維生素 Vitamins 1 Vitamins comprise a diverse group of organic compounds that are nutritionally essential micronutrients. Vitamins function in vivo in several ways, including: a) as coenzymes or their precursors (niacin, thiamin, riboflavin, biotin, pantothenic acid, vit B6, vit B12 and folate). b) as components of the antioxidative defense system (ascorbic acid, certain carotenoids and vit E). c) as factors involved in genetic regulation (vit A, D and potentially several others). d) in specialized functions such as vit A in vision, ascorbate in various hydroxylation reactions, and vit K in specific carboxylation reactions. 2
Since vitamins are quantitative minor constituents in foods, it is interested in maximizing vit retention by minimizing aqueous extraction (leaching) and chemical changes such as oxidation and reaction with other food constituents. Vitamins also influence the chemical nature of food, by functioning as reducing agents, radical scavengers, reactants in browning reactions, and flavor precursors. Potential toxicity of vitamins A, D and B6 occurs as over- consumption of nutritional supplements. 3 4
Addition of nutrient to foods: Restoration: addition to restore the original concentration of key nutrients. Fortification: addition of nutrients in amounts significant enough to render the food a good to superior source of the added nutrients. Enrichment: addition of specific amounts of selected nutrients in accordance with standard or identity by US FDA. Nutrification: a general term to include any addition of nutrients to food. 5 6
Dietary recommendation: 7 8
Bioavailability of vitamins: Factors affecting the bioavailability of vitamins include: a) composition of the diet. => influence intestinal transit time. b) form of the vitamin. => differ in rate or extent of absorption, ease of conversion to metabolically active form, or metabolic functionality. c) interactions between a vitamin and components of the diet (e.g., proteins, starches, dietary fiber, lipids). => interfere with intestinal absorption of the vitamin. 9 General causes of variation/losses of vitamins in foods: Beginning at the time of harvesting, all foods inevitably undergo some loss of vitamins. => Most processing, storage and handling methods are intended to minimize vit. losses. High-temperature, short-time treatments improve retention of labile nutrients during blanching and thermal processes. * Fat-soluble vitamins: - Vitamin A: retinol and related compounds and carotenoids. Vit A activity is in the form of retinol or its esters, retinal and to a lesser extent, retinoic acid. The concentration of vit A is greatest in liver. 10
11 Vit A degradation parallels the oxidative degradation of unsaturated lipids. Factors promoting oxidation of unsaturated lipids enhance vit A degradation by direct oxidation or indirect effects of free radicals. b-carotene and other carotenoids have the ability to act as antioxidants under conditions of reduced O concentration (<150 torr O 2 ), although they may act as pro-oxidants at higher O concentration. b-carotene act as an antioxidant by scavenging singlet O, OH, and superoxide radicals and by reacting with peroxy radicals (ROO.). 12
13 Vitamin D: vit D 2 (ergocalciferol) and D 3 (cholecalciferol) Cholecalciferol forms in animal tissues upon exposure to sunlight, and it is a multiple process involving photochemical modification of 7-hydroxycholesterol followed by nonenzymatic isomerization. Ergocalciferol is formed by commercial irradiation of photosterol (ergosterol in fungi including mushrooms) with UV light. 14
1,25-dihydroxy derivatives of cholecalciferol is the main physiologically active form, involved in the regulation of calcium absorption and metabolism. 25-Hydroxy cholecalciferol is also a significant amount of naturally occurring vit D activity in meat and milk products. Fortification of most fluid milk products with vit D 2 and D 3 makes a significant contribution to dietary needs. - Vitamin E: tocopherols and tocotrienols. 15 16
Tocopherols and tocotrienols are very nonpolar and present in the lipid phase of foods, when not esterified, have the ability to act as antioxidants. => quenching free radicals by donating the phenolic H and an electron. Tocopherols are a natural constituent of all biological membranes and are though to contribute membrane stability through their antioxidant activity. a-tocopheryl acetate has no antioxidant activity and does exhibit vit E activity and in vivo antioxidant effect as a result of enzymatic cleavage of the ester. Tocopherols also contribute the stability of highly unsaturated vegetable oils. 17 Biological potency: α>β>γ>δ; antioxidant potency: δ> γ>β> α 18
Vitamin K: naphthoquinones. Menadione: unsubstituted form. Phylloquinones (vit K 1 ) is a product of plant origin. Menaquinones (vit K 2 ) of varying chain length are products of bacterial synthesis, by intestinal microflora. Vit K deficiency is rare and associated with malabsorption syndromes or the use of pharmacological anticoagulants. 19 20
Water-soluble vitamins: - Ascorbic acid: vit C L-Ascorbic acid is acidic, highly polar and water-soluble, and reducing properties are contributed by 2,3-enediol moiety. L-Ascorbic acid (AA) and L-dehydroascorbic acid (DHAA) exhibit vitamin C activity, whereas L-isoascorbic acid and D-AA have no vit C activity (no nutritional value). 21 L-Isoascorbic acid and D,L-AA are used as food ingredients for their reducing and antioxidant activity. 22
Fatty acid esters such as ascorbyl palmitate and AA acetals are lipid soluble and provide a direct antioxidant effect in lipid environments. Functions of ascorbic acid in foods: a) as an essential nutrient, b) as a food ingredient/additive due to its reducing and antioxidant properties, c) to inhibit enzymatic browning by reducing o-quinone products, d) reductive action in dough conditioners, e) protection of certain oxidizable compounds (e.g., folates) by reductive effects, free radical and O scavenging, f) inhibition of nitrosamine formation in cured meats, and g) reduction of metal ions. 23 Antioxidant role of ascorbic acid: a) scavenging singlet O, b) reduction of O- and C-centered radicals, c) preferential oxidation of ascorbate and d) regeneration of other antioxidants, such as through reduction of tocopherol radical. - Thiamin: A substituted pyrimidine linked through a methylene bridge (-CH 2 -) to a substituted thiazole. 24
Losses of thiamin from foods are favored when a) conditions favor leaching of the vitamin into surrounding aqueous media, b) ph is neutral or greater and/or c) exposure to a sulfating agent occurs. 25 26
Thiamin exhibits excellent stability under conditions of low water activity at ambient temperature. 27 Riboflavin: vit B 2, 7,8-dimethyl-10(1 -ribityl)isoalloxazine 28
All derivatives of riboflavin are given as flavins. Phosphorylation of 5 -position of ribityl side chain yields flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD) has an additional 5 -adenosyl monophosphate moiety. FMN and FAD function as coenzymes in various oxidation-reduction processes. 29 Riboflavin as the free vit and during coenzymatic function undergoes redox cycling among three chemical species. 30
Riboflavin exhibits its greatest stability in acidic ph, is less stable at neutral ph and rapidly degrades in alkaline ph. Rates of degradation increase measurably at aw above the monolayer value when temperatures are above ambient. - Niacin: pyridine 3-carboxylic acid (nicotinic acid) and derivatives (nicotinamide, pyridine 3-carboxamide). 31 Coenzyme forms of niacin: nicotinamide adenine dinucleotide phosphate (NADP) and (NAD). Heat, especially under acid and alkaline conditions, converts amide to acid form without loss of vit activity. Niacin loss occurs by leaching during washing, blanching, and processing/preparation, and by exudation of fluids. Niacin is not affected by light, and no thermal losses occur under conditions relevant to food processing. Niacin is abundant in foods, and high protein diet can provide tryptophan for metabolic conversion to nicotinamide. 32
Vitamin B 6 : group of 2-methyl-3-hydroxy-5-hydroxymethyl pyridines containing vit activity of pyridoxine 33 Three basic forms: Pyridoxine (PN, pyridoxol), pyridoxal (PL) and pyridoxamine (PM). Vit B 6 (PLP, less PMP) functions as a coenzyme in over 100 enzymatic reactions involving in the metabolism of amino acids, carbohydrates, neurotransmitters and lipids. All vit B6 compounds are susceptible to lightinduced degradation. => causing losses during food processing, preparation and storage and during analysis. Vit B 6 is measured by microbiological assay or HPLC. 34
Folate: folic acid (pteroyl-l-glutamic acid) or folacin. Natural forms: folates, 5,6,7,8-tetrahydrofolates (H4 folates) and 7,8-dihydroxyfolates (H2 folates). 35 Folate analogues with a 4-amino group or other modification frequently are potent antagonists used in chemotherapy for cancer and autoimmune disease. All folates are subject to oxidative degradation. => Reducing agents such as ascorbic acid and thiols exert multiple protective effects on folates. Folates are among the most limiting of vitamins. => due to: a) poor diet selection, folates-rich foods (citrus, green leafy vegetables and organ meats), b) losses during food processing and/or home preparation by oxidation, leaching or both, and c) incomplete bioavailability in many human foods. 36
Folates are measured by microbiological growth method, HPLC and competitive-binding radioassay procedures. - Biotin: a bicyclic, water-soluble vitamin functions as an coenzyme in carboxylation and transcarboxylation reactions. 37 Biotin is very stable to heat, light and oxygen and quite well retained in foods. Absorption of biotin is prevented by consumption of raw egg albumin, which contains biotin-binding protein avidin. 38
- Pantothenic acid: D-N-(2,4-dihydroxy-3,3-dimethylbutyryl)-b-alanine Pantothentic acid is most stable at ph 5-7 in solution, and exhibits good stability during food storage, especially at reduced water activity. Leaching of pantothentic acid or its loss in tissue fluids is very significant. 39 Vitamin B 12 : cyanocobalamin Vitamin B 12 are produced only by microbial biosynthesis and in animal tissues consists mainly of the coenzyme forms, methylcobalamin and 5 -deoxyadenosylcobalamin. 40
Under most conditions of food processing, preservation and storage, loss of vit B 12 is little nutritionally significant. Presence of ascorbic acid accelerates degradation of vit B 12. => foods containing vit B 12 do not contain significant amounts of ascorbic acid. 41 Conditionally essential vitamin-like compounds: - Choline: as free compound and a constituent of phosphatidylcholine, sphingomyelin and acetylcholine. 42
Carnitine: synthesized by human body and found in food of animal origin. Carnitine functions in transportation of organic acid across biological membranes and facilitates their metabolic utilization and/or disposal. Pyrroloquinone quinone: synthesized by intestinal bacteria. 43 Optimization of vitamin retention: Inevitable losses of nutritional value occur during the course of the postharvest handling, cooking, processing and storage of foods. Losses of nutrients frequently occur during thermal processing procedures intended to provide a shelf-stable product. Such losses often involve both chemical degradation and leaching. 44
Examples: The use of acidulants, if compatible with the product, would promote the stability of thiamin and ascorbic acid. Reduction in ph would decrease the stability of certain folates. Cooking or commercial processing under conditions that minimize exposure to oxygen and excess liquid lessens the oxidation of many vitamins and the extraction (i.e., leaching) of vitamins and minerals. 45 High-temperature, short-time (HTST) conditions will cause less vitamin degradation than will conventional thermal processes of equal thermal severity (based on microbial inactivation). Certain combination of ingredients can enhance retention of several nutrients, e.g., the presence of natural antioxidants would favor retention of many vitamins. 46