Heat treatment blanching and canning

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Heat treatments are responsible for irreversible denaturation of cellular tissue in fruits or vegetables causing softening and juice loss. Vacuum infusion technology was consequently used before heat treatment such as blanching, pasteurising and canning with an aim of limiting thermal damages in the product. It is of particular interest to note the treatment of button mushrooms (McArdle et al., 1974; Gormley and Walshe, 1986; Demeaux et al., 1988), strawberries (Main et al., 1986), apricots (French et al., 1989) and turnips (Moreira et al., 1994).

McArdle et al. (1974) showed that vacuum impregnation of mushroom with only water before blanching and canning improved the weight yield in the final product. The water retention resulting in this case could be also improved thanks to the preliminary infusion of a hydrocolloid like xanthan gum (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. Moreover, the pretreatment with xanthan led to a more acceptable, less tough texture of canned mushrooms. Demeaux et al. (1988) indicated that the use of gelling agents such as egg white proteins are much more effective than xanthan gum which does not gel, in terms of weight loss reduction of canned mushroom.

The firmness of turnip dices stabilised by blanching (97°C, 3 min) and consecutive acidification (220 min dip in acetic acid solution at different constant temperatures: 20, 50, 70 and 90°C) was improved only by preliminary vacuum infusion of water which plays a protective role explained by the change in cell turgor (Moreira et al., 1994).

Calcium lactate infusion in fresh whole or sliced strawberries improved their texture and reduced their weight loss measured after canning (Main et al., 1986) owing to the presence of calcium which reinforces the cell wall structure by forming pectates (see above). This improvement in texture by calcium infusion was also observed by French et al. (1989) on canned apricot - Patterson cultivar fruits - even if the chelator effect of exogenic or endogenous citrate tended to limit calcium effectiveness especially on low maturity fruits because of their stronger acidity.

The vacuum infusion of exogenous pectinmethylesterase (PME) in fruit was found to be effective in increasing firmness in thermally processed foods. PME is a cell wall-bound enzyme in fruits and vegetables, which de-esterifies pectin. In post-harvest ripening of fruits, PME activity precedes depolymerisation by polygalacturonase, resulting in fruit softening. However, the PME is postulated to increase firmness of fruits and vegetables by demethylation of endogenous pectin and subsequent chelation of divalent cations by ionised carboxylic acid groups on adjacent pectic acid chains (Suutarinen et al., 1999). In the presence of calcium, the firming effect is proportional to the PME activity preceding the thermal treatment and can be reinforced by vacuum-assisted infusion of exogenous PME. In blanched (95°C, 30s) or blanched-canned (104°C, 12min) peaches, vacuum-infused citrus PME and calcium increased the firmness of these thermally processed products up to a value nearly four times that of un-infused controls (Javeri et al., 1991).

18.6 Freezing

Freezing/thawing cycles applied to fruits or vegetables cause substantial damage to the cellular structure, that is denaturation of the membranes and rupture of the cell walls, 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) showed that freshfruitsmixedwithsugarandgelling agents and consequently submitted to a vacuumstep,givefrozen/defrostedprod-ucts with better organoleptic quality. In thecaseofstrawberryslicesasproposed by this author, the use of pectin and alginatebeforefreezing madeitpossibleto maintain the shape, weight and colour of the fruit to a greater degree than untreated fruit particularly with HM pectin. In addition, Main et al. (1986) showed that preliminary calcium impregnation on whole or sliced strawberries only slightly improved the fruit resistance to shear. The low effectiveness of calcium in improving firmness was explainedbyinsufficientdemethylationofthe endogenous pectins in the fruit for the purpose of pectate formation. When the freezing/defrosting cycle was followedbyheattreatment,theeffectontexture was stronger owing to increased demethylation activated during temperature rise. Preliminary vacuum impregnation of thefruitsinsolutionscontaininggelling agents was proposed by Cierco (1994) asanewmethodforimprovementinthe quality of frozen strawberries. Using this process, the author obtained frozen/thawed strawberries that maintained the features and taste of fresh ones even after several years' storage at -20°C.Morerecently,Matringe etal.(1999) showed the possibility of introducing various gelling hydrocolloids (gelatine, pectin, alginate and starch) through the application of vacuum to fresh apple pieces before freezing. If the gelling agentuptakewassufficient,astructuring effect was observed on the defrosted product. An example of this texture modification is presented in Fig. 18.3. The 'cuttability' - defined as the force to cut a one centimetre thick apple cube measured by a texture analyser equipped with a blade - of impregnated samples with gelatine appeared to exhibit similar behaviour to 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 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 only corresponded to continuous crushing. Matringe et al. (1999) explained this phenomenon by formation of gel-filled intercellular spaces predominating over the softened structure of defrosted apple.

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