440 The nutrition handbook for food processors different influence on the stability of vitamin C during storage. The ascorbic acid content in untreated and pressurised (400 MPa/room temperature/15 min) guava puree started to decline respectively after 10 and 20 days whereas that in heated (88-90C/24s)and( 600 MPa/room temperature/15 min) pressurised guava puree remained constant during 30 and 40 days respectively (Yen and Lin, 1996) Kinetics of vitamin C degradation during storage have been studied in high pressure treated strawberry coulis. Vitamin C degradation of pressurised (400 MPa/20oC/30 min) and untreated coulis are nearly identical during storage at 4C. Moreover, it has been shown that a pressure treatment neither accelerates nor slows down the kinetic degradation of ascorbic acid during subsequent storage(Sancho et al, 1999) The effect of oxygen on ascorbic acid stability under pressure has been studied by Taoukis and co-workers(1998). At 600 MPa and 75C for 40 min exposed to air, ascorbic acid in buffer solution(sodium acetate buffer(0. 1 M: pH 3.5-4)) degraded to 45%o of its initial content while in the absence of oxygen, less vitamin loss was observed. Moreover, the addition of 10% sucrose resulted in a protec tive effect on ascorbic acid degradation. It was also noted that vitamin C loss was higher in fruit juice compared to that in buffer solutions. Vitamin C loss in pine- apple and grapefruit juice after pressurisation(up to 600 MPa and 75C)was max 70%o and 50% respectively. At constant pressure(600 MPa after 40 min), the pres- sure degradation of vitamin C in pineapple juice was temperature sensitive, e.g loss 20-25% at 40C. 45-50% at 60C and 60-70% at 75C in contrast to that in grapefruit juice. Detailed kinetics of combined pressure and temperature stability of ascorbic acid in different buffer(pH 4, 7 and 8)systems and real products(squeezed orange and tomato juices) have been carried out by Van den Broeck and co- workers(1998). At 850MPa and 50C for 1 hour, no ascorbic acid loss was observed. The high pressure/thermal degradation of ascorbic acid at 850 MPa and 65-80oC followed a first order reaction the rate of ascorbic acid degradation at 850 MPa increased with increasing temperature from 65 to 80C indicating that pressure and temperature act synergistically. Ascorbic acid in tomato juice was more stable than in orange juice. It was also reported that temperature depend- ence of ascorbic acid degradation(z value)was independent of the pressure level Based on this study, it can be concluded that ascorbic acid is unstable at high pressure(850MPa)in combination with high temperature(65-80oC) 21.5.2 Vitamin a and carotene The effect of high pressure treatment on carotene stability has been studied in carrots and in mixed juices. Based on the available literature data, we can con- clude that high pressure treatment does not affect (or affects only slightly) the carotene content in food products. a-and B-carotene contents in carrot puree were only slightly affected by pressure exposure at 600MPa and 75.C for 40 min (Tauscher, 1998). Similar findings have also been reported by de acos and co- workers(2000)showing that carotene loss in carrot homogenates and carrot pastedifferent influence on the stability of vitamin C during storage. The ascorbic acid content in untreated and pressurised (400 MPa/room temperature/15 min) guava puree started to decline respectively after 10 and 20 days whereas that in heated (88–90°C/24 s) and (600 MPa/room temperature/15 min) pressurised guava purée remained constant during 30 and 40 days respectively (Yen and Lin, 1996). Kinetics of vitamin C degradation during storage have been studied in high pressure treated strawberry coulis. Vitamin C degradation of pressurised (400 MPa/20°C/30 min) and untreated coulis are nearly identical during storage at 4°C. Moreover, it has been shown that a pressure treatment neither accelerates nor slows down the kinetic degradation of ascorbic acid during subsequent storage (Sancho et al, 1999). The effect of oxygen on ascorbic acid stability under pressure has been studied by Taoukis and co-workers (1998). At 600 MPa and 75°C for 40 min exposed to air, ascorbic acid in buffer solution (sodium acetate buffer (0.1 M; pH 3.5–4)) degraded to 45% of its initial content while in the absence of oxygen, less vitamin loss was observed. Moreover, the addition of 10% sucrose resulted in a protec￾tive effect on ascorbic acid degradation. It was also noted that vitamin C loss was higher in fruit juice compared to that in buffer solutions. Vitamin C loss in pine￾apple and grapefruit juice after pressurisation (up to 600 MPa and 75°C) was max. 70% and 50% respectively. At constant pressure (600 MPa after 40min), the pres￾sure degradation of vitamin C in pineapple juice was temperature sensitive, e.g. loss 20–25% at 40°C, 45–50% at 60°C and 60–70% at 75°C in contrast to that in grapefruit juice. Detailed kinetics of combined pressure and temperature stability of ascorbic acid in different buffer (pH 4, 7 and 8) systems and real products (squeezed orange and tomato juices) have been carried out by Van den Broeck and co￾workers (1998). At 850 MPa and 50°C for 1 hour, no ascorbic acid loss was observed. The high pressure/thermal degradation of ascorbic acid at 850 MPa and 65–80°C followed a first order reaction. The rate of ascorbic acid degradation at 850 MPa increased with increasing temperature from 65 to 80°C indicating that pressure and temperature act synergistically. Ascorbic acid in tomato juice was more stable than in orange juice. It was also reported that temperature depend￾ence of ascorbic acid degradation (z value) was independent of the pressure level. Based on this study, it can be concluded that ascorbic acid is unstable at high pressure (850 MPa) in combination with high temperature (65–80°C). 21.5.2 Vitamin A and carotene The effect of high pressure treatment on carotene stability has been studied in carrots and in mixed juices. Based on the available literature data, we can con￾clude that high pressure treatment does not affect (or affects only slightly) the carotene content in food products. a- and b-carotene contents in carrot puree were only slightly affected by pressure exposure at 600 MPa and 75°C for 40 min (Tauscher, 1998). Similar findings have also been reported by de Ancos and co￾workers (2000) showing that carotene loss in carrot homogenates and carrot paste 440 The nutrition handbook for food processors