Foodchem Lecture 12 Pics

• Decarboxylation
• Decarboxylase is the most active at lower pH's
• Results in loss of a hydrophilic group, hence reducing solubility in water and overall molecular weight, while increasing volatility

• Deamination
• Deaminase is most active at higher pH's
• Results in loss of a hydrophilic group, hence reducing solubility in water and overall molecular weight, while increasing volatility

• Fish tissue contains TMAO which has no smell but which is readily converted to volatile TMA by the enzyme trimethyl amine oxidase
• TMA is largely responsible for the "fishy" smell associated with fish - fresh fish does not smell at all

• Hydrated protein
• Water binding capacity (WBC) is strongly associated with the ionic species present (NH₃+ and COO-) and hydrogen bonding sites (C=O, N-H) present in a protein that are readily hydrated

• Hydrated protein - in detail
• Water binding capacity (WBC) is strongly associated with the ionic species present (NH3+ and COO-) and hydrogen bonding sites (C=O, N-H) present in a protein that are readily hydrated

• A moisture sorption isotherm illustrates the equilibrium moisture associated with a material as a fucntion of relative humidity at a constant temperature
• Three stages (ideally):
• A) Bound water
• B) Water in looser form
• C) 'Free water'

• Center shows a protein
• Blue portion is the bound water (bound to the protein)
• Green portion is transitional water (associated with the bound water, but in a looser form
• Yellow portion is the free/bulk water (unstructured)

• The Maillard reaction - non enzymatic browning
• Involves the reaction of an aldehyde group of a reducing sugar and a free amino group of an amino acid or the free amino group on a peptide or protein, such as that from lysine or the amide group of asparagine
• The one pictured is the reaction with the free epsilon amino group of lysine

The sugar portion undergoes scission and breaks up into low molecular weight compounds (flavor/aroma) which readily polymerize into brown pigments (color) termed melanoidins

• Net result of Maillard reaction is that lysine will be 'bound'
• 'Bound' lysine causes steric hindrance and limits enzymatic digestion of the protein

• Cross linking reactions: lysine and asparagine
• (If reducing sugars are mainly absent, the cross linking of proteins can occur at higher temperatures by the formation of amide bonds with free COOH and NH₂ groups)

• Cross linking reactions: lysine and glutamic acid/aspartic acid
• (If reducing sugars are mainly absent, the cross linking of proteins can occur at higher temperatures by the formation of amide bonds with free COOH and NH2 groups)

• Hexapeptide
• The amide bond in cross linking reactions cannot be hydrolyzed by digestive proteolytic enzymes and if such a bond is formed intra or inter protein, it will leave a hexapeptide which cannnot be utilized

• Loss of cystine
• Cystine is one of the more heat sensitive amino acids and will readily hydrolyze and break down as follows
• This reaction can be significant if the limiting amino acid of the protein is cystine - also produces a typical 'cooked' flavor (milk, meat)

Cystine

• The formation of dehydroalanine from cystine
• Preparation of protein concentrates and isolates often calls for the treatment of proteins with alkali to solubilize them in the presence of heat to modify their properties
• Under such harsh conditions, 'new' amino acids can be formed if free amino acids are present or produced by base hydrolysis

• Reaction of dehydroalanine with lysine
• The unnatural amino acid lysinoalanine is poorly absorbed and may be toxic having been shown to cause renal failure in rats

• Pallets of casein powder
• One of the 2 categories of milk proteins
• Comes in 2 forms, isoelectric casein (acid casein) and paracasein

• A whey protein supplement
• One of the 2 categories of milk proteins
• Proteins left in solution after the casein has been precipitated out (enzymatically or via pH adjustment)
• Again a complex mixture of proteins

• aS1 casein
• Alpha fraction of casein is predomiant and can be further subdivided into alpha S1 and kappa casein
• 

• k Casein
• Alpha fraction of casein is predomiant and can be further subdivided into alpha S1 and kappa casein
• 

• Milk micelle
• Colloidal spherical protein particles
• 80-300 nanometers in diameter
• Made up of aS1 and k casein held together by Ca++ and inorganic phosphate

• Casein sub-micelle model
• 10-100 casein molecules (sub-micelles)

• Rennin attacks k casein at a specific site, cleaving off a hydrophilic glycomacropeptide
• Micelles are destabilized and the hydrophobic proteins associate to form a curd

• Beta lactoglobulin has been studied extensively
• It is a heat labile protein and contains 2 disulfide bonds (cystine) and one cysteine, which provides the protein with a reactive free SH group

When whey proteins are denatured their SH groups are exposed and are capable of interacting with the SH and S-S bonds in gluten, to strengthen the dough matrix by sulfhydryl interchange