Processed fruit firming by infusion of gelling agents

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The infusion of gelling agents in the fruit or vegetable structure allows texture improvement either by generating intercellular bridges in the pores complementing the plant cell wall network or by forming bonds between the added hydrocolloids and the cell wall components. However, the intercellular gel can modify the texture response only where the porous structure is high and the solute gain is significant, especially as mass transfer phenomena are generally limited by the high viscosity of gelling agent solutions. In addition, gel formation for many hydrocolloids (pectin or alginate) works with calcium, and the use of the cation is usually needed. Because of this the combination of the two reagents in the same solution is not appropriate due to the risk of causing thickening or gel formation before infusion. Thus two vacuum infusion cycles are generally necessary: the first with the solution containing the gelling agent to fill a large fraction of pores and the second with the calcium to complement the residual free spaces. Otherwise, infused gelling agents could react with the endogenous calcium or with the calcium present in the food medium - e.g. a sauce or a syrup - where the fruit or vegetable pieces would be incorporated. Other medium conditions, such as concentration, temperature or pH, exist to make the gelling effective. Different mechanisms are thus involved depending on the hydrocolloid type. This complexity makes the control of the application more difficult, as was the case for the firming agents proposed in the previous sections.

Among the well-known applications, the vacuum treatment of button mushroom with xanthan gum before blanching and canning has been shown to improve the weight yield and the organoleptic quality of the final product (Gormley and Walshe, 1986). Xanthan impregnation tended to decrease the shrinkage of mushroom during the blanching/canning cycle and thus to reduce the product weight loss. The pre-treatment with xanthan led to a more acceptable and less tough texture of canned mushrooms. This 'softening' effect of the vacuum treatment on canned mushroom is a desirable feature since canned mushrooms often have a 'hard' texture. The benefit is due presumably to the thickening property of the xanthan gum solution (0.5-1 % w/w) which occupies the wide-open hyphae structure of mushroom and prevents expulsion during blanching and retorting. Some of the xanthan molecules might also be bound by the mushroom proteins. Demeaux et al. (1988) indicated that in terms of weight loss reduction of canned mushroom, the use of gelling agents such as egg white proteins is much more effective than xanthan gum which does not gel.

The freezing/thawing cycle applied to fruits or vegetables causes substantial damage to the cellular structure, that is denaturation of the membranes and rupture of the cell walls by ice crystals, leading to loss of turgor and rigidity. This generally results in a strong juice exudation when defrosting the product. With the aim of limiting these problems, Barton (1951) attested that fresh fruits mixed with sugar and gelling agents and consequently submitted to a vacuum step give frozen/defrosted products with better organoleptic quality.

In the case of strawberry slices, as proposed by Barton, the use of pectin and alginate before freezing made it possible to maintain the shape, weight and colour of the fruit to a greater degree than was the case for untreated fruit this was particularly so when high methylated (HM) pectin was used. Preliminary vacuum impregnation of strawberries in solutions containing gelling agents was proposed by Cierco (1994) as a new method for improving the quality of frozen fruits. Using this process, frozen/thawed strawberries were obtained which maintained the features and the taste of fresh ones even after several years' storage at -20 °C and that are usable for traditional pastry-making.

More recently, Matringe et al. (1999) showed the possibility of introducing various gelling hydrocolloids (gelatine, pectin, alginate and starch) through the application of vacuum on fresh apple pieces before freezing. Texture measurements by compression test on apple samples infused in that way showed firmness improvement with certain gelling agents just after vacuum treatment (Fig. 15.4). If the gelling agent uptake is sufficient, a structuring

Fig. 15.4 Firmness of apple slices (1 cm thickness, 2 cm diameter) after vacuum impregnation with water and different texture agents. Compression speed: 0.5 mm/s. Relative deformation: 15%. *double impregnation effect is also observed on the frozen-defrosted product. An example of this texture modification is presented in Fig. 15.5. The 'cuttability' - defined as the force to cut a 1 cm thick apple cube measured by a texture analyser equipped with a blade - of samples impregnated with gelatine appeared to exhibit behaviour similar to that with a simple hydrocolloid gel. Indeed, apple dices treated with gelatine before freezing definitely showed higher gel strength (the slope of the curve is steeper). Then, the frozen impregnated sample showed a tendency to be cut like a gel (there is a breaking point before the end of the measurement), which was completely different from the control case for which the gel strength value corresponded only to a continuous crushing. The measurement of gel strength under compression gave similar results (Fig. 15.5). These phenomena were explained by the formation of gel-filled intercellular spaces predominating over the softened structure of defrosted apple.

Thereafter, work carried out on pasteurised fruit preparations for dairy products described in the FAIR European programme, referenced in Section 15.5, showed significant texture improvement for products containing pear or strawberry pieces enriched with pectin or alginate. The improved organoleptic qualities of the processed products were validated by sensory analysis (Cattaneo et al., 2000; Avitabile Leva et al., 2000).

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Distance (mm)

Fig. 15.5 Texture analysis profiles of frozen-defrosted 1 cm3 apple cubes, vacuum infused with gelatine and non-infused, representing shearing force ('cuttability') or strength force under compression versus distance.

Distance (mm)

Fig. 15.5 Texture analysis profiles of frozen-defrosted 1 cm3 apple cubes, vacuum infused with gelatine and non-infused, representing shearing force ('cuttability') or strength force under compression versus distance.

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