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FOOD CHEMISTRY 3 FCHE30

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  1. FOOD CHEMISTRY 3FCHE30 Faculty of ScienceDept. of Horticulture and Food Technology Semester 2 Module 5 Effect of Heat and pH on Proteins

  2. Protein • Protein Occurrence • Polymers of some 20 different amino acids • Joined together by peptide bonds • Different proteins have different chemical properties • Because of widely different secondary and tertiary structures • Amino Acids • Grouped on the basis of the chemical nature of the side chains • Side chains my be polar or non-polar • High levels of polar amino acid residues in a protein increase water solubility

  3. Amino Acids • Amino acids are the building blocks (monomers) of proteins. 20 different amino acids are used to synthesize proteins. The shape and other properties of each protein is dictated by the precise sequence of amino acids in it. • Each amino acid consists of an alpha carbon atom to which is attached • a hydrogen atom • an amino group (hence "amino" acid) • a carboxyl group (-COOH). This gives up a proton and is thus an acid (hence amino "acid") • one of 20 different "R" groups. It is the structure of the R group that determines which of the 20 it is and its special properties.

  4. The amino acid shown here is Alanine.

  5. Amino Acids • Most polar side chains are those of the basic amino acidic amino acids • Present at high levels in soluble albumins and globulins • Wheat proteins, gliadin and glutenin, have low levels of polar side chains and are quite insoluble in water • Acidic amino acids may also be present in proteins in the form of their amides, glutamine and asparagine • This increases the nitrogen content of the protein • Hydroxyl groups in the side chains may become involved in ester linkages with phosphoric acid and phosphates • Sulfur amino acids may form disulfide cross-links between neighbouring petide chains or between different parts of the same chain

  6. Amino Acids • Joined together by peptide bonds: Form the primary structure of proteins • The amino acid composition established the nature of secondary and tertiary structures • These influence the functional properties of food proteins and their behavior during processing • 20 Amino acids : Only about half essential for human nutrition • Amount of essential amino acids present in a protein and their availability determine the nutritional quality of the protein • Animal proteins are higher quality than plant proteins • Egg protein • One of the best quality proteins • Biological value of 100 • Widely used as a standard, Protein Efficiency Ratio (PER) sometimes use egg white as a standard

  7. Amino Acids • When hydrolyzed by strong mineral acids or with the aid of certain enzymes, proteins can be completely decomposed into their component amino acids • Simplest amino acid: Glycine, The “R”-group is H (Hydrogen) • Aliphatic monoamino monocarboxylic amino acids • Glycine • Alanine • Valine • Leucine • Isoleucine • Serine • Threonine • Proline

  8. Amino Acids • Sulfur-containing amino acids • Cysteine • Cystine • Methionine • Monoamino dicarboxylic amino acids • Aspartic acid • Glutamic acid • Basic amino acids • Lysine • Arginine • Histidine

  9. Amino Acids • Aromatic amino acids • Phenylalanine • Tyrosine • Tryptophan

  10. Protein Classification • Classification of Proteins • Based mostly on the solubility of proteins in different solvents • More recent criteria being used includes: • Behaviour in the centrifuge • Electrophoretic propterties • Proteins are divided into the following main groups • Simple Proteins • Conjugated Proteins • Derived Proteins

  11. Protein Classification • Simple Proteins • Yield only amino acids on hydrolysis and include the following classes • Albumins • Globulins • Glutelins • Prolamins • Sclereproteins • Histones • Protamines

  12. Protein Classification • Conjugated Proteins • Contain an amino acid part combined with a non-protein material such as a lipid, nucleic acid, or carbohydrate • Some of the major conjugated proteins are as follows: • Phosphoproteins • Lipoproteins • Nucleoproteins • Glycoproteins • Chromoproteins

  13. Protein Classification • Derived Proteins • These are compounds obtained by chemical or enzymatic methods and are divided into primary and secondary derivatives • Primary derivatives • Slightly modified and are insoluble in water • Ex. Rennet-coagulated casein • Secondary derivatives • Changed more extensively, include proteoses, peptones, and petides • Difference between these breakdown products is in size and solubility • All are soluble in water • Not coagulated by heat • Proteoses can be precipitated with saturated ammonium sulfate solution • Peptides contain two or more amino acid residues

  14. ProteinStructures • Proteins are macromolecules with different levels of structural organization • Primary Structure • the peptide bonds between component amino acids • and also the amino acid sequences in the molecule • Secondary Structure • Involves folding the primary structure • Hydrogen bonds between amide nitrogen and carbonyl oxygen are the major stabilizing force • Bonds may be formed between different areas of the same polypeptide chain, or adjacent chains • The secondary structure may be either a-helix or sheet • Helical structures are stabilized by intramolecular hydrogen bonds • Sheet structures are stabilized by intermolecular hydrogen bonds

  15. ProteinStructures • Tertiary Structure • Involves a pattern of folding of the chains into a compact unit • Stabilized by • hydrogen bonds • van der Waals forces • disulfide bridges • hydrophobic interactions • This structure results in the formation of a tightly packed unit with most polar amino acid residues located on the outside and hydrated • Internal part with most of the apolar side chains and virtually no hydration • Large molecules of molecular weights above about 50 000 may form quaternary structures by association of subunits • These structures my be stabilized by hydrogen bonds, disulfide bridges and hydrophobic interactions

  16. Denaturation • The denaturation process • Changes the molecular structure without breaking any peptide bonds of a protein • This process is peculiar to proteins and affects different proteins to different degrees, depending on the structure of a protein • Denaturation can be brought about by a variety of agents • Heat (most important), causes the destruction of enzyme acticvity • pH • Salts • Surface effets • Denaturation usually involves loss of biological activity and significant changes in some physical or functional properties, such as solubility • Usually non-reversible

  17. Denaturation • Heat denaturation is sometimes desirable • The denaturation of whey proteins for the production of milk powder used in baking • Proteins of egg white are readily denatured by heat and by surface forces when egg white is whipped to a foam • Meat proteins are denatured in the temperature range 57 to 75C, which has a profound effect on texture, water holding capacity and shrinkage • Denaturation may result in flocculation of globular proteins and may lead to the formation of gels • Protein denaturation and coagulation are aspects of heat stability that can be related to the amino acid composition and sequence of the protein

  18. Denaturation • DEFENITION: Denaturation is a major change in the native structure that does not involve alteration of the amino acid sequence • Effect of heat usually involves a change in the tertiary structure, leading to a less ordered arrangement of the polypeptide chains • The temperature range in which denaturation and coagulation of most proteins takes place is about 55 to 75C • Casein and gelatin are examples of proteins that can be boiled without apparent change in stability

  19. Denaturation • The exceptional Stability of Casein • Makes it possible to boil, sterilize, and concentrate milk, without coagulation • In the first place • restricted formation of disulfide bonds due to low content of cystine and cysteine results in increased stability • Casein with its extremely low content of sulfur amino acids are less likely to become involved in the type of sulfhydryl agglomeration • The heat stability of casein is also explained by the restraints against forming a folded tertiary structure • These restraints are due to the relatively high content of proline and hydroxyproline in the heat stable proteins

  20. Protein Quality • What is Protein Quality? • Proteins with a relatively high content of essential amino acids are called first class proteins or high quality proteins • This type of proteins are quite expensive to produce • Ex. Red meat, white meat, dairy products, eggs and a few legumes (peas and soya beans) • Animal proteins usually contain much more of the essential amino acids than do plant proteins • Intermediate quality proteins are those derived from plant material • Potatoes, rice, wheat etc. • Poor quality proteins are derived from millet, sorghum, cassava and other roots and tubers

  21. Protein Quality • Protein poor diet supplies limited amino acids to the consumer • Many processes actually leads to a further decline in the protein quality of food products • These effects need to be monitored and, where necessary, controlled to ensure a safe and nutritious food supply to the population

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

  23. 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

  24. Environmental Effects on Protein Quality • Environment changes that can adversely affect proteins include • Heat in the presence and absence of carbohydrate • Extremes in pH (particularly alkaline) • 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 • The amino acid composition of food protein is of fundamental importance in determining nutritional quality and functionality

  25. Environmental Effects on Protein Quality • Influences of Processing on Proteins • Different proteins and food systems have very different susceptibilities to damage resulting from processing • In order to obtain measurable responses, experimental conditions are frequently much harsher than those to which a food might be exposed during commercial processing • 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

  26. Environmental Effects on Protein Quality • Physical and Chemical environments that a protein is exposed to during processing can result in wide variety of changes • Changes in amino acid side chains • Amino acid razemize and develop new cross-links in alkaline solution • Losses in nutritional quality • Significant changes in functionality • Arginine, Cystine, Threonine, Serine, and Cysteine are destroyed • Glutamine and Asparagine are deaminated under alkaline conditions • In Acid Solutions, Tryptophan is rapidly destroyed, and Serine and Threonine are slowly destroyed • Ultraviolet light destroys Tryptophan, Tyrosine and Phenylalanine

  27. Environmental Effects on Protein Quality • 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

  28. Influence of Heat on Protein • Susceptibility to heat damage varies among different protein sources • Susceptibility is increased in the presence of various carbohydrates and other food constituents • Nutritional value of proteins can be significantly affected even when there is little or no apparent significant difference in amino acid composition • The Thermally related changes in proteins can be broken into four basic catagories

  29. 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, it is likely, but not inevitable, that changes in tertiary structure will reduce or eliminate enzymatic activity • Thermal denaturation is of great significance in food technology because of the changes in the chemical and physical properties of the proteins

  30. Influence of Heat on Protein • Globular proteins will exhibit changes (generally losses) in • Solubillity • Viscosity • Osmotic properties • Electrophoretic mobilty • Immunosensitivity • Chemical sensitivity • The changes are due to new reactive side chains of the protein being exposed as a result of the increased random coil being introduced to the protein • Fibrillar proteins when heated will suffer changes in • Elasticity • Flexibility • Fibrillar length

  31. Influence of Heat on Protein • Thermal changes will alter the properties of the food, improving or destroying the functional properties • Enzyme inhibitors, such as Trypsin inhibitors, are denaturated • Avidin is denaturated by heat • Egg albumin becomes insoluble (but in a useful form for consumption) • Gluten is an example of a Protein that losses its dough-forming properties as a result of too much heat • Most of these changes, however, have no measurable effect on the nutritional quality of the proteins themselves.

  32. Influence of Heat on Protein • (2) Non-enzymatic browning / Maillard reaction • The reaction most of us think of first when considering damage to proteins during food processing • Has the most significance from the nutritional point of view • (The denaturation of protein may be more significant in terms of effect on protein functionality) • 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

  33. Influence of Heat on Protein • Keep in mind that while the browning reaction can account for substantial losses in nutritional quality of proteins, it is also critical to the development of flavor in foods • Environment of protein or food can have a substantial effect on the nature and extent of the observed browning • 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

  34. 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

  35. Influence of Heat on Protein • The browning of bread during the baking process is essential to the development of what we consider to be bread flavor • Lysine is the first limiting amino acid in wheat and therefore in bread • This limiting factor can be aggravated by the baking process • The next table and data represents breads baked at different temperatures, but clearly illustrate the loss in nutritive value as a result of intense heating

  36. Influence of Heat on Protein Table: PER (protein efficiency ratio) of Bread and Toast

  37. Influence of Heat on Protein • When looked at the influences of high-temperature short-time heating of pizza doughs on the amino acid profiles, the results show • Lysine, and to a lesser extent cystine, tyrosine, and threonine are lost in the crust after baking • The losses range from 7.1% in whole-wheat pizza crust to 19.4% in commercial pizza crust • It was proposed that the losses in nutritive value of pizza crust could be correlated with losses in lysine

  38. 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

  39. 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

  40. 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

  41. 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

  42. Influence of Heat on Protein • Solubility • One of the most easily observed thermal changes in protein is the change in conformation which affects solubility • Generally, protein solubility decreases with increases in the time and temperature of heat treatment • Thermal denaturation of protein occurs when hydrogen and other non-covalent bonds, such as ionic and van der Waals bonds within the protein, are disrupted by the heating process • Thus, the normal secondary, tertiary, and quaternary structure of the protein is disrupted, and the protein becomes “denatured”

  43. 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

  44. Influence of Heat on Protein Solubility • Moist heat frequently exerts more involved reactions in the protein than dry heat and is influenced greatly by pH and ionic stregth • Dry proteins denature at different rates, but minimum solubility was reached for most proteins by 153°C • The effects of moist heat were found more complex than the effects of dry heat • Proteins receiving dry heat exhibited a linear loss in solubility as function of increasing temperature from 110°C to 115°C • Wet-heated samples showed a sigmoidal curve (minimum of 120°C) with the solubility plot over the same temperature range • The solubility then decreased sharply after 145°C • the 115°C sample being nearly as insoluble as the dry-heated protein

  45. Influence of Heat on Protein • Coaggregration • Since food systems typically contain many different proteins, thermal processing can cause co-aggregation among proteins in the mixture • Such co-aggregation can be important in determining the characteristics of the food • The aggregations are also extremely difficult to study because they are chemically very stable • When ĸ-casien is heated in the presence of ß-lactoglobulin, a disulfide link is formed between the two proteins, which reduces the thermal denaturation of the normally stable ß-lactoglobulin • The functionality of milk powder for baking is significantly enhanced by heating the milk before drying to enhance the formation of the disulfide links between ĸ-casien and ß-lactoglobulin

  46. Influence of Heat on Protein • Digestibility and Nutritional value of heat-damaged Proteins • It has been known for some time that heat induces a number of changes in the physical properties of proteins and that such changes can influence the digestibility and hence nutritional value of proteins • The reduction in the nutritional quality of heated proteins is attributed to the isopeptide cross-links formed as a result of the heat treatment • Homogenates of the small intestine showed considerable activity toward the isopeptides • Glutamyllysine will pass the gut wall • The decreased protein digestibility reduces the apparent bio-availability of most of the amino acid residues in the protein • AS the intensity of heating increases, the level of isopeptides also increases • The severity of damage to the remainder of the protein increases with increasingly intense heat treatment

  47. 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

  48. 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

  49. Photo-oxidation of Proteins • Mechanisms of free radical initiation and subsequent damage to protein • Free radicals plays an important role in photolytic reactions both in direct dissociations and in photosensitized oxidations • Several factors influence the pathway and extent of free radical damage • The nature of the sensitizer • The nature of the substrate and redox potential of the substrate • Concentration of sensitizer and oxygen in the system • The nature of the medium (potential for diffusion) • The aliphatic amino acids, although they absorb light only to a small extent, can be damaged significantly by the radiation in the absence of a photosensitizer

  50. Photo-oxidation of Proteins • The damage results from short wavelengths • Glycine is not damaged at wavelengths above 2265 Å • The precise changes and pathways of destruction are influenced by • Irradiation wavelength • Irradiation dose • Reaction conditions • Individual amino acid being irradiated • The sulfur amino acids exhibit more measurable photo-decomposition than the aliphatic amino acids • The aromatic amino acids can act as photosensitizers in the protein, particularly sensitizing the sulfur amino acids