As already discussed, all vitamins exhibit a degree of instability, the rate of which is affected by a number of factors. Naturally-occurring vitamins in foods are susceptible to many of these factors during the harvesting, processing and storage of the food and its ingredients. It is particularly important that the effects of processing are taken into consideration when assessing vitamin stability in foods, as the food may have been subjected to a number of adverse factors during processing. The most common factor during processing is the application of heat, which in some cases, such as canning, can be for a relatively long time. Most of the work on the stability of vitamins in fruits and vegetables during blanching and canning was carried out during the 1940s and 1950s. Although there have since been refinements both in processing and analytical techniques, many of the conclusions drawn from this research are still valid.
In terms of blanching it was found that a high temperature-short time water blanch gave a better vitamin retention than a low temperature-long time blanch and that, overall, steam blanching was superior to water blanching. The addition of sulphite to the blanching water has been shown to affect significantly thiamin levels in fruits and vegetables. Beta carotene was found to be the best survivor during blanching. Riboflavin had retentions in the range 80 to 95%;
vitamin C was in the range 70 to 90% under optimum conditions and niacin 75
Studies on the heat processing of fruits and vegetables in both tin and glass containers showed significant losses of both vitamin C and thiamin. In some cases, the vitamin C levels assayed immediately after the heat processing were between 15 and 45% of the fresh product and these values further reduced during storage.
Thiamin reduced by about 50% during heat processing and further declined to between 15 and 40% of the original level after 12 months' storage. Riboflavin losses were between 12 and 15% during processing but levels of about 50% of the original were observed after 12 months. Niacin was more stable with initial losses of 15 to 25% but with much less than riboflavin being lost during storage. Beta carotene was found to be relatively stable.
In milk, the fat soluble vitamins A and D are relatively stable to the heat treatments used for the processing of milk, as are the water soluble vitamins riboflavin, niacin, pantothenic acid and biotin. Vitamin C, thiamin, vitamin B6, vitamin B12 and folic acid are all affected by the heat processing of milk, with the more severe the process, the greater the loss. With the exception of vitamin C, vitamin losses are generally less than 10% after pasteurisation of milk and between 10 and 20% after Ultra High Temperature (UHT) treatment. Average losses following sterilisation of milk are reported as 20% for thiamin, vitamin B6 and vitamin B12 and 30% for folic acid. Studies have shown that the stability of vitamin C during the processing of milk is also affected by the oxygen content of the milk. Average losses for the vitamin C were 25% after pasteurisation, 30% after UHT and 60% after sterilisation. However, vitamin C appears to be particularly well retained in condensed full cream milk.10
B-vitamin stability during the heat processing and cooking of meats varies widely. Cooking conditions can have a marked effect on stability and the retention of thiamin in beef and pork is related to roasting temperatures. If the vitamin content of the drippings is taken into consideration, it is generally found that riboflavin, niacin and vitamin B12 are stable during the cooking of meat. Pan-tothenic acid losses in cooked meat are usually less than 10% although high losses of folate (both free and total) of over 50% have been found in pork, beef and chicken that had been boiled for 15 minutes. Post-mortem ageing of beef can result in up to a 30% loss of niacin over seven days, although the remaining niacin is relatively stable on cooking. The baking of bread can induce losses of about 20% for thiamin, up to 17% for vitamin B6 and up to one third of the natural folate content. Niacin and pantothenic acid are normally stable during baking.
Although most of the vitamins are stable in frozen fruits and vegetables for periods of up to a year, losses of vitamin C have been found to occur at temperatures as low as -23°C.
262 The nutrition handbook for food processors 10.7.4 Dehydration
Studies on the dehydration of blanched vegetables show that the dehydration process can result in additional losses. The dehydration of blanched cabbage (unsulphited) gave an additional 30% reduction in vitamin C content, 5 to 15% in the niacin content and about 15% of the thiamin.
10.7.5 Effect of irradiation on vitamin stability in foods
The use of ionising radiation (irradiation) as a sterilisation technique for foods has been accepted in a number of countries, including the European Union. In many countries the foods and ingredients that are allowed to be irradiated are restricted by law and the process is normally only used for foods at risk of high levels of microbiological contamination.
It has been shown that vitamin levels in a food can be affected by irradiation and the losses can, in general, be related to the dose. At low doses (e.g. up to 1 kilogray), the losses for most vitamins are not significant. At higher doses (3-10kGy) it has been shown that vitamin losses can occur in foods that are exposed to air during the irradiation and subsequent storage. At the highest permitted radiation doses, care has to be taken to protect the food by using packaging that excludes the air and by carrying out the irradiation process at a low temperature.
There is evidence that the fat-soluble vitamins A, E and K and the water-soluble thiamin are the most sensitive to irradiation, whereas niacin, riboflavin and vitamin D are relatively stable. There is conflicting evidence for vitamins with some foods showing significant losses and others almost none. If it is intended that nutrition claims are to be made for irradiated foods, it is essential that studies are carried out on the content and stability of the vitamins after the treatment with the ionising radiation.
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