Synthetic dipeptide aspartame (aspartylphenylalanyl methyl ester) is widely used as a sweetener in light (low calorie) foods and diets for diabetics. The effect of high pressure on aspartame stability has been reported by Butz and co-workers (1997). Aspartame (0.5 g/L corresponding to the concentration in commercial diet cola and chocolate milk) in full cream milk (pH 6.8) lost almost 50% of active substances after pressure treatment at 600 MPa and 60°C for 3 minutes while the non-sweet compounds, i.e. aspartylphenylalanine and diketopiperazine, were formed. One of the important factors influencing the pressure stability of aspar-tame was the pH. It was stated that low pH foods containing aspartame could be treated by high pressure without great loss of active substances while high pressure treatment of dairy products (at neutral pH) such as chocolate milk and ice cream may create problems (possibly toxicological ones). After pressure treatment at 600MPa and 60°C (pH 7), 1.15mM of diketopiperazine (corresponding to 300mg/L) could be present after 5 minutes. In this case, a human individual of 40 kg consuming 1L of pressurised chocolate milk would ingest the upper limit of diketopiperazine (acceptable daily intake, ADI) of 7.5 mg/kg of body weight. As a consequence, it would be inadvisable to compensate the pressure related aspartame loss by adding higher aspartame doses prior to pressure treatment because it results in even higher diketopiperazine concentrations in the end-product.
Information on the effect of high pressure on the mineral content of food products is very scarce. Pressure treatment (600MPa/room temperature/20 minutes) could increase up to 50% the soluble iron content of liver suggesting the break up of the protein coat surrounding the cluster of hydrous ferric oxide in ferritin. The soluble iron content in spinach and soya flour was unaffected by pressure treatment. In beef muscle, the soluble iron was decreased by up to 50% and 67% respectively after pressure and heat treatments. Based on this study, the evidence of increasing rate of lipid oxidation in pressure treated meats (see section 21.6) could be supported by iron complexes released from ferritin or perhaps haemosidrin (Defaye and Ledward, 1999).
21.7.3 Human anti-mutagenic and anti-carcinogenic compounds
Diets rich in fruit and vegetables are found to be associated with a low incidence of many types of human cancer. Unfortunately, most of the antimutagenic activity in fruit and vegetables is reduced by heat treatment. The effect of high pressure treatment at different temperatures on the antimutagenic activity of fruit and vegetables has been studied in detail by Butz and co-workers (1998). In strawberry and grapefruit, heat (100°C/10min) and pressure (400-800MPa/25-35°C/ 10 minutes) had no effect on the antimutagenic activity. Carrots, kohlrabi, leek, spinach and cauliflower were characterised by strong antimutagenic potencies that are sensitive to heat but not to pressure. Antimutagenic activity of tomatoes and beets was affected by pressure but extreme high pressure/temperature conditions were required (tomatoes: 600MPa/50°C/10min and 800MPa/35°C/ 10min; beets: 800MPa/35°C/10min). It can be concluded that high pressure processing of vegetable juices offers advantages compared with thermal processing regarding their antimutagenicity.
In broccoli, isothiocyanates have been shown to have cancer protective properties. Application of high pressure (600MPa) combined with temperature (25, 40, 60 and 75°C) increased the degradation rate of both allyl and benzyl isoth-iocyanate up to 4 times compared with treatment under ambient pressure. In addition, the isothiocyanate degradation impaired such qualities as colour, flavour and some physiological properties. Therefore, high pressure technology may have limited application potential for food products containing isothiocyanates (Grupe et al, 1997).
In dairy and egg-based products, pressure induced conformational changes of albumin in relation to its technological and nutritional functionality were examined by determining the susceptibility of the treated protein to trypsin. High pressure treatment (600-800MPa/25°C/5-10min) of ovalbumin solutions in the absence of salt (NaCl) and sucrose did not modify the susceptibility of the residual soluble protein to trypsin. Pressure insolubilised ovalbumin was not digested by trypsin. Pressure treatment at neutral pH in the presence of NaCl or sucrose resulted in a pressure dependent increase of the susceptibility of ovalbumin to trypsin. The highest increase in proteolysis was observed for ovalbumin treated at 800 MPa and 25°C for 10min in the presence of 10% sucrose (Iametti et al, 1998). Similar phenomena have been reported for purified egg albumin. The presence of sucrose in pressurised albumin (400-600 MPa/25°C/5min) increased the susceptibility to proteolysis and the increase was more pronounced than in the presence of NaCl (Iametti et al, 1999).
Digestibility of pressurised foodstuffs has been studied in vitro (using digestibility tests) and in vivo (feeding trial in young pigs). Feeding trials (using a mixture of potatoes, carrots, meat, peas and vegetable oil) showed no changes in the digestibility of the individual nutrient fractions of pressurised foodstuffs as compared to fresh (untreated) ones. High pressure (500MPa/20°C/10min) did not affect the digestibility of the nitrogen free extract content, fats and crude extract fibre. The nitrogen retention in animals was only 45.4% of the nitrogen consumed when the heat treated feed was given while it was 58.6% using pressurised food and 57.9% using an untreated feed. In vitro studies showed no significant differences between the high pressure treated, heat treated (100 °C) and untreated pork samples on digestibility. Pressure treated soybean had a better digestibility than the untreated sample and the lowest digestibility was found in heat treated samples (100°C) (Schoberl et al, 1999).
For meat and lupin protein, the effect of high pressure on protein digestibility has been studied using in vitro tests. Protein digestibility of pressurised meat was higher than that of heat treated meat. The effectiveness of food processing on protein digestibility in meat could be ranged in the following order: untreated > pressure treated (500MPa/10°C/10min) (70% of digestibility) > pressure treated (200MPa/10°C/10min) (67% of digestibility) > heat treated (95°C/30min) samples (43% of digestibility). For lupin proteins, the pressure induced digestibility was more remarkable than for meat proteins and the ranking was different, i.e. pressure treated (500MPa/10°C/10min): digestibility up to 430% > heat treated (95°C/30min): digestibility up to 300% > pressure treated (200MPa/ 10°C/10min): digestibility up to 140% > untreated samples (de Lamballerie-Anton et al, 2001).
Most foods contain both major and minor allergens. The majority of food-allergic individuals are sensitive to one or more of the major allergens present in common allergic foods. The effect of different food processing unit operations on the immunochemical stability of celery allergens has been studied in detail using in vitro and in vivo tests by Jankiewicz and co-workers (1997). High anti-genic and allergenic activity in native celery was reduced by heat treatment and only mildly reduced by non-thermal processing such as high pressure (600 MPa/ 20°C), high voltage pulse treatment and irradiation.
In dairy and egg based products, modification of epitopic regions of ovalbu-min in pressure-treated ovalbumin has been studied by Iametti and co-workers (1998 and 1999). Pressure treatment (600-800 MPa/25°C/5min) resulted in modifications of the epitopic regions of the protein (determined by direct and non-competitive ELISA). Increasing the pressure level caused an increased loss of recognisability. Under pressure, ovalbumin in the presence of sucrose presented a lower recognisability than in the presence of NaCl. Samples treated at
600-800 MPa/25°C/5min in the presence of NaCl showed an affinity towards antibodies that was 40% lower than that of untreated protein. When comparing with the result determined by direct competitive ELISA, it can be concluded that pressure treatment did modify epitopic regions of ovalbumin. In the presence of sucrose, increasing protein concentrations led to a decrease in the specific content of antibody recognition sites per unit mass protein while no effect was found in the presence of NaCl.
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