A characteristic shared by most fruits and low-pH foods is their high acidity. Although most species of bacteria are inhibited by the resulting hydrogen ion concentration, lactic acid bacteria, yeast and moulds are more aciduric and many find these pH values to be tolerable, if not optimum, for growth. It is because of acidity, therefore, that fungi and lactic acid bacteria are the principal spoilage microorganisms of fruit and fruit products. High pressure processing is a potentially useful way of helping to inactivate spoilage bacteria and control enzymatic activity. However, as has been indicated, it cannot be used in isolation.
Pasteurisation or sterilisation of low-acid foods using high pressure, for example, is only feasible when combined with other preservation techniques which enhance inactivation. Factors such as heat, antimicrobials, ultrasound and ionising radiation can potentially be used in combination with high pressure. These approaches will not only help to accelerate the rate of inactivation, but can also be useful in reducing the pressure level and, hence, the cost of the process, while eliminating the commercial problems associated with sublethal injury and survivor tails.
As an example, studies of residual PPO activity in fruit purées after HP treatments suggest that inhibition of undesirable enzymatic reactions, such as browning, requires the combination of pressurisation with one or more additional factors, such as low pH, blanching or refrigeration temperatures to inhibit (or at least reduce significantly) enzyme activity (Lopez-Malo et al., 1998; Palou et al., 2000; Lopez-Malo et al., 2000). Other research suggests that blanching, for example, is important for pressure treatment of fruit and vegetables to minimise enzymatic and oxidative reactions (Hoover, 1993). The effects of blanching and HP treatments on PPO activity of banana purée adjusted to pH 3.4 and water activity, aw 0.97, showed PPO activity was reduced during steam blanching and further reduced after HP treatment (Palou et al., 1999a).
A key role for high pressure processing is in reducing the severity of the processes traditionally used to preserve foods. The use of high pressure in combination with mild heating has considerable potential (Palou et al., 1999b; Lopez-Malo et al., 2000). The antimicrobial effect of high pressure can be increased with heat, low pH, carbon dioxide, organic acids and bacteriocins such as nisin (Palou et al., 1997a, 1997b, 1997c; Papineau et al., 1991; Mallidis and Drizou, 1991). Knorr (1995a, 1995b), Papineau et al. (1991) and Popper and Knorr (1990) reported enhanced pressure inactivation of microorganisms when combining pressure treatments with additives such as acetic, benzoic or sorbic acids, sulphites, some polyphenols and chitosan. These combination treatments allow lower processing pressure, temperature and/or time of exposure. It has been suggested that some food preservatives show enhanced activity when subjected to high pressure, though others may be adversely affected (Tauscher, 1995; Palou et al., 1997a). The use of high pressure as one amongst several hurdles provides a way, for example, of reducing the dependence on sulphites as antibrowning and antimicrobial agents. It has also been suggested that the efficiency of high pressure enzyme inactivation be improved by applying pressure cycles. Successive applications of HP treatments resulted in higher inactivation of many enzymes (Hen-drickx et al., 1998). Enzyme activity after a multicycle process was lower than that of a single-cycle process of the same total duration (Ludikhuyze et al., 1997).
A number of examples illustrate the potential application of high pressure treatment. Lopez-Malo et al. (1999) evaluated the effects of high pressure treatments at 345, 517 or 689MPa for 10, 20 or 30min at initial pHs of 3.9, 4.1 or 4.3 on (PPO) activity, colour and microbial inactivation in avocado purée during storage at 5, 15 or 25°C. Standard plate, as well as yeast and mould counts of high pressure-treated purées, were <10cfug-1 during 100 days of storage at 5, 15
or 25°C. Significantly less (p < 0.05) residual PPO activity was obtained with increasing pressure and decreasing initial pH. Avocado purée with a residual PPO activity <45% and stored at 5°C maintained an acceptable colour for at least 60 days and achieved a shelf-life of 35 days when stored at 15°C.
Palou et al. (2000) have analysed the effects of continuous and oscillatory high pressure treatment on guacamole. Significantly less (p < 0.05) residual PPO and LOX activity was obtained by increasing the process time and number of pressurisation-decompression cycles. LOX was inactivated with a 15min continuous treatment of oscillatory high pressure. The lowest residual PPO activity value (15%) was obtained after four high pressure cycles at 689MPa with 5 minutes of holding time each. Standard plate as well as yeast and mould counts of high pressure-treated guacamole were <10cfug-1. Sensory acceptability and colour of high pressure guacamole were not significantly different (p > 0.05) from those of a guacamole control. Browning during storage was related mainly to changes in the hue attributed to a decrease in the green contribution to the colour. A shelf-life of 20 days was achieved at <15°C.
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