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 straightforward 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, ascorbic acid and its oxidised form, dehydroascorbic acid. The former is relatively heat stable but the oxidised form is much more heat labile; losses of vitamin C are therefore related to the degree of oxidation of the vitamin rather than to the severity of the heat process (Burton, 1988). This is known to apply to other products, especially fruit juices (Ryley and Kadja, 1994) and vegetables.
There are some vitamin interactions which occur. In milk, vitamin B12 and folic acid interact with vitamin C during heat treatment so that losses of these vitamins again are not a simple function of the degree of heat treatment. Folic acid is protected by ascorbic acid, the reduced form of vitamin C, which, if oxidised, will lead to higher losses of folate. Vitamin B12 losses depend on the oxidative degradation of ascorbic acid, so these losses depend on the availability of oxygen and ascorbic acid, as well as on the thermal process.
Although these changes are dependent on the degree of thermal process applied to the foodstuff, the remainder of the process must also be considered when evaluating the nutritional quality of the product. Changes during storage of the product over the usual three to six month shelf-life at ambient temperatures can be considerable, even though the container is designed to exclude light and oxygen. In addition, there is usually some pre-processing during manufacture of products; for instance, soups, stews and cook-in sauces are usually cooked or fried before sterilisation. Finally, handling of the product in the home is usually out of the manufacturer's control and overheating or long standing time at warm temperatures can easily have a more severe effect on nutritional quality than any of the procedures described above.
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