The stability of vitamins during food processing P Berry Ottaway, Berry Ottaway and Associates Ltd 10.1 ntroduction Vitamins, by their definition, are essential to health and have to be obtained from the diet on a regular basis because, with the exception of vitamin D, they cannot be produced by the body. In terms of medicine and nutrition, our knowledge of vitamins is relatively recent. Although James Lind discovered an association between limejuice and scurvy in 1753, it was over 170 years later that vitamin C was eventually isolated. The understanding of vitamin B1 goes back only to the 1950s and new roles for folates were still being discovered in the late 1990s Man's supply of vitamins is obtained from a varied diet of vegetables, cereals fruits and meats and the quantities of vitamins that are present in the dietary sources can be affected significantly by the processing and storage of the food 10.2 The vitamins Vitamins are a heterogeneous group of substances and are vital nutrients that must be obtained from the diet. Although a number of these were termed vitamins between the 1930s and 1950s, nutritional science now recognises only 13 sub- stances, or groups of substances, as being true vitamins. The 13 substances are divided into two categories, the fat-soluble vitamins of which there are four (vitamins A, D, E and K)and the water-soluble vitamins of which there are nine (vitamins C, Bl, B2, B6, B12, niacin, pantothenic acid and biotin ). They are listed in Table 101. even within the two sub-categories the vitamins have almost no common attributes in terms of chemistry, function or daily requirements. In terms of requirements some, such as vitamins C, E and niacin, are needed in tens of
10 The stability of vitamins during food processing P. Berry Ottaway, Berry Ottaway and Associates Ltd 10.1 Introduction Vitamins, by their definition, are essential to health and have to be obtained from the diet on a regular basis because, with the exception of vitamin D, they cannot be produced by the body. In terms of medicine and nutrition, our knowledge of vitamins is relatively recent. Although James Lind discovered an association between limejuice and scurvy in 1753, it was over 170 years later that vitamin C was eventually isolated. The understanding of vitamin B12 goes back only to the 1950s and new roles for folates were still being discovered in the late 1990s. Man’s supply of vitamins is obtained from a varied diet of vegetables, cereals, fruits and meats and the quantities of vitamins that are present in the dietary sources can be affected significantly by the processing and storage of the food. 10.2 The vitamins Vitamins are a heterogeneous group of substances and are vital nutrients that must be obtained from the diet. Although a number of these were termed vitamins between the 1930s and 1950s, nutritional science now recognises only 13 substances, or groups of substances, as being true vitamins. The 13 substances are divided into two categories, the fat-soluble vitamins of which there are four (vitamins A, D, E and K) and the water-soluble vitamins of which there are nine (vitamins C, B1, B2, B6, B12, niacin, pantothenic acid and biotin). They are listed in Table 10.1. Even within the two sub-categories, the vitamins have almost no common attributes in terms of chemistry, function or daily requirements. In terms of requirements some, such as vitamins C, E and niacin, are needed in tens of
248 The nutrition handbook for food processors Table 10.1 Vitamins and some commonly used synonyms Vitamin Synonyms vitamin A retinol vitamin D, Vitamin colecalciferol vitamin e alpha, beta and gamma tocopherols and alpha tocotrienol vitamin K menadione vitamin K menadione Water-soluble vitamin B1 thiamin vitamin B riboflavin vitamin B6 pyridoxal, pyridoxine, pyridoxamine vitamin B1 balamins, cyanocobalamin, hydroxocobalamin nicotinic acid(vitamin PP) acidamide nicotinamide(vitamin PP) antothenic acid lain(vitamin M) vitamin h vitamin c ascorbic acid milligrams a day whilst others, such as vitamins D and B12, are only needed in single microgram amounts. It can be seen from these examples that there is no relationship between the form of delivery (i.e. fat or water soluble)and the daily requirements. The heterogeneity also applies to the chemical structure and the functions of the vitamins. Chemically, there are no similarities between the sub- stances. Some are single substances such as biotin, whilst others, such as vitamin E, are groups of compounds all exhibiting vitamin activity 10.3 Factors affecting vitamin stability One of the very few attributes that the vitamins have in common is that none is completely stable in foods. The stability of the individual vitamins varies from the relatively stable, such as in the case of niacin, to the relatively unstable, such as vitamin B12. The factors that affect stability vary from vitamin to vitamin and the principal ones are summarised in Table 10.2. The most important of these factors are heat, moisture, oxygen, pH and light The deterioration of vitamins car place naturally during the storage of vegetables and fruits and losses can
milligrams a day whilst others, such as vitamins D and B12, are only needed in single microgram amounts. It can be seen from these examples that there is no relationship between the form of delivery (i.e. fat or water soluble) and the daily requirements. The heterogeneity also applies to the chemical structure and the functions of the vitamins. Chemically, there are no similarities between the substances. Some are single substances such as biotin, whilst others, such as vitamin E, are groups of compounds all exhibiting vitamin activity. 10.3 Factors affecting vitamin stability One of the very few attributes that the vitamins have in common is that none is completely stable in foods. The stability of the individual vitamins varies from the relatively stable, such as in the case of niacin, to the relatively unstable, such as vitamin B12. The factors that affect stability vary from vitamin to vitamin and the principal ones are summarised in Table 10.2. The most important of these factors are heat, moisture, oxygen, pH and light. The deterioration of vitamins can take place naturally during the storage of vegetables and fruits and losses can occur during the processing and preparation 248 The nutrition handbook for food processors Table 10.1 Vitamins and some commonly used synonyms Vitamin Synonyms Fat-soluble vitamin A retinol vitamin D2 ergocalciferol vitamin D3 cholecalciferol vitamin E alpha, beta and gamma tocopherols and alpha tocotrienol vitamin K1 phylloquinone, phytomenadione vitamin K2 farnoquinone, menaquinone vitamin K3 menadione Water-soluble vitamin B1 thiamin vitamin B2 riboflavin vitamin B6 pyridoxal, pyridoxine, pyridoxamine vitamin B12 cobalamins, cyanocobalamin, hydroxocobalamin niacin nicotinic acid (vitamin PP) niacinamide nicotinamide (vitamin PP) pantothenic acid — folic acid folacin (vitamin M) biotin vitamin H vitamin C ascorbic acid
The stability of vitamins during food processing 249 Table 10.2 Factors affecting the stability of vitamins Factor perature Presence of metallic ions(e.g. copper, iron) Oxidising and reducing agents Presence of other vitamins Other components of food(e.g. sulphur dioxide) Combinations of the above of foods and their ingredients, particularly those subjected to heat treatment. The factors that affect the degradation of vitamins are the same whether the vitamins are naturally occurring in the food or are added to the food from synthetic source However, the form in which a synthetic source is used(e.g. a salt or ester)may enhance its stability. For example, the vitamin E(tocopherol) esters are more stable than the tocopherol form itself. with the increased use of nutritional labelling of food products, vitamin levels in foods have become the subject of label claims that can be easily checked by the enforcement authorities. This poses a number of problems for the food tech- nologist. When more than one vitamin is the subject of a quantitative label claim for a food, it is very unlikely that the vitamins will deteriorate at the same rate If the amounts of these vitamins are included in nutritional labelling the shelf life of the food is determined by the life of the most unstable component In order to comply with the legal requirements of maintaining the label claim throughout the declared life of a food product, the food technologist needs to obtain a reasonably accurate estimation of the stability of each of the vitamins in the product. This has to be evaluated in the context of the food system(solid liquid, etc. ) the packaging and probable storage conditions and is achieved by conducting well-designed stability tests 10.4 Fat-soluble vitamins 10.4.1 Vitamin a Nutritionally, the human body can obtain its vitamin A requirements from two sources: from animal sources as forms of retinol, and from plant sources from B-carotene and related carotenoids. Both sources provide a supply of vitamin A but by different metabolic pathways. In terms of stability the two sources are different from each other Vitamin a is one of the more labile vitamins and retinol is less stable than the
of foods and their ingredients, particularly those subjected to heat treatment. The factors that affect the degradation of vitamins are the same whether the vitamins are naturally occurring in the food or are added to the food from synthetic sources. However, the form in which a synthetic source is used (e.g. a salt or ester) may enhance its stability. For example, the vitamin E (tocopherol) esters are more stable than the tocopherol form itself. With the increased use of nutritional labelling of food products, vitamin levels in foods have become the subject of label claims that can be easily checked by the enforcement authorities. This poses a number of problems for the food technologist. When more than one vitamin is the subject of a quantitative label claim for a food, it is very unlikely that the vitamins will deteriorate at the same rate. If the amounts of these vitamins are included in nutritional labelling, the shelf life of the food is determined by the life of the most unstable component. In order to comply with the legal requirements of maintaining the label claim throughout the declared life of a food product, the food technologist needs to obtain a reasonably accurate estimation of the stability of each of the vitamins in the product. This has to be evaluated in the context of the food system (solid, liquid, etc.), the packaging and probable storage conditions and is achieved by conducting well-designed stability tests. 10.4 Fat-soluble vitamins 10.4.1 Vitamin A Nutritionally, the human body can obtain its vitamin A requirements from two sources: from animal sources as forms of retinol, and from plant sources from b-carotene and related carotenoids. Both sources provide a supply of vitamin A, but by different metabolic pathways. In terms of stability the two sources are different from each other. Vitamin A is one of the more labile vitamins and retinol is less stable than the The stability of vitamins during food processing 249 Table 10.2 Factors affecting the stability of vitamins Factor • Temperature • Moisture • Oxygen • Light • pH • Presence of metallic ions (e.g. copper, iron) • Oxidising and reducing agents • Presence of other vitamins • Other components of food (e.g. sulphur dioxide) • Combinations of the above
250 The nutrition handbook for food processors retinyl esters. The presence of double bonds in its structure makes it subject to isomerisation, particularly in an aqueous medium at acid pH. The isomer with the highest biological activity is the all-trans vitamin A. The predominant cis isomer is 13-cis or neovitamin A which only has a biological activity of 75%o of the all-trans isomer; and 6-cis and 2, 6-di-cis isomers which may also form during isomerisation have less than 25%o of the biological activity of the all-trans form of vitamin A. The natural vitamin A sources usually contain about one-third neovitamin a while most synthetic sources generally contain considerably less For aqueous products where isomerisation is known to occur, mixtures of vitamin A palmitate isomers at the equilibrium ratio have been produced commercially Vitamin A is relatively stable in alkaline solutions Vitamin A is sensitive to atmospheric oxygen with the alcohol form being less table than the esters. The decomposition is catalysed by the presence of trace minerals. As a consequence of its sensitivity to oxygen, vitamin A is normally available commercially as a preparation that includes an antioxidant and often a protective coating. While butylated hydroxyanisole(BHA)and butylated hydrox toluene(BHT) are permitted in a number of countries for use as antioxidants in vitamin A preparations, the recent trend has been towards the use of tocopherols (vitamin E). Both retinol and its esters are inactivated by the ultraviolet compo- nent of light. In general, vitamin A is relatively stable during food processing involving heating, with the palmitate ester more stable to heat than retinol. It is normally regarded as stable during milk processing, and food composition tables give only small differences between the retinol contents of fresh whole milk. sterilised and ultra high temperature (UHT)treated milk. However, prolonged holding of milk or butter at high temperatures in the presence of air can be shown to result in a significant decrease in the vitamin A activity A provitamin is a compound that can be converted in the body to a vitamin and there are a number of carotenoids with provitamin A activity. Carotenoids are generally found as naturally occurring plant pigments that give the charac- teristic yellow, orange and red colours to a wide range of fruits and vegetables Some can also be found in the liver, kidney, spleen and milk. The provitamin A with the greatest nutritional and commercial importance is B-carotene. The sta- bility of the carotenoids is similar to vitamin A in that they are sensitive to oxygen, light and acid media It has been reported that treatment with sulphur dioxide reduces carotenoid destruction in vegetables during dehydration and storage. A study with model systems showed that the stability of B-carotene was greatly enhanced by sulphur dioxide added either as a sulphite solution to cellulose powder prior to B-carotene absorption or as a headspace gas in containers of B-carotene. While it was found that the B-carotene stability was improved by increasing the nitrogen levels in he containers, the stability was even greater when the nitrogen was replaced by ulphur dioxide. Comparative values for the induction period were 19 hours for B-carotene samples stored in oxygen only, 120 hours in nitrogen and 252 hours in sulphur dioxide. 2
retinyl esters. The presence of double bonds in its structure makes it subject to isomerisation, particularly in an aqueous medium at acid pH. The isomer with the highest biological activity is the all-trans vitamin A. The predominant cis isomer is 13-cis or neovitamin A which only has a biological activity of 75% of the all-trans isomer; and 6-cis and 2, 6-di-cis isomers which may also form during isomerisation have less than 25% of the biological activity of the all-trans form of vitamin A. The natural vitamin A sources usually contain about one-third neovitamin A while most synthetic sources generally contain considerably less. For aqueous products where isomerisation is known to occur, mixtures of vitamin A palmitate isomers at the equilibrium ratio have been produced commercially. Vitamin A is relatively stable in alkaline solutions. Vitamin A is sensitive to atmospheric oxygen with the alcohol form being less stable than the esters. The decomposition is catalysed by the presence of trace minerals. As a consequence of its sensitivity to oxygen, vitamin A is normally available commercially as a preparation that includes an antioxidant and often a protective coating. While butylated hydroxyanisole (BHA) and butylated hydroxytoluene (BHT) are permitted in a number of countries for use as antioxidants in vitamin A preparations, the recent trend has been towards the use of tocopherols (vitamin E). Both retinol and its esters are inactivated by the ultraviolet component of light. In general, vitamin A is relatively stable during food processing involving heating, with the palmitate ester more stable to heat than retinol. It is normally regarded as stable during milk processing, and food composition tables give only small differences between the retinol contents of fresh whole milk, sterilised and ultra high temperature (UHT) treated milk.1 However, prolonged holding of milk or butter at high temperatures in the presence of air can be shown to result in a significant decrease in the vitamin A activity. A provitamin is a compound that can be converted in the body to a vitamin and there are a number of carotenoids with provitamin A activity. Carotenoids are generally found as naturally occurring plant pigments that give the characteristic yellow, orange and red colours to a wide range of fruits and vegetables. Some can also be found in the liver, kidney, spleen and milk. The provitamin A with the greatest nutritional and commercial importance is b-carotene. The stability of the carotenoids is similar to vitamin A in that they are sensitive to oxygen, light and acid media. It has been reported that treatment with sulphur dioxide reduces carotenoid destruction in vegetables during dehydration and storage. A study with model systems showed that the stability of b-carotene was greatly enhanced by sulphur dioxide added either as a sulphite solution to cellulose powder prior to b-carotene absorption or as a headspace gas in containers of b-carotene. While it was found that the b-carotene stability was improved by increasing the nitrogen levels in the containers, the stability was even greater when the nitrogen was replaced by sulphur dioxide. Comparative values for the induction period were 19 hours for b-carotene samples stored in oxygen only, 120 hours in nitrogen and 252 hours in sulphur dioxide.2 250 The nutrition handbook for food processors
The stability of vitamins during food processing 251 Investigations into the effect of sulphur dioxide treatment on the B-carotene stability in dehydrated vegetables have given varying results and it has been pos- tulated that the effects of the drying and storage conditions on the stability of the ulphur dioxide has a consequential effect on the stability of the B-carotene in dehydrated products. Studies on the heat stability of both a-carotene and B- carotene showed that the B-carotene was about 1.9 times more susceptible than a-carotene to heat damage during normal cooking and blanching processes Products containing B-carotene should be protected from light and headspace air kept to the minimum 10.4.2 Vitamin E A number of naturally occurring substances exhibit vitamin E activity, including the a.B. y and 8 tocopherols and a tocotrienols. Dietary sources of vitamin E are found in a number of vegetables and cereals, with some vegetable oils such as wheatgerm, sunflower seed, safflower seed and maize oils being particularly good sources. Both synthetic and naturally-sourced forms of vitamin E are available commercially. Whilst the natural sources of the tocopherols, which also have the highest biological activity, are in the d form, the synthetic versions can only be produced in the dl form. Both the d and dl forms are also commercially available There is a considerable difference in the stability of the tocopherol forms of vitamin E and the tocopherol esters. While vitamin E is regarded as being one of the more stable vitamins, the unesterified tocopherol is less stable due to the free phenolic hydroxyl group Vitamin E is unusual in that it exhibits reduced stability at temperatures below freezing. The explanation given for this is that the peroxides formed during fat oxidation are degraded at higher temperatures but are stable at temperatures below 0C and as a consequence can react with the vitamin E. It has also been shown that a-tocopherol may function as a pro-oxidant in the presence of metal ons such as iron a-Tocopherol is readily oxidised by air. It is stable to heat in the absence of air but is degraded if heated in the presence of air and is readily oxidised dur ing the processing and storage of foods. One of the most important naturally- occurring sources of tocopherols are the vegetable oils, particularly wheat germ and cotton-seed oils. While deep-frying of the oils may result in a loss of vitamin E of around 10%, it has been found that the storage of fried foods, even at tem peratures as low as-12 C, can result in very significant losses. DL-a-Tocopheryl acetate is relatively stable in air but is hydrolysed by mois ture in the presence of alkalis or strong acids to free tocopherols 10.4.3 Vitamin D Present in nature in several forms dietary vitamin d occurs predominantly in animal products with very little being obtained from plant sources. Vitamin D
Investigations into the effect of sulphur dioxide treatment on the b-carotene stability in dehydrated vegetables have given varying results and it has been postulated that the effects of the drying and storage conditions on the stability of the sulphur dioxide has a consequential effect on the stability of the b-carotene in dehydrated products.3 Studies on the heat stability of both a-carotene and bcarotene showed that the b-carotene was about 1.9 times more susceptible than a-carotene to heat damage during normal cooking and blanching processes.4 Products containing b-carotene should be protected from light and headspace air kept to the minimum. 10.4.2 Vitamin E A number of naturally occurring substances exhibit vitamin E activity, including the a, b, g and d tocopherols and a tocotrienols. Dietary sources of vitamin E are found in a number of vegetables and cereals, with some vegetable oils such as wheatgerm, sunflower seed, safflower seed and maize oils being particularly good sources. Both synthetic and naturally-sourced forms of vitamin E are available commercially. Whilst the natural sources of the tocopherols, which also have the highest biological activity, are in the d form, the synthetic versions can only be produced in the dl form. Both the d and dl forms are also commercially available as esters. There is a considerable difference in the stability of the tocopherol forms of vitamin E and the tocopherol esters. While vitamin E is regarded as being one of the more stable vitamins, the unesterified tocopherol is less stable due to the free phenolic hydroxyl group. Vitamin E is unusual in that it exhibits reduced stability at temperatures below freezing. The explanation given for this is that the peroxides formed during fat oxidation are degraded at higher temperatures but are stable at temperatures below 0°C and as a consequence can react with the vitamin E.5 It has also been shown that a-tocopherol may function as a pro-oxidant in the presence of metal ions such as iron. a-Tocopherol is readily oxidised by air. It is stable to heat in the absence of air but is degraded if heated in the presence of air and is readily oxidised during the processing and storage of foods. One of the most important naturallyoccurring sources of tocopherols are the vegetable oils, particularly wheat germ and cotton-seed oils. While deep-frying of the oils may result in a loss of vitamin E of around 10%, it has been found that the storage of fried foods, even at temperatures as low as -12°C, can result in very significant losses. dl-a-Tocopheryl acetate is relatively stable in air but is hydrolysed by moisture in the presence of alkalis or strong acids to free tocopherols. 10.4.3 Vitamin D Present in nature in several forms, dietary vitamin D occurs predominantly in animal products with very little being obtained from plant sources. Vitamin D3 The stability of vitamins during food processing 251