《食品化学 Food Chemistry》课程教学课件（讲稿，英文版）Modifications of Food Proteins

Modifications of Food Proteins

Modifications of Food Proteins

Food Processing and Storage z Cooking and Industrial processing – Overall beneficial effect z Decrease spoilage z Increases shelf stability – Unfavorable reactions at primary structure level z Losses in nutritional quality z Losses in functionality z Increased risk of toxicity z Desirable and undesirable flavor changes

Food Processing and Storage z Cooking and Industrial processing – Overall beneficial effect z Decrease spoilage z Increases shelf stability – Unfavorable reactions at primary structure level z Losses in nutritional quality z Losses in functionality z Increased risk of toxicity z Desirable and undesirable flavor changes

Processing & Storage z Factors that can adversely affect proteins include – Heat – Extremes in pH – Exposure to oxidative conditions z Caused by oxidizing lipids z Other oxidizing agents – Reaction with CHOs

Processing & Storage z Factors that can adversely affect proteins include – Heat – Extremes in pH – Exposure to oxidative conditions z Caused by oxidizing lipids z Other oxidizing agents – Reaction with CHOs

Moderate Heat Treatments z Globular proteins – Reduces solubility – No disruption or formation of covalent bonds – Primary structure unaffected z Beneficial – Inactivation of enzymes – Destruction of toxins or anti-nutritional factors – Improve digestibility

Moderate Heat Treatments z Globular proteins – Reduces solubility – No disruption or formation of covalent bonds – Primary structure unaffected z Beneficial – Inactivation of enzymes – Destruction of toxins or anti-nutritional factors – Improve digestibility

z Thermal treatments > 115C – Partial destruction of cysteine and cystine z Formation of hydrogen sulfide, dimethylsulfide and cysteic acid – Deamidation reactions (>100C) z Release of ammonia z Change in pI of proteins z Covalent Cross-links – Thermal treatments in presence of oxygen z Partial destruction of tryptophan residues Moderate Heat Treatments

z Thermal treatments > 115C – Partial destruction of cysteine and cystine z Formation of hydrogen sulfide, dimethylsulfide and cysteic acid – Deamidation reactions (>100C) z Release of ammonia z Change in pI of proteins z Covalent Cross-links – Thermal treatments in presence of oxygen z Partial destruction of tryptophan residues Moderate Heat Treatments

Severe Heat Treatments z Temperatures > 200C as well as alkaline conditions – Isomerization z ß-elimination z Reduces nutritional value z Digestibility – Cyclic derivatives z Strong mutagenic action z Tryptophan ⇒ carbolines – Destruction of Aas at alakaline conditions z Arginine z Cysteine z Ser, Thr, Lys

Severe Heat Treatments z Temperatures > 200C as well as alkaline conditions – Isomerization z ß-elimination z Reduces nutritional value z Digestibility – Cyclic derivatives z Strong mutagenic action z Tryptophan ⇒ carbolines – Destruction of Aas at alakaline conditions z Arginine z Cysteine z Ser, Thr, Lys

Influence of Severe Heat z Lysine and Arginine side chains react with the free acids of glutamic and aspartic acid – isopeptide cross-links which can impede digestion and exhibit major effects on functionality z Temperatures of 180 – 300C – Such as occur in roasted coffee, meat, fish and in the baking of some biscuits z These reactions also account for some of the flavor and color developed as a result of the roasting process

Influence of Severe Heat z Lysine and Arginine side chains react with the free acids of glutamic and aspartic acid – isopeptide cross-links which can impede digestion and exhibit major effects on functionality z Temperatures of 180 – 300C – Such as occur in roasted coffee, meat, fish and in the baking of some biscuits z These reactions also account for some of the flavor and color developed as a result of the roasting process

MeIQx z Possible formation of toxic products – Mutagenic activity on flame￾broiled fish and beef – Several mutagens are of protein and amino acid origin z These compounds are only formed at temperatures in excess of 300°C Influence of Severe Heat PhIP

MeIQx z Possible formation of toxic products – Mutagenic activity on flame￾broiled fish and beef – Several mutagens are of protein and amino acid origin z These compounds are only formed at temperatures in excess of 300°C Influence of Severe Heat PhIP

Table 1.Reported Levels of PhIP and MelQx in Selected Meats,Poultry, and Fish Products PhIP(ng/g MelQx(ng/g product) product) Hamburgers,pan-fried("home-cooked") 0-32 0-10 Beef steak,broiled/grilled/fried 0.6-48.5 0.5-8.3 Chicken breasts,pan-fried 12-70 0 Chicken breasts,grilled (barbecued) 27-480 0-9 Fish,broiled/grilled 2-73 0-5 Fish,pan-fried 1.7-69 1.4-6 Fast food hamburgers(fried or charbroiled) 0.1-0.6 0-0.3 Fast food chicken(chicken breast 0 0 sandwiches and deep-fried chicken) Fast food breakfast sausages 0 0-0.3 Fast food fish(sandwiches) 0 0

Alkaline pH z Thermal treatments at alkaline – Covalent Cross-Links – Condensation reaction of Lys, Cys or Ornithine with DHA (cysteine or phosphoserine) – Lower nutritional value – Toxicity?

Alkaline pH z Thermal treatments at alkaline – Covalent Cross-Links – Condensation reaction of Lys, Cys or Ornithine with DHA (cysteine or phosphoserine) – Lower nutritional value – Toxicity?