Preventing enzymatic browning of potato by microwave blanching
Carla SEVERINI1, *, Teresa DE PILLI1, Antonietta BAIANO1, Dino MASTROCOLA2, Roberto MASSINI3
RÉSUMÉ Traitement de blanchiment par micro-ondes pour la prévention du brunissement enzymatique de pommes de terre.
Différents échantillons de pomme de terre de type Primura, ont été coupés en rondelles et en cubes, puis blanchis par différents traitements : immersion dans l’eau bouillante, dans une solution hypertonique de sirop de maïs, par micro-ondes et micro-ondes après immersion dans l’eau, avec et sans chlorure de sodium et de calcium. Ces expériences ont été réalisées à des tempéra- tures de 98 °C et 100 °C et à une puissance de 850 Watts pour les micro- ondes, le tout pour des durées d’exposition variables. On a évalué différentes caractéristiques physiques des pommes de terre telles que la couleur et la consistance des échantillons, ainsi que les transferts d’eau et de matière dus au procédé. Les résultats obtenus ont montré que les échantillons coupés en cubes, plongés dans l’eau, et blanchis par micro-ondes présentaient une cou- leur plus stable et une consistance plus molle que les autres échantillons (ceci étant probablement dû à un plus grand degré de gélatinisation de l’amidon des pommes de terre). La consistance des produits blanchis par micro-ondes a été améliorée en employant des solutions de chlorure de sodium et de calcium pour le bain de blanchiment.
Mots clés : blanchiment, brunissement enzymatique, micro-onde, pomme de terre.
SUMMARY
Samples of potato slices and cubes, cultivar Primura, were blanched by dip- ping them in boiling water and hypertonic corn syrup solution, by microwaves and microwaves upon water immersion, with or without sodium chloride and/or calcium chloride. Treatments were performed at 98°C, 100°C and 850 Watts,
1. Istituto di Produzioni e Preparazioni Alimentari, University of Foggia, via Napoli 25, 71100 Foggia, Italy.
2. Dipartimento Scienze degli Alimenti, University of Udine, via Marangoni 97, 33100 Udine, Italy.
3. Dipartimento Ingegneria Industriale, University of Parma, Parco Area delle Scienze 181/A, 43100 Parma, Italy.
* Correspondance.
respectively, for different times. Some physical characteristics (colour and firm- ness of samples) as well as mass and water transfers were evaluated. The results showed that the samples of potato cubes put in water and blanched by microwaves were the most stable in terms of colour but the least consistent, probably because of the accelerated gelatinization of potato starch. The firm- ness of product blanched by microwave was improved by using a calcium chloride and sodium chloride aqueous solution as blanching bath.
Key words: blanching, enzymatic browning, microwave, potato.
1 - INTRODUCTION
Browning of potato, due to enzymatic activity, is usually prevented by blan- ching, by adding antioxidants such as potassium bisulphite or ascorbic acid or by acidification with citric acid (SAPERSand MILLER, 1993; MARTINEZand WHITA- KER, 1995). Dipping in boiling water is the most common method of blanching before frying and freezing and it is also used in order to decrease the reducing sugar content at the surface of the product; in fact, the presence of reducing sugars leads to changes in colour due to caramelization and Maillard reactions (SMITH, 1975). Moreover, the boiling water blanching shows some disadvan- tages, such as soluble solid loss, firmness decrease, high water consumption and, consequently, high volume of water to drain away. Blanching of potatoes is also performed by steam and microwaves (COLLINS and MCCARTY, 1969; CHEN et al., 1971; RAMASWAMYand VAN DE VOORT, 1990; PONNEet al., 1991): in these cases, the product has to be stored at room temperature for several weeks to reduce the high content of sugars before treatment.
The most dramatic result of enzymatic activity is due to polyphenoloxidase (PPO), which, in the presence of molecular oxygen, reacts with phenolic com- pounds (mainly tyrosine) to form brown polymers (MAPSON, 1963; COLLINS and MCCARTY, 1969). Moreover, peroxidase activity leads in particular to the forma- tion of off flavours (PINSENT, 1962). Problems linked to changes in colour are particularly significant if the product is frozen or dried at low/medium tempera- tures; in fact, enzymatic activity remains significant at low temperatures (MAN- ZOCCO et al., 1999) and it is strongly encouraged at 40-50°C (CHEFTEL and CHEFTEL, 1988).
Heat treatment of potatoes also causes changes in textural characteristics, probably because of swelling of gelatinized starch. Other phenomena connec- ted to the use of high temperature, such as loss of cells turgor pressure or acti- vation/inhibition of pectic enzymes could also affect product firmness (MALTINI et al., 1993). COLLINSand MCCARTY(1969) found that although the softening of the product seemed to be more evident after microwave blanching than after water blanching, an effective treatment against enzymatic activity was obtained in a blanching time three times lower by using microwaves.
Some authors (DI CESARE et al., 1997) evaluated the effects on colour, fla- vour, firmness and sensorial characteristics of apple cubes blanched by micro- waves with or without water, as compared to samples blanched by a traditional method in boiling water. The best results were obtained by microwave blan-
ching performed at 80°C without water; this process yielded a product with a better flavour and firmness than for traditionally blanched samples.
The aim of this work was to study the changes in colour and firmness of potato slices and cubes blanched by different processes, such as microwaves, dipping in boiling water and dipping in hypertonic corn syrup solution, in order to define the most effective process. Blanching treatments were performed at different temperatures for different times, and some characteristics of potatoes such as changes in colour (red value – a* –) and firmness were evaluated as well as mass and water transfers due to the treatments.
2 - MATERIALS AND METHODS
2.1 Raw material
Trials were performed on potatoes (var. Primura) purchased from the local market. This variety is characterised by early ripening, mass production and a big tuber with a regular shape and good resistance to cooking.
2.2 Sample preparation
Potato tubers were selected and washed in tap water, then each tuber was hand peeled and cut. Samples were cut into slices of 0.5 cm thick (first lot) or in cubes of 1 ×1 ×1 cm (second lot).
2.3 Blanching treatments
The blanching was carried out in different ways:
Dipping in sodium chloride aqueous solution (3% w/w)
Sliced and cubed samples (about 200 g of potatoes) were blanched at 88°C and 98°C for 1, 3 and 5 min. The product: solution ratio was 1:5 (w/w). After blanching, the samples were cooled in water for 1 min and then drained and wiped with blotting paper. All samples and control (unblanched potatoes) were kept at room temperature and exposed to light for 8 h.
Dipping in hypertonic corn syrup solution with sodium chloride
The blanching was carried out on sliced samples with industrial corn syrup (Glicosa), which had the following characteristics: refractometer grade
= 80.6°Brix, pH value = 5.1 at 20°C and dry matter 80.7%. The chemical com- position of corn syrup “Glicosa” used for the “osmo-blanching” is the following:
30% dextrose, 46% maltose, 10% trisaccharides, 14% polysaccharides. The corn syrup was diluted down to 70°Brix, with distilled water then 3% (w/w) of sodium chloride was added. The sugar solution: product ratio was 1:5 (w/w).
The blanching temperatures were 88°C, 98°C and 100°C, while the lengths of treatments were 1, 3 and 5 min. Afterwards the samples were treated as those dipped in sodium chloride solution.
Microwaves
The sliced potatoes (about 200 g) were placed in a glass container and radiated with microwaves by a “Sfornatutto De Longhi” mod. Combi & Function Convection (De Longhi, Milano, Italy) at a power of 850 Watts for different lengths of time (from 20 to 180 s).
The cubed potatoes (about 200 g) were blanched upon immersion in water, sodium chloride (3% w/w), calcium chloride (0.02% w/w) plus sodium chloride (3%) and calcium chloride (0.02% w/w) aqueous solution in a glass container and radiated for 5 min with microwaves in the same way as the sliced potatoes.
The product: solution ratio was 1:5 (w/w). Afterwards the samples were treated as sliced products.
The temperature in the centre of the samples and in the solution used for blanching treatments was measured with a Hanna thermocouple mod. HI 93530 (Hanna Instruments, Rocchi di Villafranca, PD, Italy).
2.4 Analyses
On samples (raw, immediately after blanching and, at regular intervals, over a period of 8 h of storage at room temperature) colour determinations were car- ried out. The colour analysis was performed by a tristimulus colorimeter (Chro- mameter-2 Reflectance, Minolta, Osaka, Japan) equipped with a CR 300 measuring head. The colorimeter was calibrated on a standard tile (L*: 93.5; a*:
–1.0; b*: 0.8) before each series of measurements. Colour index was expressed as red value (a* parameter on the Hunter scale). For each sample 5 determina- tions were performed. The coefficients of variation were less than 8%.
The following analyses were also performed:
– weight loss (g): weighing samples before and after treatments;
– firmness: using a Universal Testing Machine (Instron International LTD, High Wycombe, UK), model 4301, equipped with a standard plunger (φ 3 mm) at a crosshead speed of 200 mm/min, compression force was taken as the maximum peak of recorded force expressed as Newtons (N) (MAL- TINI et al., 1993); for each treatment 10 determinations were carried out, the coefficients of variation were less than 10%;
– moisture (%): placing a carefully weighed sample in a static oven (ISCO s.r.l. Milan Italy) at 105°C until it reached constant weight (AOAC method, 1984).
2.5 Data analysis
All the data reported are the average of at least three replications. Correla- tion equations and correlation coefficients (R) were calculated from linear regression of samples colour vs. exposure time at room temperature using Excel 97 (Microsoft Corporation, USA). The coefficients of variation have been calculated for all the measurements, taken as the ratio of the standard deviation to the mean value.
3 - RESULTS AND DISCUSSION
As shown in table 1, in all the trials, the longer the treatment time, the lower the a* value; these results confirm the effectiveness of blanching process in the brightening of the colour of treated products.
Table 1
a* values of raw sliced potato samples (time 0) and immediately after blanching by:
dipping in sodium chloride aqueous solution (3%) at 98°C, in “Glicosa” solution (70° Bx) plus sodium chloride (3%) at 100°C, and microwaves at 850 Watts Blanching time Water + NaCl (3%) Glicosa (70°Bx) + Microwaves
(s) at 98°C NaCl (3%) (850 Watts)
000 – 6,82 – 6,82 – 6,82
020 — — – 7,45
050 — — – 8,53
060 – 7,43 – 7,44 —
100 — — – 9,23
180 – 7,62 – 7,28 – 9,67
300 – 9,17 – 9,39 —
Figure 1 shows the changes in red value (a*) of sliced potato samples blan- ched by dipping in sodium chloride aqueous solution at 98°C for 1, 3, and 5 min as a function of the storage time at room temperature (8 h). The a* value of the raw material progressively increased, leading to the unacceptability of the pro- duct. Trials carried out for 1 and 3 min showed that the blanching time was not enough for complete polyphenoloxidase inactivation, in fact the colour of the slices became red-brown. Only the treatment performed for 5 min resulted in a stable colour. Trials performed at 88°C did not allow complete enzymatic inacti- vation (data not shown).
Blanching in corn syrup solution (Glicosa 70° Brix), suitable as pre-treatment before drying in order to obtain a partial dehydration and enzymatic inactivation (MASTROCOLAand DALL’AGLIO, 1993), was used for trials at 88°C, 98°C (data not shown) and 100°C; but the complete enzymatic inactivation was obtained only at the last temperature. The use of a corn syrup aqueous solution as blanching bath for vegetables is possible because of its low content of monosaccharides;
this characteristic avoid the sugar uptake by the treated products and, conse- quently, do not influence their sensorial properties (MASTROCOLA et al., 1987).
Red values of potato samples, blanched in Glicosa solution at 100°C for 1, 3 and 5 min are shown in figure 2. Also in this case, only the heat treatment car- ried out for 5 min was found to be effective in preventing enzymatic browning of the samples. In fact, potato slices treated for 1 min showed changes in colour similar to those of raw potatoes, while samples treated for 3 min were more
similar to those treated for 5 min, although its colour was less stable during sto- rage after treatment.
Figure 1
Changes in a* value of raw sliced potato samples and sliced potato samples blanched by dipping in sodium chloride aqueous solution (3%) at 98°C for 1, 3
and 5 min, during storage at room temperature after the treatment Storage time (hours)
raw material 20’’ 50’’ 100’’ 180’’
Figure 2
Changes in a* value of raw sliced potato samples and sliced potato samples blanched by dipping in “Glicosa” solution (70° Bx) and sodium chloride (3%) at 100°C for 1, 3 and 5 min, during storage at room temperature after the treatment
Storage time (hours)
raw material (100°C;1’) (100°C;3’) (100°C;5’)
A third series of potato slices was blanched by microwaves and the results are shown in figure 3. The a* values of the sample treated for 180 s were the lowest and the most stable during storage after blanching; on the contrary the a* value of the sample treated for 20 s increased as the time of storage increa- sed, showing a rate of change similar to those of the raw material. During sto- rage at room temperature, the colour of the sample blanched for 50 s showed an anomalous behaviour with more intense browning than those of raw pota- toes. In fact, black and unhomogeneous spots appeared on the slice surface instead of a red homogeneous colour (typical of natural enzymatic browning).
This behaviour could be connected to the low homogeneity of the electroma- gnetic field that left surface areas with active polyphenoloxidase able to cata- lyse the formation of anomalous coloured compounds. The different substrates of enzyme catalysis could derive from the partial gelatinization of potato starch, induced by microwave treatment.
Rate kinetic constants, calculated from linear regression of curves shown in figures 1, 2 and 3, are reported in table 2. In all the trials, with the exception of the samples blanched by microwaves for 50 s, the longer the treatment time, the lower the kinetic constants, according to SAPERSandDOUGLAS, 1987; MAS- TROCOLAand LERICI, 1991.
Change in firmness was also a sign of the modified internal structure of the sample treated by microwaves for 50 s (figure 4). Figure 4 shows the changes in firmness of all the samples as a function of the treatment time. A dramatic decrease was observed after 50 s in samples treated by microwaves compared to the gradual decrease of the firmness of potato slices blanched by water and Glicosa.
Figure 3
Changes in a* value of raw sliced potato samples and sliced potato samples blanched by microwaves at 850 Watts for 20, 50, 100 and 180 s, during storage
at room temperature after the treatment Storage time (hours)
raw material 20’’ 50’’ 100’’ 180’’
Figure 4
Changes in firmness (N) of raw sliced potato samples and sliced potato samples blanched by dipping in sodium chloride aqueous solution (3%) at 98°C, dipping
in Glicosa solution (70° Bx) and sodium chloride (3%) at 100°C and blanched by microwaves at 850 Watts, as a function of the treatment time (s)
Time (s)
water blanching 98°C glicosa 100°C microwave 850 watts Table 2
Kinetic constants (K) and correlation coefficient (R) calculated from linear regression of curves shown in figures 1, 2 and 3
Kinetic constants (k) and correlation coefficient (R)
NaCl aqueous solution Sugar solution Microwaves
(glicose 70° Brix)
K; R K; R K; R
T.Q. – 0.449; 0.919 T.Q. – 0.449; 0.919 T.Q. – 0.449; 0.919 (98°C; 1’) – 0.176; 0.827 (100°C; 1’) – 0.412; 0.956 20” – 0.340; 0.966 (98°C; 3’) – 0.121; 0.870 (100°C; 3’) – 0.034; 0.266 50” – 0.926; 0.970 (98°C; 5’) – 0.038; 0.824 (100°C; 5’) – 0.026; 0.538 100” – 0.056; 0.604 180” – 0.025; 0.396
The same experiments were performed on potato cubes. Results confirmed that the most stable samples in terms of colour were those blanched in water at 98°C for 5 min and in corn syrup solution (Glicosa 70° Brix) at 100°C for 5 min (data not shown). The microwave blanching of potato cubes was carried out placing the samples in water or sodium chloride aqueous solution (3%), in order to avoid the product burning due to the “edge effect”. In fact, from the results of previous trials, the browning due to the edge effect was proved in potato cubes
treated by microwaves without water (CANNARSI, 2000), probably because of the unhomogeneous heating rate of microwaves apparatus when used on dry and cubed products. Trials were carried out for 3 (data not shown) and 5 min, but only those performed for 5 min were found to be effective in preventing brow- ning of the product.
Immediately after blanching the cube samples showed similar colour to that of the raw product (a* value: – 6.48 and – 6.65 versus – 6.42 for the raw samples).
Figure 5
Changes in a* value of raw cubed potato samples and cubed potato samples blanched by microwaves at 850 Watts for 5 min upon immersion in water and in
sodium chloride aqueous solution (3%) as a function of storage time at room temperature after the treatment
Storage time (hours)
raw material microwave (850 watts; 5’) in water
microwave (850 watts; 5’) in sodium chloride aqueous solution
In figure 5, changes in red value of potato cube samples blanched by micro- waves in water and sodium chloride aqueous solution during storage are shown in comparison with changes in colour of raw samples. Even though the variabi- lity of the a* values of the cubes was greater than those of slices, it was pos- sible to observe the increase of browning of the raw sample as the storage time increased, whereas the colour of the blanched cubes was nearly the same before and after storage (for raw cubes: kinetic constant 0.474, R 0.865; for potato cube sample blanched by microwaves in water: kinetic constant 0.077, R 0.500; for potato cube sample blanched in sodium chloride aqueous solution:
kinetic constant 0.006, R 0.064). Furthermore, the firmness of the blanched sample put in sodium chloride aqueous solution was greater than that of water blanched sample (table 3); this is likely because of the salt transfer from the solution to the product by osmosis, which could affect water transfer and starch gelatinization.
In order to improve the firmness of processed products, trials with different salt solutions as blanching bath were carried out. In table 3 the firmness of pro- duct expressed as raw/treated ratio is shown.
Table 3
Firmness (N) of potato cubes blanched by dipping in sodium chloride aqueous solution (3%), in calcium chloride aqueous solution (0.02%), in sodium chloride (3%)
plus calcium chloride (0.02%) aqueous solutions at 98°C for 5 min and by microwaves at 850 Watts for 5 min upon immersion in the same solutions,
expressed as raw/treated sample ratio
Firmness (N raw/ N treated)
Treatments Solutions
3% NaCl 0.02% CaCl2 3% NaCl + 0.02% CaCl2
98°C; 5’ 0.57 0.54 0.48
MW (850 Watt; 5’) 0.47 0.34 0.74
The data showed that the firmness of product treated by microwaves (with 3% sodium chloride or 0.02% calcium chloride aqueous solution) was lower than that of boiling water blanched products. On the contrary, the firmness of potato cubes blanched by microwave “wet” process in 3% of sodium chloride and 0.02% of calcium chloride aqueous solution was decidedly improved.
Table 4
Mass and water transfer balance of raw cubed samples and cubed samples blanched by dipping in sodium chloride aqueous solution (3%) at 98°C, dipping
in Glicosa solution (70° Bx) and sodium chloride (3%) at 100°C and blanched by microwaves at 850 Watts
Weight loss Dry matter Dry matter Water
Samples
(g/100g) (g) H2O(g) gain/loss loss gain/loss (g/100g) (g/100g)
Raw — 21.5 78.5 — —
H2O + NaCl (3%) – 2.72 17.45 82.55 – 4.53 + 1.8
Glicosa + NaCl (3%) – 15.17 26.07 73.93 + 0.61 – 15.78
MW in H2O – 3.67 15.11 84.89 – 6.95 + 3.27
MW in H2O + NaCl (3%) – 0.8 20.18 79.82 – 1.48 + 0.68
Mass and water transfers were calculated in cubed samples. Data (table 4) showed that all the samples lost weight, in particular when blanched by Glicosa solution (–15.17 g/100 g), which caused the partial dehydration of the product.
In this case the weight loss was principally due to the water loss and, only in low percentage, to the solids transfer from Glicosa to potato pieces. The oppo- site phenomenon, water gain and dry matter loss, was observed for the other
samples; in fact the potato cubes absorbed water from the blanching bath, while they released dry matter (mainly starch) into water. Furthermore, the dry matter loss was the balance between starch release and sodium chloride gain.
The highest weight loss was found for the sample put in water and blanched by microwaves (– 3.67 g/100 g), whereas the lowest was in sample put in sodium chloride solution and blanched by microwaves (– 0.8 g/100 g); this is likely because the osmotic pressure of sodium chloride solution decreased the water gain and balanced the starch loss of the product.
4 - CONCLUSIONS
Results of our trials confirmed that the blanching process by microwaves could effectively substitute the traditional process, performed by boiling water, in the case of potato samples. In fact, potato slices, when treated for a suitable length of time, showed stable colour. Previous experiments proved that potato cubes cannot be treated without water, because of the unhomogeneous heating rate of microwaves apparatus when used on dry and cubed products. When microwave blanching was carried out by the “wet” process with the addition of sodium chloride, colour of the samples was acceptable, even though firmness was lower than those of the samples blanched in water or Glicosa, probably because of the faster starch gelatinization, loss of cells turgor pressure or acti- vation of pectic enzymes. In our experimental conditions the firmness of pro- duct blanched by microwave was improved by using a calcium chloride and sodium chloride aqueous solution as blanching bath. Moreover the optimisation of microwave “wet” process could lead to a lower consumption of water during the process and a lower volume of water to drain a way.
Received 21 August 2000, accepted 9 March 2001.
REFERENCES
AOAC, 1984. Official Methods of Analysis, 14th ed. Association of Official Analytical Chemists, Arlington, VA.
CHEFTEL J.C., CHEFTEL H., 1988. Biochi- mica e Tecnologia degli Alimenti. Vol. 1, Eda- gricole, Bologna, Italy.
CANNARSI M., 2000. Pretratamenti alla disi- dratazione di prodotti vegetali: studio su alcune condizioni di processo e sulla qualità del prodotto. Graduation Thesis, University of Foggia, Agriculture Faculty.
CHEN S.C., COLLINS J.L., MCCARTY I.E., JOHNSTON M.R., 1971. Blanching of white potatoes by microwave energy followed by boiling water. J. Food Sci., 36, 742-743.
COLLINS J.L., MCCARTY I.E., 1969. Compa- rison of microwave energy with boiling water for blanching whole potatoes. Food Tech., 23, 63-66.
DI CESARE L.F., NANI R., PROIETTI M., 1997. Impiego delle microonde come tecnica alternativa per il blanching di cubetti di mela:
valutazioni quali-quantitative. In: 3th Italian Congress on Food Science and Technology (CISETA) «Ricerche e Innovazioni nell’Indus- tria Alimentare». Vol. II. 140-149, Chiriotti Ed., Pinerolo (TO), Italy.
MALTINI E., TORREGGIANI D., RONDO BROVETTO B., BERTOLO G., 1993. Functio- nal properties of reduced moisture fruits as ingredients in food systems. Food Res. Int., 26, 413-419.
MANZOCCO L., NICOLI M.C., ANESE M., PITOTTI A., MALTINI E., 1999. Polypheno- loxidase and peroxidase activity in partially frozen systems with different physical pro- perties. Food Res. Int., 31, 363-370.
MAPSON L.W., SWAIN T., TOMALIN A.W., 1963. Influence of variety, cultural conditions and temperature of storage on enzymic browning of potato tubers. J. Sci. Food Agric., 13, 673-676.
MARTINEZ M.V., WHITAKER J.R., 1995. The biochemistry and control of enzymatic brow- ning. Tr. Food Sci. Tech., 6, 195-200.
MASTROCOLA D., DALL’AGLIO G., 1993.
Problematiche relative all’essiccazione della frutta. Ind. Alim., 32, 1-6.
MASTROCOLA D., LERICI C.R., 1991. Colo- rimetric measurements of enzymatic and
non-enzymatic browning in apple purees. Ital.
J. Food Sci., 3, 219-229.
MASTROCOLA D., SEVERINI C., LERICI C.R., SENSIDONI A., 1987. Osmotic deydra- tation of carrot cubes. Ind. Alim., 2, 133-138.
PINSENT B.R.W., 1962. Peroxidase regene- ration and its effect on quality in frozen peas and thawed peas. J. Food Sci., 27, 120-122.
PONNE C.T., VAN REMMEN H.H.J, BAR- TELS P.V., 1991. Application of electroma- gnetic energy in processing of whole potatoes. Voedingsmiddelentechnologie, 24, 44-46.
RAMASWAMY H., VAN DE VOORT F.R., 1990. Microwave application in food proces- sing. Can. Inst. Food Sci. Tech. J., 23, 17-23.
SAPERS G.M., DOUGLAS F.W. Jr., 1987.
Measurement of enzymatic browning at cut surfaces and in juice of raw apple and pear fruits. J. Food Sci., 52, 11258-1262, 1285.
SAPERS G.M., MILLER R.L., 1993. Control of enzymatic browning in pre-peeled potatoes by surface digestion. J. Food Sci., 57, 1132- 1135.
SMITH O., 1975. Potato Processing. Talburt, W.F. & Smith, O. (Ed.) Ch. 10, Avi Publishing Co., Westport, CT.
