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  1. Nutrient • The nutritients account for more than 99.9% of the food. The main classes of nutrients are : • carbohydrates, proteins, fats, vitamins, and minerals, • and all of them may pose toxicological risks to the consumer.

  2. Most of the food is treated in some way to improve its shelf life, texture, palatability or appearance. It would be difficult to change this situation. So, it is important to know what happens to the various food components on the way from raw material to consumer. From raw materials to consumer: Chemical, microbiological and technological aspects of food

  3. Nutrients Macronutrients 1 Fats Changes in dietary fats during storage and processing of raw materials, and during manufacturing, preparation and storage of food Rancidity Oxidation of fats and oils, and adverse health consequences

  4. Carbohydrates Changes in dietary carbohydrates during manufacturing and storage of food Proteins Changes in proteins during processing of raw materials, and during manufacturing, preparation and storage of food Pyrolysis products occurring in food

  5. Lipid oxidation • Lipid oxidation results in the production of off-flavours and odours, which is referred to as oxidative rancidity. • Other effects of lipid oxidation include a decrease in nutritive value, colour changes, and sometimes the production of toxic compounds.

  6. Effects of Lipid Oxidation • Rancidity formation • Loss of essential fatty acids • Loss of fat soluble vitamins • Possible toxic compounds formation

  7. Lipid Oxidation • Initiation: RH + O2 -->R· + ·OH R· + O2 --> · + ROO· • Propagation: ROO· + RH --> R· + ROOH ROOH--> RO· + HO· • Termination: R· + R· --> RR R· + ROO·--> ROOR ROO· + ROO· --> ROOR + O2

  8. Lipid Oxidation • Where RH is any unsaturated fatty acid; • R· is a free radicalformed by removing a labile hydrogen from a carbon atom adjacent to a double bond; • ROOH is a hydroperoxide, one of the major initial oxidation products that decompose to form compounds responsible for off-flavors and odors. • Such secondary products include hexanal, pentanal, and malonaldehyde.

  9. oleic acid as an example, a hydrogen could be removed from either C-8 or C-10, as these positions are located alpha to the double bond. Lipid Oxidation of a Monoenoic Acid

  10. Lipid Oxidation of a Monoenoic Acid • Using oleic acid as an example, a hydrogen could be removed from either C-8 or C-10, as these positions are located alpha to the double bond. Abstraction from carbon 8 results in the two radicals A and B which are positional isomers of each other stabilized by resonance

  11. Lipid Oxidation of a Monoenoic Acid • Oxygen can be added to each radical to form peroxy radicals at C-8, C-9, C-10 or C-11. Addition to the 8 and 10 positions yield the peroxy radicals shown above

  12. Lipid Oxidation of a Monoenoic Acid • The addition oxygen at the 11 and 9 positions results in the peroxy radicals

  13. Lipid Oxidation of a Monoenoic Acid • These radicals may abstract hydrogens from other molecules to yield the hydroperoxides shown

  14. Lipid Oxidation • This reaction scheme is capable of generating aldehydes, ketones, alcohols and hydrocarbons. Many of the volatile compounds formed during lipid oxidation originate through similar dismutations.

  15. Mechanisms of Chemical-induced Toxicity • Direct effects • Effects on the cytoskeleton, resulting in plasma membrane permeability changes • Effects upon mitochondrial membranes and enzyme

  16. Covalent Binding Theory of Chemical Toxicity • Covalent binding of reactive metabolite to critical cellular nucleophiles (protein SH, NH, OH groups) • Inactivation of critical cell function (e.g., ion homeostasis) • Cell death

  17. Chemical Disruption of Ca++ Homeostasis • Release from mitochondria quinones, hydroperoxides, Fe+2, Cd+2 • Release from endoplasmic reticulum • CCl4, bromobenzene, quinones hydroperoxides, aldehydes

  18. Protein Quality The Essential Amino Acids • Histidine • Isoleucine • Leucine • Lysine • Methionine (and/or cysteine) • Phenylalanine (and/or tyrosine) • Threonine • Tryptophan • Valine

  19. Environmental Effects on Protein Quality • The environment can exert profound changes on the functionality and nutritional quality of the protein • Degradative reactions can result from the processing or storage environment which can cause undesirable changes in proteins • As a result of these reactions protein can exhibit: • Losses in functionality • Losses in nutritional quality • Increased risk of toxicity • Desirable and undesirable flavor changes

  20. Environmental Effects on Protein Quality • Environment changes that can adversely affect proteins include • Heat in the presence and absence of carbohidrate • Exposure to oxidative conditions • Caused by light and • Caused by oxidizing lipids • Nutrients are destroyed when foods are processed, largely because they are • Sensitive to pH of solvent • Sensitive to oxygen, light, heat or combinations of these

  21. Environmental Effects on Protein Quality • Influences of Processing on Proteins • Most commercial processes such as dehydration, canning, baking and domestic cooking have only small effects on nutritional quality of proteins • There are the exceptions, for example, conditions where foods are exposed to • Very high pH • Extreme heat • Peroxidizing lipids

  22. Environmental Effects on Protein Quality • In Acid Solutions, Tryptophan is rapidly destroyed, and Serine and Threonine are slowly destroyed • Ultraviolet light destroys Tryptophan, Tyrosine and Phenylalanine • Sulfur amino acids are damaged by reaction products from lipid oxidation or by the addition of bleaching or oxidizing agents • All amino acids, especially Lysine, Threonine, and Methionine, are sensitive to dry heat, browning, and radiations

  23. Influence of Heat on Protein • The Thermally related changes in proteins can be broken into four basic catagories

  24. Influence of Heat on Protein • (1) Alteration in the tertiary structure of the protein • Requires only mild heating • Exerts no nutritional effect • Tertiary changes can have significant influence on functionality • Ex. Loss in Solubility • If the protein is an Enzyme, changes in tertiary structure will reduce or eliminate enzymatic activity

  25. Influence of Heat on Protein • (2) Non-enzymatic browning / Maillard reaction • This reaction occurs primarily between the ∈-amino group of Lysine and a carbohydrate • The lysine after the very earliest stages of the reaction, becomes unavailable • Therefore, with the resulting Maillard reaction product bound to the protein • The solubility of the protein changes • Colour will change as the melanoidin pigments are formed

  26. Influence of Heat on Protein • The Maillard reaction occurs during both • Storage • Heat treatment • The reaction is slow at room temperature and increases with temperature • The loss of the essential amino acid Lysine serves as the best single indicator of damage to the protein from the browning reaction

  27. Influence of Heat on Protein • The pH can also influence the browning reaction of protein • Acidification inhibits the browning reaction • Raising the pH above 7.0 greatly enhances browning • The Maillard reaction increases approximately linearly from pH 3 to 8.0 • This is also the region where most foods are subject to heat treatment

  28. Influence of Heat on Protein • A major environmental factor which influences the extent of browning in proteins is the Water Content of the System • Anhydrous protein is fairly stable to heat and storage in the presence of carbohydrate • At water activities of 0.4 – 0.7 the browning reaction proceeds rapidly • The reaction then slows as the protein is diluted • Liquid milk, therefore, is more stable to heating effects than powdered milk with residual moisture

  29. Influence of Heat on Protein It is clear that heating and/or storage of protein in the presence of reducing sugars and limited water is an environment that will facilitate rapid degradation of the protein, particularly the ∊–amino group

  30. Influence of Heat on Protein • (3) More severe heat treatment • Particularly lysine and cystine are sensitive to this type of thermal decomposition. • Lysine and Arginine side chains react with the free acids of glutamic and aspartic acid or with the amides to yield isopeptide cross-links which can impede digestion and exibit major effects on functionality • Cystine is relatively sensitive and is converted to dimethyl sulfide as well as other products at temperatures of 115°C • A lactone ring is formed between a terminal carboxyl group and hydroxel amino acids

  31. Influence of Heat on Protein • (4) Heat damage on the outside surface of roasted foods • The result of roasting is racemization of amino acid residues in the protein • Or in the case of extensively heated material, complete destruction of the amino acids • Temperatures of 180 – 300C • Such as occur in roasted coffee, meat, fish and in the baking of some biscuits • These reactions also account for some of the flavor and colour developed as a result of the roasting process

  32. Influence of Heat on Protein Solubility • During solubility changes the protein goes through stages and some changes are observable • Interactions with different functional groups become more prevalent as the protein unfolds • Sulfhydryl • Dislufide • Tyrosyl • These changes lead to a diverse and complicated series of reactions that ultimately lead to the precipitation of the protein • The interaction of water along with heat causes various ionic and polar groups in the protein to exert considerable influence on the folded conformations of the proteins

  33. Influence of Heat on Protein • Thermal decomposition • Several amino acids has been studied • Free radicals are formed in protein or Lysine heated at 200°C for 22 minutes • It is of concern that these free radicals appear to be stable in water and in digestive juices • An aspect of thermal decomposition that must be considered is the possible formation of toxic products • Mutagenic activity on flame-broiled fish and beef • Several mutagens are of protein and amino acid origin • Two of the most toxic mutagens are derived from tryptophan • It is important to note that these compounds are only formed at temperatures in excess of 300°C

  34. Photo-oxidation of Proteins • Photo-chemical reactions • Amino acid side chains that are readily modified by photo-oxidation are • Sulhydryl • Imidazole • Phenoxyindole • Thiol ether • Data indicated that there are losses in the oxidizable amino acids, but that aspartic acid and valine are stable to photo-oxidation

  35. Interaction of Protein with Lipids • Maximum interaction or degradation of the protein takes place when the lipid oxidation is at the stage of maximum peroxide formation • Losses in available lysine appeared to take place in the initial induction period and during the induction of peroxides • Oxidizing lipids or peroxides in the environment of the protein clearly cause significant change in the protein • Oxidations and cross-links generated tend to adversely affect • Solubility • Enzyme activity • Nutritive quality

  36. Interaction of Protein with Lipids • Lipid hydroperoxides cause a number of interesting reactions with various reactive amino acid residues in protein • These various reactions all help account for the polymerization of proteins • Lipid peroxidation free radicals serve as initiators of the polymerization • Substantial losses in amino acids when proteins were exposed to peroxidizing lipids • Methionine, histidine, cystine and lysine were the most vulnerable to damage • Losses in digestibility and biological value of the proteins after oxidation

  37. BP = Benzo[a] pyrene A polyclycic aromatic hydrocarbon

  38. DNA repair after binding of mutagen, e.g. a polycyclic hydrocarbon

  39. Advanced Glycation Endproducts (AGE) • When proteins are exposed to elevated levels of glucose the following series of non-enzymatic chemical reactions occur • Glycation of proteins and formation of both Amadori adducts and AGE compounds can have biological consequences Glycation Rearrangement Cross-linking hours days weeks/months Glucose Glycated Amadori + proteins Adducts AGES Protein-NH2 (Schiff Base) Cell Activation Tissue Structural Changes

  40. INTOXICATION MECHANISMS I Genotoxic compounds. Compounds that bind to DNA (adducts) may cause mutations when repair mechanisms fail PAHs, vinyl chloride, aflatoxin after activation by MFO Neurotoxic compounds

  41. MITOCHONDRIAL TOXICITY Dissipation of proton motive force: uncoupling by weak acids Inhibition of enzyme complexes: cyanide, rotenone, cadmium

  42. Consequences of Disruption of Ca++ Homeostasis • Activation of proteases • Degradation of cytoskeletal and membrane proteins • Activation of endonucleases • DNA fragmentation, cell death • Possible mechanism of mutation induction by cytotoxic agents