HP processing fruit and vegetable quality

HP processing has a range of effects on:

• vitamin content.

17.6.1 Texture

In general, pressures up to 350MPa can be applied to plant systems without any major effect on overall texture and structure (Knorr, 1995b). Several studies revealed that pressure treatment of fruit and vegetables can cause both firming and softening (Basak and Ramaswamy, 1998), the effects being dependent on pressure level and pressurisation time. In general, the softening curves revealed that texture changes caused by pressure occurred in two phases: a sudden loss as a result of the pulse action of pressure followed by further loss of gradual recovery during pressure holding phase. At low pressure (100MPa), instantaneous pressure softening was caused by compression of cellular structures without disruption, while at higher pressure (>200MPa) severe texture loss occurs owing to rupture of cellular membranes and consequent loss of turgor pressure. During pressure holding time, the instantaneous texture loss can be gradually recovered and some products become even firmer than their fresh counterparts. In many cases, pressure-treated vegetables do not soften during subsequent cooking, which is attributed to the action of PME that is only partially inactivated by pressure. Simultaneous disruption of cell structures allows interaction of the enzyme with the pectic substance. Hence, the de-esterfied cell wall pectin can crosslink with divalent ions, leading to increased compactness of cellular structure.

17.6.2 Colour

For many fruit and vegetable products such as fruit jam, strawberries, tomato juice, guava purée, avocado purée and banana purée, high pressure treatment was noted largely to preserve fresh colour (Watanabe et al., 1991; Poretta et al., 1995; Donsi et al., 1996; Yen and Lin, 1996; Lopez-Malo et al., 1998). The brightness (L-colour value) and redness/greenness (a-colour value) of pressure-treated products were found to be superior compared with their thermally treated counterparts. However, during storage of guava and banana purée, the green colour gradually decreased because of browning as a result of residual PPO activity (Lopez-Malo et al., 1998; Palou et al., 1999a). The longest acceptability storage time was achieved by using high pressure, low pH and refrigerated storage. A detailed kinetic study regarding the combined effect of pressure and temperature on colour of broccoli juice revealed that the chlorophyll content and green colour (a-value) were stable for up to 4h treatment at 800 MPa and 40°C. Only when high pressure is combined with temperature higher than 50°C, were some colour changes noted. Degradation of chlorophyll content was described by a first order model, with chlorophyll a being less stable than chlorophyll b. On the other hand, loss of green colour was described by a consecutive step model because both conversion of chlorophyll to pheophytin and further conversion to pyropheophytin occurred (Van Loey et al., 1998; Weemaes et al., 1999).

17.6.3 Flavour

For most fruit juices, the potential benefits of using high pressure mainly arise from the fact that fresh flavour can be maintained during pressure treatment.

Many authors reported that trained sensory panels were unable to differentiate between fresh and pressurised juice made from the same raw material (Ogawa et al., 1990; Watanabe et al., 1991; Bignon, 1996). For tomato and onions, however, some flavour defects caused by pressure treatment were perceived: tomato had a rancid taste while onions smelled less intensely and more like fried onions (Butz et al., 1994; Poretta et al., 1995). In the former case, the rancid flavour was attributed to a marked increase in n-hexanal which is largely responsible for fresh tomato flavour in a concentration of 1-2mgkg-1. Higher concentrations impart the rancid flavour. For onions, pressure treatment was reported to diminish dipropylsulphide, a compound responsible for pungency and the characteristic odour of fresh onions and to increase transpropenyldisulphide and 3,4-dimethylthiophene concentrations leading to a flavour of braised or fried onions.

17.6.4 Vitamin content

Bignon (1996) observed that vitamin A, C, Bj, B2 and E content of fruit and vegetable products is not significantly affected by pressure treatment in contrast to thermal treatment. Besides, in the case of strawberries and guava purée, the decrease in vitamin C content during storage after pressure treatment (400-600MPa/15-30min) was found to be much lower compared to the fresh products (Sancho et al., 1999). A more detailed kinetic study of pressure-temperature stability of ascorbic acid in buffer, orange juice and tomato juice was performed by Van den Broeck et al. (1998). They found only significant degradation of ascorbic acid when pressure of about 850MPa was combined with temperatures between 60 and 80°C, and more in tomato and orange juice than in buffer. As well as vitamins, some minor studies of other health characteristics such as antimutagenicity and toxicity have been performed. Fruit and vegetables such as carrots, cauliflower, kohlrabi, leek and spinach are characterised by strong antimutagenic potencies, which were found to be sensitive to heat but not to pressure. For beet and tomatoes antimutagenic activity was affected, but only under very extreme conditions, that is 600MPa/50°C or 800MPa/35°C (Butz et al., 1997).

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