r e e z i n g , and he showed t h a t a l e s s e r e x p e n d i t u r e of c o l d i s e x p l a i n e d by t h e f a c t t h a t super-cooling o c c u r r e d i n t h e s e c a s e s and t h a t a competfon of f r e e z i n g o c c u r s i n t h e s t o r a g e chambers,
*
The e r r o r s of e x p l a n a t i o n s u s u a l l y c i t e d i n l i t e r a t u r e e x p r e s s i n ga d e s i r e f o r r a p i d p a s s i n g of t h e tlzone of maximum c r y s t a l l f z a t f o n a o b v i o u s l y f o l l o w s from t h e above quoted t h e o r f e s and t h e r e f o r e we d i d n o t d w e l l on t h i s q u e s t i o n i n o u r a r t i c l e ,
P l a n k , i n o r d e r t o be convinced t h a t t h e t e m p e r a t u r e was e q u a l
i n a l l p a r t o f t h e food p r o d u c t , r e t a i n e d t h e f r o z e n food f o r c a l o r i - m e t r i c d e t e r m i n a t i o n f o r a prolonged p e r i o d of time a t a t e m p e r a t u r e
f o r which t h e e x p e n d i t u r e of c o l d was determined, and a t t h i s same t i m e , of c o u r s e , t h e completion o f f r e e z i n g o f t h e super-cooled li- q u i d s o c c u r r e d ,
S e v e r a l y e a r s l a t e r ~ e i s s * ~ w h i l e conducting s t u d i e s on t h e d e t e r m i n a t i o n of t h e e x p e n d i t u r e of c o l d d u r i n g q u i c k and slow f r e e z - i n g , attempted t o s o l v e t h e problem o f e x p l a i n i n g t h e r e a s o n s f o r t h e v a r i a t i o n i n t h e r e s u l t s o b t a i n e d by Plank and B a d y l g k e s , Geiss wrote t h a t i f t h e r e i s a c t u a l l y a d i f f e r e n c e i n t h e amount of c o l d r e q u i r e d f o r qufck and f o r slow f r e e z i n g , t h e n t h i s c i r c u m s t a n c e would i n i t s e l f be a g r e a t advantage f o r methods of quick f r e e z i n g , I n h i s
e x p e r i m e n t s , G e f s s a i n o r d e r t o be convinced t h a t t h e t e m p e r a t u r e of meat a t
aU
p o i n t s was t h e same as t h a t o f t h e f r e e z i n g medium, k e p t t h e meat a t t h i s t e m p e r a t u r e f o r a p e r i o d s e v e r a l t i m e s l o n g e r t h a nt h a t r e q u i r e d f o r f u l l f r e e z i n g a c c o r d i n g t o P l a n k u s formula, A s a r e s u l t Geiss found t h a t under a s t a t e of e q u f l i b r i u m t h e r e i s no d i f -
f e r e n c e i n t h e amount of c o l d s p e n t d u r i n g qufck and slow f r e e z i n g , The r e s u l t s o b t a i n e d by B a d y l g k e s , i n t h e o p i n i o n of G e i s s ,
can o n l y be e x p l a i n e d by t h e p r o b a b i l i t y t h a t B a d y l g k e s , from a p u r e l y p r a c t i c a l c o n s i d e r a t i o n
-
f o e , a d e s i r e t o f u l l y u t i l i z e t h e expen- s i v e f r e e z i n g t a n k , simply d i d n o t f u l l y f r e e z e t h e f i s h , A t t h e same t i m eit
i s obvious t h a t t o keep t h e f i s h i n t h e f r e e z i n g t a n k u n t i l complete f r e e z f n i of a l l t h e w a t e r has been a t t a i n e d , correspond- i n g t o a s t a t e o f e q u i l i b r i u m , i s completely i r ~ a t i o n a l , During an*
2, f , d , g , K a e l t e I n d u s t r i e , no, 7-9,1933
o n s e t of s u p e r - c o o l i n g t h e t e m p e r a t u r e of f i s h can b e c o n e , s f t r r a
c e r t a i n t i m e , e q u a l t o t h a t o f t h e b r i n e i n t h e t a n k , and a t t h e same t i m e f o r complete f r e e z i n g o f f i s h
it
would have been n e c e s s a r y t o r e - move s t i l l a n o t h e r determined amount o f h e a t , Likewise f o r t h e f r e e z - i n g o f w a t e r i n t h e super-cooled t i s s u e s a r a t h e r prolonged time i s n e c e s s a r y , which i s e x p l a i n e d , as we p o i n t e d o u t , by t h e i m p e n e t r a b i l -i t y of t h e c e l l w a l l s by i c e c r y s t a l s and t h e small p r o p e n s i t y of t i s -
s u e s o l u t i o n s f o r spontaneous c r y s t a l l i z a t i o n , I n t h i s r e s p e c t G e i s s i n h i s work p e r m i t t e d a n e r r o r because t h e t i m e n e c e s s a r y f o r f u l l f r e e z i n g of meat was determined by t h e r m a l c o n d u c t i v i t y f o r m u l a , Mean- w h i l e , w i t h t h e p o s s i b i l i t y o f s u p e r - c o o l i n g , one c a n n o t , o f c o u r s e , d e t e r m i n e t h e time n e c e s s a r y f o r complete f r e e z i n g because o f t h e con- d i t i o n s of t h e r m a l c o n d u c t i v i t y , I n t h i s t h e p r o p e n s i t y f o r spontaneous c r y s t a l l i z a t i o n i s t h e d e c i s i v e f a c t o r , I n d e e d , i n c e r t a i n c a s e s t h e s t a t e of s u p e r - c o o l i n g i s maintained o v e r a v e r y prolonged p e r i o d o f
tfme, So, a c c o r d i n g t o t h e o b s e r v a t i o n s of B a d y l o k e s , t h e completion o f f r e e z i n g o f l i q u i d s i n q u i c k l y f r o z e n f i s h can proceed f o r s e v e p a l d a y s , From t h e above
it
i s a p p a r e n t t h a tit
i s s u p e r - c o o l i n g t h a t is t h e d e t e r m i n i n g f a c t o r f o r t h e s i z e of i c e c r y s t a l s formed d u r i n g f r e e z i n g , Up t o t h e p r e s e n t t i m e , f o r t h e purpose of o b t a i n i n g a f i n e c r y s t a l l i n e s t r u c t u r e , r e s e a r c h e f f o r t s and i n v e n t i v e t h i n k i n g have been d i r e c t e d toward i n c r e a s i n g t h e rate of f r e e z i n g , A h i g h e rr a t e of f r e c z i n g however, i s o n l y one of t h e f a c t o r s i n o b t a i n i n g s u p e r - c o o l i n g l e a d i n g t o a f i n e c r y s t a l l i n e s t r u c t u r e , From o u r work i t f o l -
lows t h a t t h e o b t a i n i n g of a f i n e c r y s t a l l i n e s t r u c t u r e could be accom-
p l i s h e d a t r e l a t i v e l y high f r e e z i n g t e m p e r a t u r e s by d i r e c t l y s u b j e c t i n g
t h e food p r o d u c t s t o s u b s t a n c e s t h a t d e c r e a s e t h e r a t e of c r y s t a l l i z a t i o n b u t i n c r e a s e t h e number of c r y s t a l l i z a t i o n n u c l e i t h a t a r i s e i n c e l l s , The r e s u l t would be t h a t f r e e z i n g would begin i n c e l l s a t v e r y s l i g h t
s u p e r - c o o l i n g and d e h y d r a t i o n o f c e l l s would p r a c t i c a l l y n o t o c c u r ,
A
d e c r e a s e i n t h e r a t e of c r y s t a l l i z a t f o n and an i n c r e a s e i n t h e number of n u c l e i would a t t h e same t i m e l e a d t o t h e o b t a i n i n g of a f i n e c r y s t a l - l i n e s t r u c t u r e even a t a s l i g h t d e g r e e of s u p e r - c o o l i n g , A t t h e p r e s e n t time t h e f r e e z i n g of f r u i t i s c a r r i e d o u t i n s u g a r s y r u p s of v a r y i n g c o n c e n t r a t i o n , V e g e t a b l e s a r e f r o z e n i n s a l t s o l u -t i o n s , From t h e d i s c u s s i o n p r e s e n t e d above one may conclude t h a t
it
i s n e c e s s a r y t o i n v e s t i g a t e t h e i n f l u e n c e of t h e c o n c e n t r a t i o n of s u g a r
and s a l t on t h e c u r v e s of r a t e s o f c ~ y s t a l l i z a t i o n and on t h e p r o p e n s i t y
f o r spontaneous c r y s t a l l i z a t i o n ,
I n t h e p r e s e n t paper we have g i v e n a t h e o r e t i c a l b a s i s f o r o b t a i n - i n g a f i n e c r y s t a l l i n e s t r u c t u r e by q u i c k f r e e z i n g of food p r o d u c t s which have i n common t h e f o l l o w i n g c h a r a c t e r ; t h e y c o n s i s t of c e l l s and d u r i n g f r e e z i n g t h e w a t e r c o n v e r t s t o c r y s t a l s , It i s c l e a r t h a t
t h e o b t a i n i n g of a f i n e c r y s t a l l i n e s t r u c t u r e i n t h e p r e p a r a t i o n of o t h e r
f o o d s , e o g o , c h o c o l a t e , v a r i o u s k i n d s of i c e d e s s e r t s , e t c , , a l s o depends on t h e same d e g r e e of s u p e r - c o o l i n g d u r i n g which c r y s t a l l i z a t i o n p r o g r e s s e s ,
T e m p e r a t u r e i n O C . F i g . 1. The d e p e n d e n c e o f t h e number o f n u c l e i on t e m p e r a t u r e f o r p i p e r i n e ( m e l t i n g t e m p e r a t u r e 1 2 g 0 ) . C d u p e r - Iblelting p o i n t i , 2 The d e p e n d e n c e o f t h e l i n e a r r a t e of c r y s t a l - l i z a t i o n on s u p e r - c o o l i n ~ .
n. c r .
S u p e r -
P i & . 3 .
