8: Food Proteins Flashcards
Explain the composition of milk
Made up of fat; casein; whey protein; lactose, etc. etc.
What is casein?
Composed of alpha, beta (phosphoproteins) & K-caseins (glycoproteins)
- Caseins self assemble into protein packets called casein micelles (a/b caseins are held together inside by hydrophobic interactions, whereas K-caseins remain at the surface to allow suspension in milk)
- Micelles are in close association with calcium phosphate, giving milk white appearance
Explain the importance of casein in the food industry
- casein is the most dominant protein in bovine milk
- the caseins are dispersed in water solvent and carry a significant amount of calcium
- the dairy industry is able to separate these proteins = wide array of functional ingredients
How is the separation of casein from skimmed milk achieved?
Using a centrifuge. Separate cream from skim milk (plasma phase)
What are the casein-based ingredients that are obtained by methods that separate caseins?
- skimmed milk powder (SMP
- acid casein
- rennet casein
- caseinate
- milk protein concentrates (MPC)
- phosphocasein
(all obtained as powder)
Skimmed milk powder (SMP) is the major milk derived ingredient. Describe the method by which it is obtained
- requires no major separation processes except removing fat component by centrifugation
- skimmed milk is then pasteurised = heat induced denaturation in whey protein, which attach to the casein micelle
- samples are then evaporated and subjected to spray drying, producing a powder
- SMP is soluble following reconstitution & can be used for further processing e.g. UHT
How is acid casein produced?
(preliminary step in the production of yoghurt)
- caseins self aggregate by changing the pH of skimmed milk from 6.7 (natural pH) to 4.6 (isoelectric point)
- pH is reduced by using hydrochloric acid; sulphuric acid & lactobacillus sp.
- allows for concentration of caseins & elimination of lactose, whey proteins & minerals as these are soluble at low pH
- water washing facilitates casein concentration before being pasteurised & dried
How is rennet casein produced?
(preliminary step in the production of cheese)
- caseins self aggregate due to the action of rennet, resulting in formation of curd & whey
- calcium is recovered
- subsequent treatment of curd is similar to acid casein manufacture
acid/rennet caseins : very efficient method of producing high casein ingredients but dried protein is insoluble when reconstituted in water
How is caseinate produced?
Neutralisation of acid/rennet casein through alkali addition
- this pH change converts insoluble acid caseins into casein dispersions = recovery of functional properties e.g. ability to bind water, rehydrate or dissolve
Reagents used to increase pH = sodium hydroxide, calcium hydroxide, ammonium hydroxide, sodium carbonate
- If pH is adjusted with sodium hydroxide, insoluble acid casein becomes a ‘soluble’ translucent dispersion called sodium caseinate
- if pH is adjusted with calcium hydroxide, resulting caseinate is a white colloidal dispersion called calcium caseinate
How are milk protein concentrates (MPC) produced?
Concentration of casein by membrane technology
- contains both casein and whey proteins and low lactose conc.
- if at least 90% of protein (casein and whey), the product is named milk protein isolate
- calcium fraction is in its native micellar form and therefore carried significant quantities of calcium phosphate
- used for nutritional properties: suitable for protein fortified beverages
How is phosphocasein produced?
concentration of casein using micro filtration
- only contains micellar casein with calcium phosphate as whey proteins, lactose and soluble salt fractions are eliminated
- following evaporation & drying, a 75-80% protein powder is produced; rehydration possible
- however stability at processing temps is limited : reintroduction of soluble milk salts restore stability
- phospchocasein exhibits a physical-chemical behaviour similar to milk in terms of rennet/acid gelation
What are the uses and application of casein-based ingredients?
(Depends on the required end result : liquid or solid)
Ability to be stable to high heating treatments is an important property
* acid gels (sodium/calcium caseinate; phosphocasein & SMP. Control gel behaviours during fermentation by bacterial cultures. e.g. yoghurts and cream cheeses)
* recombined milk products (SMP due to stability at high temps. Stored in cans or UHT treated to ensure long life)
* food emulsions (help with flavour, texture, colour. Sodium caseinate used due to high pressure stability. Both oil in water emulsions e.g. mayo, ice cream; and water in oil emulsions e.g. table spreads)
* cheese analogues (rennet casein used. needs to go through disaggregation by adding calcium salts and then rehydrated in water)
* cream liqueur
* chocolate (composed of SMP and whey powders)
* bakery (SMP and whole milk powder often used)
What are the most abundant whey proteins?
Beta-lactoglobulin
alpha-lactalbumin
Glycomacropeptides
What are the 3 whey-protein based ingredients?
- whey protein concentrates = obtained by ultrafiltration to remove water, lactose & minerals. Loss of a-lactalbumin may happen
- whey protein isolates = obtained by ion exchange separation; wash out lactose, minerals & caseinomacropeptide
- Whey hydrolysates = protein hydrolysis by incubating with proteolytic enzymes at elevated temp. (37-40 degrees)
Whey proteins are normally supplied as dry powders
What considerations need to be made as a result of the manufacturing process?
a) effect of heat treatments : pasteurisation & drying. Denaturation of immunoglobulins and serum albumin
b) changes during storage of dry powder : whey proteins can undergo “dry” Maillard reactions
c) flavour of whey protein hydrolysates : presence of hydrophobic peptides with characteristic bitter flavours
what are the functional properties of whey protein ingredients?
- solubilisers (clear sports drink folmulation)
- gelation (when heated at conc. over 10%)
- foaming (formation of aerated foods e.g. meringues)
- emulsification (in simple oil in water emulsion systems, comparable with sodium caseinate)
what are the nutritional properties of whey protein ingredients?
- infant formulations (hydrolysates : hydrolysis breaks down possible allergenic proteins such as B-lactoglobulin)
- sports nutrition (high levels of branches chain amino acids, which are preferentially metabolised by muscle rather than in the liver. Sports drinks & nutrition bars)
Describe the nutritional value of meat
- high biological value (composition close to human proteins)
- contains essential amino acids (e.g. leucine, lysine, valine, etc.)
- source of vitamins & minerals e.g. vit A/B, Fe, Cu, Zn, Se
- high in saturated fatty acids
What are the health concerns about eating meat?
- energy dense diet low in fibre: increased fat consumption
- chronic diseases: CVD, cancer
- nitrites used in producing cured meats = potential carcinogen
- food safety (salmonella, E. coli, Campylobacter)
- bacterial resistance to antibiotics
Explain the classifications of meat protein
- STROMAL PROTEINS - CONNECTIVE TISSUES 10-15% (collagen, elastin, reticulin)
- MYOFIBRILLAR PROTEINS 55% (actin, myosin)
- SARCOPLASMIC PROTEINS 30% (water soluble proteins : haemoglobin, myoglobin)
Describe the structure and functional uses of collagen
- has a triple helix structure
- non polar, non charged
- not valuable in processed meats (little binding ability), but chopped, ground, powdered collagen is useful as a gel (sausage casing, contact lenses, Pharmaceuticals)
- degrades to gelatin at 65-80 degrees C -> is used as a food ingredient in many food products e.g. jams & jellies
Explain how enzymes and pigments (sarcoplasmic proteins) impact meat
Enzymes e.g. calpain = tenderisation, flavour contribution
Pigments e.g. myoglobin (stores O2 in muscles); haemoglobin (transports O2 in blood) = responsible for colour variations in meat
Describe the stages in post-mortem acidification
- animal dies, oxygen levels fall
- supply of oxygen to the muscles stops, respiration stops
- ATP is generated from glycogen: anaerobic glycolysis
- lactic acid accumulates and the muscle acidifies (from 7.2 to 5.5 pH)
Why is acidification important?
- Muscle proteins denature as pH falls
- myosin & actin reach their isoelectric point: no electrical charge
- loss of biological activity : water holding capacity and solubility
- exudation of water from muscle fibres
- light scattering properties of contractile elements increase
- meat changes from dark and translucent to pale and opaque
