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
252 The nutrition handbook for food processors or cholecalciferol is derived in animals, including man, from ultra-violet irra- diation of 7-dehydrocholesterol found in the skin. Human requirements are obtained both from the endogenous production in the skin and from dietary sources. Vitamin D,(ergocalciferol) is produced by the ultraviolet irradiation of ergosterol, which is widely distributed in plants and fungi. Both vitamins D2 and D3 are manufactured for commercial use. Both vitamins D2 and D3 are sensitive to light and can be destroyed relatively pidly if exposed to light. They are also adversely affected by acids. Prepara- tions of vitamin D in edible oils are more stable than the crystalline forms, and the vitamin is normally provided for commercial usage as an oil preparation or stabilised powder containing an antioxidant(usually tocopherol). The prepara tions are normally provided in lightproof containers with inert gas flushing The presence of double bonds in the structure of both forms of vitamin D can make them susceptible to isomerisation under certain conditions. Studies have shown that the isomerisation rates of ergocalciferol and cholecalciferol are almost equal. Isomerisation in solutions of cholecalciferol resulted in an equilibrium being formed between ergocalciferol and precalciferol with the ratios of the isomers being temperature dependent. The isomerisation of ergocalciferol has been studied in powders prepared with calcium sulphate, calcium phosphate, talc and magnesium trisilicate. It was found that the isomerisation was catalysed by he surface acid of these additives 6 Crystalline vitamin D2 is sensitive to atmospheric oxygen and will show signs of decomposition after a few days storage in the presence of air at ambient temperatures. Crystalline cholecalciferol, D,, is also destroyed by atmospheric oxygen but is relatively more stable than D,, possibly due to the fact that it has one less double bond The vitamin D3 naturally occurring in foods such as milk and fish, appears to be relatively stable to heat processing. 104. 4 Vitamin K Vitamin K occurs in a number of forms. Vitamin K(phytomenadione or phyl loquinone) is found in green plants and vegetables, potatoes and fruits, while vitamin K2(menaquinone) can be found in animal and microbial materials. The presence of double bonds in both vitamins K and K, makes them liable to isomerisation. Vitamin K has only one double bond in the side chain in the 3-position whereas in K double bonds recur regularly in the side chain. Vitamin KI exists in the form of both trans and cis isomers. The trans isomer is the naturally occurring form and is the one that is biologically active. The cis form has no significant biological activity The various forms of vitamin K are relatively stable to heat and are retained after most cooking processes. The vitamin is destroyed by sunlight and is decomposed by alkalis. Vitamin Ki is only slowly decomposed by atmospheric oxygen. Vitamin K is rarely added to food pre roducts and the most common commer-
or cholecalciferol is derived in animals, including man, from ultra-violet irradiation of 7-dehydrocholesterol found in the skin. Human requirements are obtained both from the endogenous production in the skin and from dietary sources. Vitamin D2 (ergocalciferol) is produced by the ultraviolet irradiation of ergosterol, which is widely distributed in plants and fungi. Both vitamins D2 and D3 are manufactured for commercial use. Both vitamins D2 and D3 are sensitive to light and can be destroyed relatively rapidly if exposed to light. They are also adversely affected by acids. Preparations of vitamin D in edible oils are more stable than the crystalline forms, and the vitamin is normally provided for commercial usage as an oil preparation or stabilised powder containing an antioxidant (usually tocopherol). The preparations are normally provided in lightproof containers with inert gas flushing. The presence of double bonds in the structure of both forms of vitamin D can make them susceptible to isomerisation under certain conditions. Studies have shown that the isomerisation rates of ergocalciferol and cholecalciferol are almost equal. Isomerisation in solutions of cholecalciferol resulted in an equilibrium being formed between ergocalciferol and precalciferol with the ratios of the isomers being temperature dependent. The isomerisation of ergocalciferol has been studied in powders prepared with calcium sulphate, calcium phosphate, talc and magnesium trisilicate. It was found that the isomerisation was catalysed by the surface acid of these additives.6 Crystalline vitamin D2 is sensitive to atmospheric oxygen and will show signs of decomposition after a few days storage in the presence of air at ambient temperatures. Crystalline cholecalciferol, D3, is also destroyed by atmospheric oxygen but is relatively more stable than D2, possibly due to the fact that it has one less double bond. The vitamin D3 naturally occurring in foods such as milk and fish, appears to be relatively stable to heat processing. 10.4.4 Vitamin K Vitamin K occurs in a number of forms. Vitamin K1 (phytomenadione or phylloquinone) is found in green plants and vegetables, potatoes and fruits, while vitamin K2 (menaquinone) can be found in animal and microbial materials. The presence of double bonds in both vitamins K1 and K2 makes them liable to isomerisation. Vitamin K1 has only one double bond in the side chain in the 3-position whereas in K2 double bonds recur regularly in the side chain. Vitamin K1 exists in the form of both trans and cis isomers. The trans isomer is the naturally occurring form and is the one that is biologically active. The cis form has no significant biological activity. The various forms of vitamin K are relatively stable to heat and are retained after most cooking processes. The vitamin is destroyed by sunlight and is decomposed by alkalis. Vitamin K1 is only slowly decomposed by atmospheric oxygen. Vitamin K is rarely added to food products and the most common commer- 252 The nutrition handbook for food processors
The stability of vitamins during food processing 253 cially available form is Ki(phytomenadione), which is insoluble in water. A water-soluble K, is available as menadione sodium bisulphite 10.5 Water-soluble vitamins The water-soluble vitamin group contains eight vitamins collectively known as the B-complex vitamins plus vitamin C (ascorbic acid 10.5.1 Thiamin(vitamin B1) Thiamin is widely distributed in living tissues. In most animal products it occurs in a phosphorylated form, and in plant products it is predominantly in the non- phosphorylated form. Commercially it is available as either thiamin hydrochlo ride or thiamin mononitrate. Both these salts have specific areas of application and their use depends on the product matrix to which they are added A considerable amount of research has been carried out on the heat stability of thiamin and its salts, particularly in the context of cooking losses. Early work on thiamin losses during bread-making showed an initial cleavage of the thiamin to pyrimidine and thiazole. The destruction of thiamin by heat is more rapid in alkaline media. Vitamin B, losses in milk, which has an average fresh content of 0.04 mg thiamin per 100 g, are normally less than 10% for pasteurised milk, between 5 and 15% for Uht milk and between 30 and 40%o for sterilised milk Between 30 and 50% of the vitamin B, activity can be lost during the production of evaporated milk. Losses of thiamin during the commercial baking of white bread are between 5 and 20%. Part of this loss is due to the yeast fermentation, which can convert thiamin to cocarboxylase, which is less stable than thiamin. Thiamin is very sensitive to sulphites and bisulphite as it is cleaved by sulphite. This reaction is rapid at high pH, and is the cause of large losses of the vitamin in vegetables blanched with sulphite, and in meat products such as comminuted meats where sulphites and bisulphite are used as preservatives. Where the pH is low, such as in citrus fruit juices, the bisulphite occurs mainly as the unionised acid, and thiamin losses in such systems are not significantly different from those in prod ucts not containing bisulphite. Studies on the rate of sulphite-induced cleavage of thiamin during the prepa ration and storage of minced meat showed that losses of thiamin were linear with sulphur dioxide concentrates up to O.1%. The storage temperature did not have a significant effect on the losses. It has also been reported that thiamin is cleaved by aromatic aldehydes. Thiamin is decomposed by both oxidising and reducing agents. If it is allowed to stand in alkaline solution in air it is oxidised to the disulphide and small amounts of thiothiazolone. A range of food ingredients has been shown to have an effect on the stability of thiamin. In general, proteins are protective of the vitamin, particularly food proteins such as egg albumin and casein. When heated with glucose, either
cially available form is K1 (phytomenadione), which is insoluble in water. A water-soluble K3 is available as menadione sodium bisulphite. 10.5 Water-soluble vitamins The water-soluble vitamin group contains eight vitamins collectively known as the B-complex vitamins plus vitamin C (ascorbic acid). 10.5.1 Thiamin (vitamin B1) Thiamin is widely distributed in living tissues. In most animal products it occurs in a phosphorylated form, and in plant products it is predominantly in the nonphosphorylated form. Commercially it is available as either thiamin hydrochloride or thiamin mononitrate. Both these salts have specific areas of application and their use depends on the product matrix to which they are added. A considerable amount of research has been carried out on the heat stability of thiamin and its salts, particularly in the context of cooking losses. Early work on thiamin losses during bread-making showed an initial cleavage of the thiamin to pyrimidine and thiazole.7 The destruction of thiamin by heat is more rapid in alkaline media. Vitamin B1 losses in milk, which has an average fresh content of 0.04 mg thiamin per 100 g, are normally less than 10% for pasteurised milk, between 5 and 15% for UHT milk and between 30 and 40% for sterilised milk.7 Between 30 and 50% of the vitamin B1 activity can be lost during the production of evaporated milk. Losses of thiamin during the commercial baking of white bread are between 15 and 20%. Part of this loss is due to the yeast fermentation, which can convert thiamin to cocarboxylase, which is less stable than thiamin. Thiamin is very sensitive to sulphites and bisulphites as it is cleaved by sulphite. This reaction is rapid at high pH, and is the cause of large losses of the vitamin in vegetables blanched with sulphite, and in meat products such as comminuted meats where sulphites and bisulphites are used as preservatives. Where the pH is low, such as in citrus fruit juices, the bisulphite occurs mainly as the unionised acid, and thiamin losses in such systems are not significantly different from those in products not containing bisulphite.8 Studies on the rate of sulphite-induced cleavage of thiamin during the preparation and storage of minced meat showed that losses of thiamin were linear with sulphur dioxide concentrates up to 0.1%. The storage temperature did not have a significant effect on the losses. It has also been reported that thiamin is cleaved by aromatic aldehydes. Thiamin is decomposed by both oxidising and reducing agents. If it is allowed to stand in alkaline solution in air it is oxidised to the disulphide and small amounts of thiothiazolone. A range of food ingredients has been shown to have an effect on the stability of thiamin. In general, proteins are protective of the vitamin, particularly food proteins such as egg albumin and casein. When heated with glucose, either as a The stability of vitamins during food processing 253
254 The nutrition handbook for food processors ry mixture or in solution, a browning analogous to a Maillard reaction can occur. This reaction is similar to the reaction between sugars and amino acids and may be important in the loss of thiamin during heat processing. Work has shown that fructose, invertase, mannitol and inositol can actually retard the rate of destruc ion of thiamin .g Thiamin is unstable in alkaline solutions and becomes increasingly unstable as the pH increases. The stability of the vitamin in low pH solutions such as for- tified fruit drinks is very good. In common with that of some other vitamins, the stability of thiamin is adversely affected by the presence of copper ions. This effect can be reduced by the addition of metal-chelating compounds such as calcium disodium ethylenediamine tetra-acetate(EDTA). The heavy metals only appear to influence thiamin stability when they are capable of forming complex anions with constituents of the medium The enzymes, thiaminase, which are present in small concentrations in a number of animal and vegetable food sources, can degrade thiamin. These enzymes are most commonly found in a range of seafoods such as shrimps, clams and raw fish. but are also found in some varieties of beans mustard seed and rice polishings. Two types of thiaminase are known and these are designated thia- minase I and thiaminase Il. The former catalyses the decomposition of the thia- mine by a base-exchange reaction, involving a nucleophilic displacement of the methylene group of the pyrimidine moiety. Thiaminase Il catalyses a simple hydrolysis of thiamin A problem associated with the addition of vitamin B, to food products is th pleasant flavour and odour of the thiamin salts. The breakdown of thiamin, particularly during heating, may give rise to off-flavours, and the compounds derived from the degradation of the vitamins are believed to contribute to the cooked'flavours in a number of foods. However, both thiamin hydrochloride and mononitrate are relatively stable to atmospheric oxygen in the absence of light and moisture, and both are normally considered to be very stable when used in dry products with light and moisture-proof packagin 10.5.2 Riboflavin (vitamin B2) Riboflavin is the most widely distributed of all the vitamins and is found in all plant and animal cells, although there are relatively few rich food sources. It is present naturally in foods in two bound forms, riboflavin mononucleotide and flavin adenine dinucleotide. Plants and many bacteria can synthesise riboflavin and it is also found in dietary amounts in dairy products. Riboflavin is available commercially as a crystalline powder that is only sparingly soluble in water. As a consequence, the sodium salt of riboflavin-5-phosphate, which is more soluble in water, is used for liquid preparations The most important factor influencing the stability of this vitamin is light, with the greatest effect being caused by light in the 420 to 560um range. Fluorescent light is less harmful than direct sunlight, but products in transparent packagin can be affected by strip lighting in retail outlets. Riboflavin and riboflavin phos
dry mixture or in solution, a browning analogous to a Maillard reaction can occur. This reaction is similar to the reaction between sugars and amino acids and may be important in the loss of thiamin during heat processing. Work has shown that fructose, invertase, mannitol and inositol can actually retard the rate of destruction of thiamin.9 Thiamin is unstable in alkaline solutions and becomes increasingly unstable as the pH increases. The stability of the vitamin in low pH solutions such as fortified fruit drinks is very good. In common with that of some other vitamins, the stability of thiamin is adversely affected by the presence of copper ions. This effect can be reduced by the addition of metal-chelating compounds such as calcium disodium ethylenediamine tetra-acetate (EDTA). The heavy metals only appear to influence thiamin stability when they are capable of forming complex anions with constituents of the medium. The enzymes, thiaminases, which are present in small concentrations in a number of animal and vegetable food sources, can degrade thiamin. These enzymes are most commonly found in a range of seafoods such as shrimps, clams and raw fish, but are also found in some varieties of beans, mustard seed and rice polishings. Two types of thiaminases are known and these are designated thiaminase I and thiaminase II. The former catalyses the decomposition of the thiamine by a base-exchange reaction, involving a nucleophilic displacement of the methylene group of the pyrimidine moiety. Thiaminase II catalyses a simple hydrolysis of thiamin. A problem associated with the addition of vitamin B1 to food products is the unpleasant flavour and odour of the thiamin salts.10 The breakdown of thiamin, particularly during heating, may give rise to off-flavours, and the compounds derived from the degradation of the vitamins are believed to contribute to the ‘cooked’ flavours in a number of foods. However, both thiamin hydrochloride and mononitrate are relatively stable to atmospheric oxygen in the absence of light and moisture, and both are normally considered to be very stable when used in dry products with light and moisture-proof packaging. 10.5.2 Riboflavin (vitamin B2) Riboflavin is the most widely distributed of all the vitamins and is found in all plant and animal cells, although there are relatively few rich food sources. It is present naturally in foods in two bound forms, riboflavin mononucleotide and flavin adenine dinucleotide. Plants and many bacteria can synthesise riboflavin and it is also found in dietary amounts in dairy products. Riboflavin is available commercially as a crystalline powder that is only sparingly soluble in water. As a consequence, the sodium salt of riboflavin-5¢-phosphate, which is more soluble in water, is used for liquid preparations. The most important factor influencing the stability of this vitamin is light, with the greatest effect being caused by light in the 420 to 560mm range. Fluorescent light is less harmful than direct sunlight, but products in transparent packaging can be affected by strip lighting in retail outlets. Riboflavin and riboflavin phos- 254 The nutrition handbook for food processors
The stability of vitamins during food processing 255 phate are both stable to heat and atmospheric oxygen, particularly in an acid medium. In this respect, riboflavin is regarded as being one of the more stable vitamins. It is degraded by reducing agents and becomes increasingly unstable with increasing pH. While riboflavin is stable to the heat processing of milk, one of the main causes of loss in milk and milk products is from exposure to light. Liquid milk exposed to light can lose between 20 and 80% of its riboflavin content in two hours, with the rate and extent of loss being dependent upon the light intensity, the temperature and the surface area of the container exposed Although vitamin B2 is sensitive to light, particularly in a liquid medium such as nilk, it remains stable in white bread wrapped in transparent packaging and kept in a lit retail area 10.5.3 Niacin The term ' niacin'is generic for both nicotinic acid and nicotinamide(naci- amide)in foods. Both forms have equal vitamin activity, both are present in variety of foods and both forms are available as commercial isolates. Niacin occurs naturally in the meat and liver of hoofed animals and also in some plants In maize and some other cereals it is found in the form of niacytin, which is bound to polysaccharides and peptides in the outer layers of the cereal grains and is unavailable to man unless treated with a mild alkali Both forms of niacin are normally very stable in foods because they are not affected by atmospheric oxygen, heat and light in either aqueous or solid systems 10.5.4 Pantothenic acid In nature, pantothenic acid is widely distributed in plants and animals, but is rarely found in the free state as it forms part of the coenzyme A molecule. It is found in yeast and egg yolk, and in muscle tissue, liver, kidney and heart of animals. It is also found in a number of vegetables. cereals and nuts. Pantothenic acid is optically active and only its dextro-rotary forms have vitamin activity. Losses of pantothenic acid during the preparation and cooking of foods are nor mally not very large. Milk generally loses less than 10% during processing, and meat losses during cooking are not excessive when compared to those of the other B vitamins Free pantothenic acid is an unstable and very hygroscopic oil. Commercial preparations are normally provided as calcium or sodium salts. The alcohol form, panthenol, is available as a stable liquid but is not widely used in foods. The three commercial forms, calcium and sodium D-pantothenate and D- pantothenol, are moderately stable to atmospheric oxygen and light when pro- tected from moisture. All three compounds are hygroscopic, especially sodium pantothenate. Aqueous solutions of both the salts and the alcohol form are ther molabile and will undergo hydrolytic cleavage, particularly at high or low pH. The compounds are unstable in both acid and alkaline solutions and maximum stability is in the pH range of 6 to 7. Aqueous solutions of D-panthenol are more stable than the salts, particularly in the pH range 3 to 5
phate are both stable to heat and atmospheric oxygen, particularly in an acid medium. In this respect, riboflavin is regarded as being one of the more stable vitamins. It is degraded by reducing agents and becomes increasingly unstable with increasing pH. While riboflavin is stable to the heat processing of milk, one of the main causes of loss in milk and milk products is from exposure to light. Liquid milk exposed to light can lose between 20 and 80% of its riboflavin content in two hours, with the rate and extent of loss being dependent upon the light intensity, the temperature and the surface area of the container exposed. Although vitamin B2 is sensitive to light, particularly in a liquid medium such as milk, it remains stable in white bread wrapped in transparent packaging and kept in a lit retail area. 10.5.3 Niacin The term ‘niacin’ is generic for both nicotinic acid and nicotinamide (niacinamide) in foods. Both forms have equal vitamin activity, both are present in a variety of foods and both forms are available as commercial isolates. Niacin occurs naturally in the meat and liver of hoofed animals and also in some plants. In maize and some other cereals it is found in the form of niacytin, which is bound to polysaccharides and peptides in the outer layers of the cereal grains and is unavailable to man unless treated with a mild alkali. Both forms of niacin are normally very stable in foods because they are not affected by atmospheric oxygen, heat and light in either aqueous or solid systems. 10.5.4 Pantothenic acid In nature, pantothenic acid is widely distributed in plants and animals, but is rarely found in the free state as it forms part of the coenzyme A molecule. It is found in yeast and egg yolk, and in muscle tissue, liver, kidney and heart of animals. It is also found in a number of vegetables, cereals and nuts. Pantothenic acid is optically active and only its dextro-rotary forms have vitamin activity. Losses of pantothenic acid during the preparation and cooking of foods are normally not very large. Milk generally loses less than 10% during processing, and meat losses during cooking are not excessive when compared to those of the other B vitamins. Free pantothenic acid is an unstable and very hygroscopic oil. Commercial preparations are normally provided as calcium or sodium salts. The alcohol form, panthenol, is available as a stable liquid but is not widely used in foods. The three commercial forms, calcium and sodium d-pantothenate and dpantothenol, are moderately stable to atmospheric oxygen and light when protected from moisture. All three compounds are hygroscopic, especially sodium pantothenate. Aqueous solutions of both the salts and the alcohol form are thermolabile and will undergo hydrolytic cleavage, particularly at high or low pH. The compounds are unstable in both acid and alkaline solutions and maximum stability is in the pH range of 6 to 7. Aqueous solutions of d-panthenol are more stable than the salts, particularly in the pH range 3 to 5. The stability of vitamins during food processing 255
256 The nutrition handbook for food processors 10.5.5 Folic acid/folates Folic acid (pteroylglutamic acid) does not occur in nature but can be produced commercially. The naturally occurring forms are a number of derivatives collec- tively known as folates or folacin, which contain one or more linked molecules of glutamic acid. Polyglutamates predominate in fresh food, but on storage these can slowly break down to monoglutamates and oxidise to less biologically avail- able folates. The folic acid synthesised for food fortification contains only one glutamic group. Most of the stability studies have been carried out with the commercially avail- tble folic acid, which has been found to be moderately stable to heat and atmos- pheric oxygen. In solution it is stable at around pH 7 but becomes increasingly unstable in acid or alkali media, particularly at pH less than 5. Folic acid decomposed by oxidising and reducing agents. Sunlight, and particularly ultra- violet radiation, has a serious effect on the stability of folic acid. Cleavage by light is more rapid in the presence of riboflavin, but this reaction can be retarded by the addition of the antioxidant BHa to solutions containing folic acid and The stability of the folates in foods during processing and storage is variable Folic acid loss during the pasteurisation of milk is normally less than 5%.Losses in the region of 20% can occur during UHT treatment and about 30% loss is found after sterilisation Uht milk stored for three months can lose over 50%o of its folic acid. The extra heat treatment involved in boiling pasteurised milk can decrease the folic acid content by 20%0. Losses of around 10% are found in boiled eggs, while other forms of cooking(fried, poached, scrambled) give between 30 and 35% loss. Total folic acid losses from vegetables as a result of heating and cooking processes can be very high A study carried out on the stability of folic acid in spinach during processing and storage showed major differences between water blanching and steam blanching, with a folate retention of 58 steam blanching and o with water. Frozen spinach was found to retain 72% folate after 3 months storage. 10.5.6 Vitamin B,(pyridoxine) Vitamin B activity is shown by three compounds, pyridoxol, pyridoxal and pridoxamine and these are often considered together as pyridoxine. Vitamin B is found in red meat, liver, cod roe and liver, milk and green vegetables. The commercial form normally used for food fortification is the salt, pyridoxine Pyridoxine is normally stable to atmospheric oxygen and heat Decomposition catalysed by metal ions. Pyridoxine is sensitive to light, particularly in neutral and alkaline solutions. One of the main causes of loss of this vitamin in milk is unlight with a 21%o loss being reported after 8 hours exposure. Pyridoxine is stable in milk during pasteurisation but about 20%o can be lost during sterilise-
10.5.5 Folic acid/folates Folic acid (pteroylglutamic acid) does not occur in nature but can be produced commercially. The naturally occurring forms are a number of derivatives collectively known as folates or folacin, which contain one or more linked molecules of glutamic acid. Polyglutamates predominate in fresh food, but on storage these can slowly break down to monoglutamates and oxidise to less biologically available folates. The folic acid synthesised for food fortification contains only one glutamic group. Most of the stability studies have been carried out with the commercially available folic acid, which has been found to be moderately stable to heat and atmospheric oxygen. In solution it is stable at around pH 7 but becomes increasingly unstable in acid or alkali media, particularly at pH less than 5. Folic acid is decomposed by oxidising and reducing agents. Sunlight, and particularly ultraviolet radiation, has a serious effect on the stability of folic acid. Cleavage by light is more rapid in the presence of riboflavin, but this reaction can be retarded by the addition of the antioxidant BHA to solutions containing folic acid and riboflavin.11 The stability of the folates in foods during processing and storage is variable. Folic acid loss during the pasteurisation of milk is normally less than 5%. Losses in the region of 20% can occur during UHT treatment and about 30% loss is found after sterilisation. UHT milk stored for three months can lose over 50% of its folic acid. The extra heat treatment involved in boiling pasteurised milk can decrease the folic acid content by 20%. Losses of around 10% are found in boiled eggs, while other forms of cooking (fried, poached, scrambled) give between 30 and 35% loss. Total folic acid losses from vegetables as a result of heating and cooking processes can be very high. A study carried out on the stability of folic acid in spinach during processing and storage showed major differences between water blanching and steam blanching, with a folate retention of 58% with steam blanching and only 17% with water. Frozen spinach was found to retain 72% folate after 3 months storage.12 10.5.6 Vitamin B6 (pyridoxine) Vitamin B6 activity is shown by three compounds, pyridoxol, pyridoxal and pyridoxamine and these are often considered together as pyridoxine. Vitamin B6 is found in red meat, liver, cod roe and liver, milk and green vegetables. The commercial form normally used for food fortification is the salt, pyridoxine hydrochloride. Pyridoxine is normally stable to atmospheric oxygen and heat. Decomposition is catalysed by metal ions. Pyridoxine is sensitive to light, particularly in neutral and alkaline solutions. One of the main causes of loss of this vitamin in milk is sunlight with a 21% loss being reported after 8 hours exposure.7 Pyridoxine is stable in milk during pasteurisation but about 20% can be lost during sterilisa- 256 The nutrition handbook for food processors