470 The nutrition handbook for food processors 22.5.1Fats UHT processing has not been found to cause any physical or chemical changes to fats in milk and milk products. Milk is usually homogenised during treatment, and some instability of the milk fat globule may occur due to denaturation of the proteins in the milk fat globule membrane. Because of this, homogenisation after he heat treatment section is usually preferred, especially in direct-heating systems, but has the potential to cause recontamination of the product due to leakage through the homogeniser seals and general difficulty with cleaning this area. It is important to have a well designed homogeniser with aseptic design and steam seals on the piston seals 22.6Ⅴ itamin The heat stability of vitamins in foodstuffs during heating is extremely variable, depending on the foodstuff and conditions of heating, e. g. presence of oxygen As vitamins often consist of different chemicals. all of which have vitamin activity but degrade at different rates, it is often difficult to measure vitamin loss in a mechanistic way; see the discussion on vitamin C below. Apart from straight forward denaturation, reactions between some of the vitamins may also occur, iving further losses The heat-sensitive vitamins are generally taken to be the fat-soluble vitamin A(with oxygen present)D, E, and B-carotene(provitamin A); and some of the water-soluble vitamins, B,(thiamin), B,(riboflavin)(in acid environment), nico- tinic acid, pantothenic acid, biotin and vitamin C(ascorbic acid). Vitamin Bl and folic acid are also heat labile but their destruction involves a complex series of reactions with each other. Vitamin B is generally little affected by heat, but storage after heat treatment can cause high losses. Niacin and vitamin k are fairl stable to heat. As before, losses through degradation in continuous-flow heat processing will be similar to those encountered during in-container processing but at a lower level In milk, vitamins A, D, E, pantothenic acid, nicotinic acid, biotin, riboflavin and niacin are all stable to heat (Burton, 1988). Thiamin(B1), B. Bi2 and vitamin C all degrade during sterilisation. Thiamin is the most heat labile of these and has been used as a chemical marker to define the UhT process for milk by Horak (1980)as the time-temperature combinations which produce a sterile product but have a loss of thiamin of less than 3%. The thermal degradation kinetics have been established and used to predict thiamin loss for different commercial UHT processes; again, thiamin was expected to have a better survival in direct heating processes than in indirect heating processes and, of the latter in processes with small heat recovery sections than those with large sections Vitamin C loss, although generally in e reglon of 25%, is not significant because milk is such a poor source of the vitamin in the diet. This compares well to losses of 90% of the vitamin during in-container sterilisation. The loss of the vitamin is less simple than a degradation: vitamin C exists in two forms, ascor- bic acid and its oxidised form, dehydroascorbic acid. The former is relatively heat470 The nutrition handbook for food processors 22.5.1 Fats UHT processing has not been found to cause any physical or chemical changes to fats in milk and milk products. Milk is usually homogenised during treatment, and some instability of the milk fat globule may occur due to denaturation of the proteins in the milk fat globule membrane. Because of this, homogenisation after the heat treatment section is usually preferred, especially in direct-heating systems, but has the potential to cause recontamination of the product due to leakage through the homogeniser seals and general difficulty with cleaning this area. It is important to have a well designed homogeniser with aseptic design and steam seals on the piston seals. 22.6 Vitamins The heat stability of vitamins in foodstuffs during heating is extremely variable, depending on the foodstuff and conditions of heating, e.g. presence of oxygen. As vitamins often consist of different chemicals, all of which have vitamin activity but degrade at different rates, it is often difficult to measure vitamin loss in a mechanistic way; see the discussion on vitamin C below. Apart from straight￾forward denaturation, reactions between some of the vitamins may also occur, giving further losses. The heat-sensitive vitamins are generally taken to be the fat-soluble vitamins; A (with oxygen present) D, E, and b-carotene (provitamin A); and some of the water-soluble vitamins, B1 (thiamin), B2 (riboflavin) (in acid environment), nico￾tinic acid, pantothenic acid, biotin and vitamin C (ascorbic acid). Vitamin B12 and folic acid are also heat labile but their destruction involves a complex series of reactions with each other. Vitamin B6 is generally little affected by heat, but storage after heat treatment can cause high losses. Niacin and vitamin K are fairly stable to heat. As before, losses through degradation in continuous-flow heat processing will be similar to those encountered during in-container processing but at a lower level. In milk, vitamins A, D, E, pantothenic acid, nicotinic acid, biotin, riboflavin and niacin are all stable to heat (Burton, 1988). Thiamin (B1), B6, B12 and vitamin C all degrade during sterilisation. Thiamin is the most heat labile of these and has been used as a chemical marker to define the UHT process for milk by Horak (1980) as the time–temperature combinations which produce a sterile product but have a loss of thiamin of less than 3%. The thermal degradation kinetics have been established and used to predict thiamin loss for different commercial UHT processes; again, thiamin was expected to have a better survival in direct heating processes than in indirect heating processes and, of the latter in processes with small heat recovery sections than those with large sections. Vitamin C loss, although generally in the region of 25%, is not significant because milk is such a poor source of the vitamin in the diet. This compares well to losses of 90% of the vitamin during in-container sterilisation. The loss of the vitamin is less simple than a degradation: vitamin C exists in two forms, ascor￾bic acid and its oxidised form, dehydroascorbic acid. The former is relatively heat