Effect of high pressure on lipids

The most interesting effect of high pressure on lipids in foods is the influence on the solid-liquid phase transition, e.g. a reversible shift of 16°C per 100 MPa for milk fat, coconut fat and lard (Buchheim et al, 1999). With respect to the nutritional value of lipids, the effect of high pressure on lipid oxidation and hydrolysis in food products is of importance. Lipid oxidation is a major cause of food quality deterioration, impairing both flavour and nutritional values (related to health risks, e.g. development of both coronary heart disease and cancer). Effect of high pressure on lipids has been reported by many authors and the available literature shows that pressure could induce lipid oxidation especially in fish and meat products but did not, or only slightly, affect lipid hydrolysis. For example, pressures up to 1000MPa and 80°C did not affect the hydrolysis of tripalmitin and lecithin. Therefore, no fat/oil hydrolysis is expected to occur under conditions relevant for food processing (e.g. 600MPa/60°C/time less than 30 minutes) (Isaacs and Thornton-Allen, 1998).

Pressure induced lipid oxidation has been studied in different model systems and food products. In model systems, pressures up to 600MPa and temperatures up to 40°C (less than 1 hour) had no effect on the main unsaturated fatty acid in milk, i.e. oleic acid. Linoleic acid oxidation was accelerated by exposure to pressure treatments of less than one hour, but the effect was relatively small (about 10% oxidation) (Butz et al, 1999). Increasing pressure (100 up to 600MPa and

40°C) lowered the decrease of alpha-linoleic acid indicating that pressure retarded lipid oxidation e.g. 15% decrease at 600MPa/40°C/15 minutes and 30% decrease at 100MPa/40°C/15 minutes. As a consequence, pressures above 600 MPa are suggested for retention of essential fatty acids, e.g. linoleic acids (Kowalski et al, 1996).

21.6.1 Vegetable oils

Pressure induced lipid oxidation of extra virgin olive and seed oil has been studied by Severini and co-workers (1997). The peroxide values, indicating the primary oxidation products, of untreated and pressure treated (700MPa/room temperature/10 minutes) olive oil were not significantly different. In seed oil i.e. sunflower and grape-stone oil, this value was evidently increased due to pressure treatment and storage (-18°C, 1 year); such effects were not found for soybean, peanut and maize oil. The two former seed oils show the highest level of unsaturated fatty acids which probably affects lipid oxidation. The para-anisidine value, indicating secondary oxidation products such as aldehydes, generally increased after high pressure treatment (700MPa/room temperature/10 minutes), e.g. olive oil (types A, B, C, D), sunflower, peanut and maize oil samples and after one year storage at -18°C, only the value in seed oil increased. The induction time (i.e. length of the initial stage of very slow oxidation) of pressure treated olive oil was generally shorter than that needed for untreated samples. Such a phenomenon was also found in the seed oils i.e. grape-stone, sunflower and peanut oils. It can be concluded that the olive oil was more pressure resistant to oxidation than was seed oil and, as a consequence, extra virgin olive oil is a better choice in high pressure processed foods.

The effect of high pressure on essential oils of spices and herbs has been reported. The essential oil content in basil can be retained by pulsed high pressure sterilisation (2 pulses of 1 minute holding time) using high pressure (>700 MPa) combined with high temperature (>65°C) processing while losses after conventional heat sterilisation were over 65%. It was stated that pulsed high pressure opens new perspectives in quality improvement of fresh spices and herbs (Krebbers et al, 2001).

21.6.2 Fish products

In fish products, some studies show occurrences of pressure induced lipid oxidation. The lipid oxidation rate (based on TCA (thiobarbituric acid) number) in cod muscle remarkably increased by pressurisation above 400 MPa (study up to 800 MPa) at 20°C for 20min. EDTA (ethylenediaminetetraacetic acid) addition (1% w/w) in minced cod muscle inhibited the increased oxidation rate induced by pressure treatment. It was suggested that release of transition metal ions such as copper or iron or their complexes occurred under pressure and subsequently catalysed the oxidation reaction. Lipid oxidation in cod muscle packed under air was limited at treatments of 200 MPa and room temperature for 20 minutes (Angsupanich and Ledward, 1998).

Production of free fatty acids in red fish meat, i.e. sardine and bonito, during storage was inhibited after pressure treatment at 200 MPa and room temperature for 30 minutes. Pressures above 200 MPa resulted in lipid degradation. This observation was explained by the degradation of myoglobin and the loss of the water holding capacity increasing the contact surface layer between oxygen and fish meat. The oxidation pattern of pressurised (100MPa/room temperature/30 minutes) sardine and bonito was almost the same for as long as 3 days of storage (5°C). Addition of antioxidants (a mixture of alpha tocopherol and rosemary) prolonged the storage for 12 days for untreated samples but only for 6 and 9 days respectively for 100 and 200MPa pressurised samples. It could be that high pressure also affects the radical scavenger function of alpha tocopherol and rosemary (Wada, 1992; Wada and Ogawa, 1996).

21.6.3 Meat products

In meat products, the induction time of pressure treated (800MPa/19°C/20 minutes) rendered pork fat (aw = 0.44) was shorter (approximately 3 days) than that of untreated samples (c 4 days). Pressure treated samples showed a higher peroxide value than untreated samples and the effect became more pronounced with increasing pressure. Furthermore, the extent of lipid oxidation at 800MPa for 20min was increased by increasing the treatment temperature. High pressure treatment inhibited lipid oxidation at all water activities except aw = 0.44. Since pork fat contains up to 1.5ppm iron and 0.4ppm copper, transition metals may be released from complexes and act as powerful pro-oxidants. In the aw range between 0.4 and 0.55, pressure becomes catalytic to the oxidation and the catalytic effect of the released metal ions probably overrides the inhibiting effect of peroxide destruction. At higher aw, free ions will hydrate with the available water, whereas at lower aw such hydration may not be complete and increases the catalytic effects of the ions. It seemed that pressure treatment at higher temperature diminished the inhibiting/protective effect on lipid oxidation and high pressure application for a short time had a significant effect on stability of pork lipids during subsequent storage indicating that high pressure leads to irreversible changes (Cheah and Ledward, 1995). Addition of citric acid (0.02%) prior to pressure (650-800MPa) treatment of rendered pork fat inhibited the increased rate of lipid oxidation while it was less effective in minced pork and washed muscle because of the pH decrease. On the other hand, the addition of EDTA was effective in inhibiting pressure induced oxidation. It indicated that releasing metal transition ions during pressure treatment was a major factor in increasing lipid oxidation in pressurised meat (Cheah and Ledward, 1997).

Kinetics of lipid oxidation during pressure treatment have been reported by Dissing and co-workers (1997). In this investigation, turkey meat has been chosen as a case study since it is rather susceptible to oxidation due to its relatively large content of membrane-associated phospholipids in combination with a low endogenous level of tocopherol. Pressure treatment (100-500MPa/ 10°C/10-30min) induced lipid oxidation in turkey thigh muscles prior to chilled storage. During storage, the increase of thiobarbituric acid reactive substances in pressurised (up to 400MPa/10°C/10-30min) meat was less pronounced than in heat (100°C/10min) treated samples. The extent of lipid oxidation depends on the pressure level and treatment time. The enhancement of lipid oxidation was dependent on the pressure level applied, at least above a certain threshold pressure (i.e. 100 MPa).

The evidence of pressure induced lipid oxidation may limit high pressure application in meat/fish based products unless antioxidants or suitable product packaging are used. Metal chelators which effectively remove the metal catalysts have been proposed as the most appropriate antioxidants to prevent lipid oxidation in meat products.

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