FOOD CHEMISTRY 3 FCHE30

1
FOOD CHEMISTRY 3FCHE30
Faculty of ScienceDept. of Horticulture and Food
Technology

• Semester 2
• Module 5
• Effect of Heat and pH on Proteins

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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

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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

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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

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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

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Amino Acids

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

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Amino Acids

• Aromatic amino acids
• Phenylalanine
• Tyrosine
• Tryptophan

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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

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Protein Classification

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

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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

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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

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Protein Structures

• 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

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Protein Structures

• 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

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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

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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

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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

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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

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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

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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

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Protein Quality

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

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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

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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

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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

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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

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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

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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

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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

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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

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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.

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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

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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

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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

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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

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Influence of Heat on Protein

• Table PER (protein efficiency ratio) of
  Bread and Toast

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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

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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

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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

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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 115C
• A lactone ring is formed between a terminal
  carboxyl group and hydroxel amino acids

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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

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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

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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

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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 153C
• 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 110C to 115C
• Wet-heated samples showed a sigmoidal curve
  (minimum of 120C) with the solubility plot over
  the same temperature range
• The solubility then decreased sharply after 145C
• the 115C sample being nearly as insoluble as the
  dry-heated protein

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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 ?-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 ?-casien and ß-lactoglobulin

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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

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Influence of Heat on Protein

• Thermal decomposition
• Several amino acids has been studied
• Free radicals are formed in protein or Lysine
  heated at 200C 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 300C

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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

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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

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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

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Photo-oxidation of Proteins

• Two of the potent photosensitizers in foods are
  riboflavin and chlorophyll
• Generally, two classes of reactions are initiated
  when a photosensitizer, oxygen, and a protein are
  present and light of the proper wavelength
  activates the system
• FIRST TYPE
• the photosensitezer is excited to the triplet
  state by light of the proper wavelength
• The excited sensitizer then univalently oxidazes
  an amino acid side chain of the protein
• initiating a free radical destruction of the
  amino acid

52
Photo-oxidation of Proteins

• SECOND TYPE
• The excited photosensitizer excites a
  ground-state oxygen which forms a highly reactive
  singlet oxygen
• The singlet oxygen can the attack lipids or
  reactive side chains of amino acids

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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

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Interaction of Protein with Lipids

• Considerable conversion of methionine to
  methionine sulfoxides when methionine is exposed
  to oxidizing lipid
• Loss of methionine in casein which was exposed to
  autoxidizing methyl linoleate in a model system
• Observed considerable browning takes place in the
  model system as the reaction continues
• Malonaldehyde, an intermediate in the
  decomposition of lipids, has been shown to react
  with sulfur amino acids, with enamine and imine
  linkages being proposed in the products
• When malonaldehyde reacts with collagen, lysine
  and tyrosine are the principal amino acids damaged

55
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

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Interaction of Protein with Lipids

• Chlorine
• Another environmental oxidizer which can damage
  protein quality
• The initial side of attack of the chlorine is the
  sulfur of methionine
• First intermediate formed is chlorosulfonium
• Second step is the formation of a carbonium ion
  intermediate and cleavage of the carbon sulfur
  bond
• The splitting yields a trichloroamino acid
  porduct
• Nutritional impact not likely to be significant,
  since foods produced from chlorinated flour are
  not generally consumed as sole sources of protein
• The loss of small amounts of methionine would not
  be significant

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Influence of alkaline conditions

• Proteins are frequently exposed to a high pH
  environment
• Even brief exposure of protein to an extreme in
  pH can result in significant changes in protein
• Alkaline treatment advantages
• Improving solubility of protein
• Destroying toxins
• Improving flavor or texture
• Undesirable aspect of alkaline
• Particularly at high temperatures
• Racemization
• Cross-links such as isopeptides or lysinoalanine

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Influence of alkaline conditions

• Racemization
• Observed in severely alkaline-treated proteins
• This reaction occurs via removal of the a-methine
  hydrogen
• Forming a carbanion intermediate
• Carbanion then reacts rapidly with proton with an
  equal likelihood of readdition of the proton
• Forming either the normal L form or the D form of
  amino acid residue
• Protein sequence may have some mediating
  influence on this at moderately alkaline pH, but
  as pH increases, the randomness of the protein
  structure would also be expected to increase
• The rate of racemization is proportional to the
  hydroxide ion concentration above pH 8.0
• Below pH 8.0 racemization is dependent on
  electron withdrawing ability or the amino acid
  side chain

59
Influence of alkaline conditions

• In addition to high pH, increases in length of
  alkaline treatment and temperature increases
  racemization
• Heat at slightly acid pH can induce significant
  racemization
• Biological effects of racemization
• Decreases in the vitro digestibility of
  alkaline-treated proteins
• Alkaline-treated casein was much more resistant
  to enzymatic hydrolysis than untreated casein

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