Article pp.27-34 du Vol.21 n°1 (2001)

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SCIENCES DES ALIMENTS, 21(2001) 27-34

1. Département de génie chimique, Université de technologie de Compiègne, BP 529, 60205 Compiègne cedex, France.

2. Laboratoire maîtrise des technologies agro-industrielles, LMTAI, Université de La Rochelle, 17071 La Ro- chelle cedex 9, France.

* Correspondance : C. Vœgel-Turenne, 8 rue St Hubert, 60610 La Croix St Ouen, France.

[email protected]

Evolution of the mechanical, textural

and rheological characteristics of the Granny Smith apple during drying

Christine VŒGEL-TURENNE^{1 *}, Sophia BENAMMAR^†, Karim ALLAF²

RÉSUMÉ Évolution des propriétés mécaniques, texturales et rhéologiques de la pomme Granny Smith au cours du séchage.

L’évolution de cinq propriétés mécaniques, texturales et rhéologiques (raideur, résistance à la déformation, élasticité, viscosité et température de transition vitreuse) a été observée sur des échantillons de pomme Granny Smith différem- ment déshydratés, en fonction de la teneur en eau. Une valeur critique de teneur en eau a été mise en évidence, définissant deux phases au cours du procédé de séchage. Lors de la première phase, de l’état de départ (650 g H₂O/100 g MS) jusqu’à environ 100 g H₂O/100 g MS, le produit se ramollit et perd de son élasticité et de sa viscosité. Il se trouve à l’état caoutchouteux couplé à un af- faissement (phénomène de retrait). Lors de la deuxième phase, en-dessous de 100 g H₂O/100 g MS, le produit durcit et gagne en élasticité et en viscosité.

Bien que la température de transition vitreuse T_g augmente, l’échantillon se trouve toujours à l’état caoutchouteux à la température de séchage.

Mots clés :pomme, dégradation thermique, séchage, transition vitreuse, texture.

SUMMARY

The evolution of five mechanical, textural and rheological properties (stiffness, resistance to deformation, elasticity, viscosity and glass transition temperature) of several partially dehydrated Granny Smith apple samples was observed versus water content. The existence of one critical water content value was poin- ted out, which defines two phases during the drying process. During the first phase, from 650 g H₂O/100 g DM (raw material) to about 100 g H₂O/100 g DM, the product softens and becomes less viscous and less elastic. It is in the rubbery state, and a collapse is observed. In the second phase, below 100 g H₂O/

100 g DM, the product hardens and becomes more elastic and more viscous.

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28 Sci. Aliments 21(1), 2001 C. Vœgel-Turenne et al.

Although the glass transition temperature T_g increases, the sample is still in the rubbery state at the drying temperature.

Key-words: apple,thermal degradation, drying, glass transition, texture.

1 - INTRODUCTION

In recent years, increasing attention is paid to mechanical and rheological properties of food materials because they lead to a better comprehension of the foodstuff behaviour during processing and storage. In fact, the physical state of foodstuffs widely influences their stability and their quality. Since viscosity is related to molecular motion, it acts on certain properties of the product such as:

diffusivity of water and aromatic molecules, lipid oxidation, enzymatic reactions, browning… Decreasing viscosity increases the molecular mobility and conse- quently the chemical reaction rates. Viscosity also determines two main states in the material: at low viscosity, a rubbery state and at high viscosity a glassy state.

Glass transition occurs only in amorphous materials or in amorphous zones of semi-crystalline solids. Most deteriorative changes such as stickiness, collapse, sugar crystallisation, aroma retention, browning… take place in the rubbery state (CHIRIFEet al., 1973; ROOS and KAREL, 1991a; LEVINE and SLADE, 1986). The temperature at which occurs the phase transition from the glassy to the rubbery state is known as the glass transition temperature (T_g). This parameter is charac- teristic of each material, though it can vary lightly with the “thermal past” and the experimental conditions. It is mainly related to the moisture content, increasing with decreasing water content since water acts as a plasticizer (LEVINE and SLADE, 1986).

During drying process, due to the thermal treatment as well as to the water loss, changes in food physical structure take place, leading to deteriorative reactions concerning all quality parameters: colour, texture, taste and crispness. It has been shown that dehydrated foods contain amorphous systems (WHITE and CAKEBREAD, 1966; LEVINE and SLADE, 1986; ROOS, 1991), so they may exist either in the glassy or in the rubbery state, depending on composition, moisture content and temperature. Thus it seems very important to study rheological changes during the drying process, i.e. the effect of moisture content on quality loss, so that the final quality of the dried product can be predicted and controlled.

2 - EXPERIMENTAL

2.1 Apples

Granny-Smith apples were bought on local markets and stored at 4°C until further use. This raw material was chosen because its flesh has a firm and crisp texture. The apples were then peeled and cut into 0.5 × 1 × 1 cm pieces. To avoid

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Physical state of the GS Apple 29

flesh heterogeneity and undesirable enzymatic browning, the central region of the apple core was set aside and the samples were rapidly processed. No other treatment was carried out to avoid enzymatic activity, since it has been shown that the browning observed was essentially a nonenzymatic browning (VŒGEL- TURENNEet al., 1999).

2.2 Drying

Drying was carried out in a ventilated oven at 90°C. In order to obtain different water contents, the samples were oven dried for different lengths of time. The samples were weighted before and after dehydration. The water content, W, is expressed in g water per 100 g of dry matter (g H₂O/100 g DM). It was calculated from the sample weight before and after drying.

2.3 Texture

The texture was evaluated using a texturometer TA-XT2 (Rhéo, France), equipped with a cylindrical tip 3 mm in diameter. The tip downstroke speed was 0.6 mm/s, the penetration inside the sample was 1.5 mm and the data acquisi- tion speed 200 pts/s. We defined a parameter we called the resistance to deformation (RD) which corresponds to the strength necessary to deform the sample by 1.5 mm. RD is expressed in Newton (N). Each result is the average of four measurements.

2.4 Mechanical and rheological characteristics

The mechanical measurement was performed by thermo-mechanical dynami- cal analysis (TMDA), with a Metravib viscoanalyser-viscoelasticimeter (Ecully, France). This apparatus is rarely used with food products, so a preliminary inves- tigation was necessary to determine the optimum value of the different parameters (VŒGEL-TURENNE, 1996). The traction-compression test in dynamic conditions at 5 Hz was finally chosen. The partially dehydrated samples were fixed to the sample holder with an ordinary cyanoacrylate glue. The displacement was 5 µm. E', the elasticity modulus (or storage modulus) and E", the viscosity modulus (or loss modulus) are expressed in Pa, while the stiffness K is expressed in N.m^–1. E*, the complex Young modulus, is calculated from the following equation: E* = E' + i E".

Thus its norm is: . E', E", as well as K were obtained at 20°C, and each point is the average of three experimental values.

For the glass transition temperature (T_g) determination, the scanned tempe- ratures depended on the water content of the sample: from – 30 to + 10°C for wet sample (W > 25 g H₂O/100 g DM) and from 25 to 60°C for dried samples (W

< 25 g H₂O/100 g DM). T_g was defined as the temperature at which tan δ = is maximum (δ being the loss angle between the stimulation and the sample res- ponse).

Each point is the average of three experimental values.

E'²+E''²

E'' ---E'

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3 - RESULTS

During drying, the stiffness K first slightly decreases (figure 1). This can be interpreted as a product softening. Below 100 g H₂O/100 g DM, K increases regularly, which means a sample hardening.

The curves obtained with the rheological parameters E' and E" are very similar (figure 2). One critical water content can also be observed at about 100 g H₂O/100 g DM, which affects the evolution of both parameters curves. Two phases can thus be defined. In the first phase (from 650 to 100 g H₂O/100 g DM), E' and E" slightly decrease with W. In the second phase (below 100 g H₂O/100 g DM), E' and E" increase regularly as W decreases.

E' is always greater than E", meaning that the apple is more elastic than viscous. However, the difference between those two parameters is more important in the first phase.

The changes observed in the first phase do not systematically mean that the product becomes less elastic and viscous. In fact, it is noteworthy that in the first stages of drying, the sample collapses, and the reduction in the sample volume has a strong bearing on specific parameters such as E' and E". It was important to discuss whether the E' and E" decrease was an artefact only due to the sample volume reduction. We first determined the region where the apple collapse takes place. E* is related to K through a form factor, following the relation:

E* = K * L/S with S: the sample surface, and L: the sample length.

The ratio K/E* is equivalent to the form factor S/L. Figure 3 shows its variation during drying. Just at the beginning of drying, the curve increase could mean a sample expansion, due to the melting of water. This hypothesis has to be confir-

Figure 1

Stiffness versus water content during drying

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med by further experimental points. Then, a sample volume reduction takes place until about 1 g H₂O/100 g DM. This has already been observed for colloidal foodstuffs, with a higher end point (25 g H₂O/100 g DM according to GORLING (1973)). It can be asserted that a collapse takes place in the first phase, and the decrease observed for the specific parameters E' and E" may partly be due to this phenomenon. However, even if the sample volume would have been cons-

Figure 2

The storage modulus (E') and the loss modulus (E") versus water content during drying

Figure 3

Evolution of the ratio K/E versus the water content, determining the sample volume reduction

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tant (for example with freeze-dried samples), since K decreases, E* and thus E' and E" would have also decreased. It can be concluded that a reduction in the viscosity and in the elasticity moduli really takes place in the first phase.

The two phases already noted are also observed for the glass transition temperature (T_g) (figure 4) and the critical water content is about the same. In the first phase (from 650 to about 60 g H₂O/100 g DM), the variations of T_g are not significant. The transition observed at about W = 60 g H₂O/100 g DM would not even- tually be the glass transition but the melting of water and its solutes. ROOS and KAREL (1991b), studying the T_g of a sucrose solution, also found a similar behaviour with a critical water content of 50 g H₂O/100 g DM. In the second phase (from 60 to 1 g H₂O/100 g DM), the T_g increases as W decreases as in Roos and Karel’s work. It is to be noted that during the two phases, at the oven temperature, the sample is in the rubbery state since the T_g value is lower than the oven temperature. It seems that, at the lowest water contents, the curve decreases slightly as decreases. The sample is in the glassy state at room temperature. T_g is still lower than the oven temperature, but this is of minor importance since the other measurements (determination of RD, K and E' and E") have been performed at room temperature.

The evolution of RD follows also the two previously mentioned phases (figure 5).

From 650 to 70 g H₂O/100 g DM, RD slightly decreases. Since this measure is the resistance opposed by the material to the tip downstroke, it can be assimilated to a hardness measurement. Thus, the first phase is a softening phase. From 70 to 1 g H₂O/100 g DM, RD drastically increases, meaning that the sample hardens. It has been observed that the variance measurement is important, due to the heterogeneity of the partially dried material. However, the evolution of the RD curve is clear and unequivocal, and is to be compared with those of E’ and E”. At the end of the drying, a strong decrease can be observed.

Figure 4

Glass transition temperature (Tg) as a function of water content during drying

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4 - DISCUSSION

For all of the five criteria studied in this work to characterise the quality changes in GS apple during the drying process, one critical water content value can be observed: 100 g H₂O/100 g DM (for K, E' and E"), 70 g H₂O/100 g DM (for RD) and 60 g H₂O/100 g DM (for T_g).

This critical point is assumed to be the same for all characteristics: the important variation in the critical water content values may be due to the lack of preci- sion in their determination, due to heterogeneity when the apple is dried in a ventilated oven.

This point define two phases in the drying process. In the first phase (from 650 to about 100 g H₂O/100 g DM ), the apple, initially crunchy, softens and deforms easily (K and RD decrease). It becomes also less and less viscous and elastic. The sample is in the rubbery state and it collapses (its volume decreases).

In the second phase (from about 100 to 1 g H₂O/100 g DM), the product hardens (K and RD increase simultaneously). In the oven, it is still in the rubbery state (since T_g is below the oven temperature), so it goes on collapsing along the drying.

At room temperature, the sample is in the glassy state for W lower than 25 g H₂O/

100 g DM. The product also becomes more and more elastic and viscous.

At the end of this second phase, two parameters, T_gand RD, seem to decrease. This could be correlated to the reduction in cohesiveness sensorially noted at the end of drying. For RD, it can also be attributed to the sample burs- ting noted when the tip penetrates very dry products. Moreover, this observation relative to very low water contents is not necessarily significant since our method for water content determination was not very precise.

Figure 5

Resistance to deformation as a function of water content during drying

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Comparing to the fresh sample, the product at the end of drying is glassy, harder, more elastic and viscous.

The results of this study allow the prediction of the mechanical, textural and rheological characteristics of the GS apple as a function of the water content during drying. Moreover, the T_g determination allows the prediction of the product stability during storage. The drying process may be stopped when the water content reaches 25 g H₂O/100 g DM, so that the sample is in the glassy state at room temperature, and the deteriorative reactions are minimised. Further studies would be necessary to explain the existence of the critical point and to relate it to some physical phenomenon affecting the product in the drying process. It will also be interesting to relate the evolution of the parameters measured to that of other characteristics such as browning, rehydration ability, water activity… as function of water content. This will be done in a following paper.

Receveid 19 November 1999, revised 2 October 2000, accepted 20 Octo- ber 2000.

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