8 food biochemistry and food phần 99

P1: SFK/UKS BLBS102-c25 P2: SFK BLBS102-Simpson March 21, 2012 13:23 Trim: 276mm X 219mm 482 Printer Name: Yet to Come Part 4: Milk Today, there are two principal classes of non-butter milk-fat based spreads; (1) full-fat products with partial replacement of milk fat by another (e.g., vegetable) fat, with physicochemical, rheological, economic or dietary advantage, and (2) reduced-fat products containing varying levels of milk fat (Frede 2002b) Such products may be manufactured either in a modified continuous buttermaking system (butter technology) or in a votator scraped-surface cooler system (margarine technology) Generally, all products are made by preparation of aqueous (e.g., skim milk or cream) and lipid phases, each containing the appropriate ingredients, followed by mixing and phase inversion of the initial o/w emulsion to a final w/o emulsion Reduction of the fat content of the mix requires increased attention to the structural characteristics of the aqueous phase, which must increasingly contribute to the texture and body of the product Aqueous phase structuring agents, such as polysaccharides and/or proteins, may be added for this purpose (Keogh 1995, Frede 2002b) Butter is a relatively stable product; the small size and high salt content of the water droplets make them an inhospitable environment for microbial growth Lower fat spreads, while also being generally stable products, may have preservatives, such as sorbates, incorporated into their formulation to ensure shelf life and safety (Delamarre and Batt 1999) Some new developments in the technology of spreads include the production of triple-emulsion (o/w/o) products (in which the droplets of aqueous phase in the product contain very small fat globules) (Frede 2002b) These systems have three distinct phases, and need two or more emulsifiers and/or the use of proteins or polysaccharides as stabilisers/thickeners, as they can be quite unstable due to the presence of at least two interfaces (Kanouni et al 2002, O’Regan and Mulvhill 2010) Production and Fractionation of Milk Fat The most purified form of milk fat commercially produced is anhydrous milk fat (AMF), which consists of >99% triglycerides, and is produced in many countries The production of AMF requires the removal of water and water-soluble components of milk, and may be achieved either by successive high-speed centrifugation, with phase-inversion at high fat levels, or by starting with butter, which is melted and centrifuged to recover the purified fat phase Lipid oxidation is a significant concern during AMF manufacture, and the presence of oxygen during the process must be avoided AMF may be fractionated to yield fractions of milk fat with different desirable properties (Illingworth et al 2009), usually on the basis of melting point The most common industrial processes for milk fat fractionation involve cooling melted milk fat to temperatures where crystallisation of some of the fat occurs, facilitating the removal of a high melting point fraction (stearin crystals) from the low melting point fraction (olein) This removal may be by centrifugation or filtration through plate-andframe membrane units This basic fractionation process may be improved through the use of detergents or solvents, such as acetone There are also a number of methods for removal of cholesterol from dairy products, either through enzymatic, microbial, chem- ical (solid–liquid extraction, complexation) or physical means (distillation and crystallisation, supercritical fluid extraction) There have been several studies of the application of such methods for the removal of cholesterol from dairy products, including homogenised milk, butter and butter oil, and reducedcholesterol dairy products have appeared on the marker since the 1990s; however, greater consumer preference for reducedfat versus specifically reduced-cholesterol products may limit their widespread appeal (Sieber & Eyer 2011) Lipid Oxidation Lipids with double bonds (i.e., unsaturated fatty acids) are inherently susceptible to attack by active O2 , that is, lipid oxidation Oxidation can give rise to a range of undesirable flavour compounds (such as aldehydes, ketones and alcohols that cause rancid off-flavours), and possibly result in toxic products Oxidation is dependent on factors such as availability of oxygen, exposure to light, temperature, presence of pro- or anti-oxidants and the nature of the fat (O’Brien and O’Connor 1995, 2002, 2011) Milk fat contains a high level of monounsaturated fatty acids (although less than many other fats), but a low level of polyunsaturated fatty acids It is susceptible to oxidation but raw milk samples differ in susceptibility to oxidation as follows: r Spontaneous: Will develop oxidised flavour within 48 hours without the addition of pro-oxidant metals, such as iron or copper r Susceptible: Will not oxidise spontaneously, requires contamination by pro-oxidant metals r Non-susceptible: Will not oxidise even in the presence of iron or copper The reasons for the difference between milk samples are not clear, but are likely to be related to lactational and dietary factors, and the enzyme xanthine oxidase in milk may be critical Of particular current interest is the oxidation of the unsaturated alcohol, cholesterol, in milk; consumption of cholesterol oxidation products (COPs) in the diet is tentatively linked to the incidence of artherosclerosis (Kumar and Singhal 1991), and hence these products are regarded as potential health hazards COPs are not found in all dairy product, but have been detected in WMP, baby foods, butter and certain cheese varieties, albeit at levels that probably not pose a risk to consumers (Sieber et al 1997, RoseSallin et al 1997) The formation of COPs is influenced by factors such as temperature and exposure of the food to light (Angulo et al 1997, RoseSallin et al 1997, Hiesberger and Luf 2000) Ice Cream Ice cream, probably the most popular dairy dessert, is a frozen aerated emulsion The continuous phase consists of a syrup containing dissolved sugars and minerals, while the dispersed phase consists of air cells, milk fat (or other kinds of fat) globules, ice crystals and insoluble proteins and hydrocolloids (Marshall 2002) The structure of ice cream is particularly complex, with several phases (e.g., ice crystals, fat globules and air P1: SFK/UKS BLBS102-c25 P2: SFK BLBS102-Simpson March 21, 2012 13:23 Trim: 276mm X 219mm Printer Name: Yet to Come 25 Biochemistry of Milk Processing bubbles, freeze-concentrated aqueous phase) coexisting in a single product The production technology for ice cream was reviewed by Goff (2002 2011), and will be summarised briefly here The base for production of ice cream is milk blended with sources of milk solids non-fat (e.g., SMP) and fat (e.g., cream), added sugars or other sweeteners, emulsifying agents and hydrocolloid stabilisers The exact formulation depends on the characteristics of the final product and, once blended, the mix is pasteurised, by either batch (e.g., 69◦ C for 30 minutes) or continuous (e.g., 80◦ C for 25 seconds) processes, and homogenised at 15.5–18.9 MPa, first stage, and 3.4 MPa, second stage The mix is then cooled and stored at 2–4◦ C for at least hours; this step is called ageing, and facilitates the hydration of milk proteins and stabilisers, and crystallisation of fat globules During this period, emulsifiers generally displace milk proteins from the milk fat globule surface Ageing improves the whipping quality of the mix and the melting and structural properties of the final ice cream After ageing, the ice cream is passed through a scrapedsurface heat exchanger, cooled using a suitable refrigerant flowing in the jacket, under high-shear conditions with the introduction of air into the mix These conditions result in rapid ice crystal nucleation and freezing, yielding small ice crystals, and the incorporation of air bubbles, resulting in a significant increase in the volume (over-run) of the product The partially crystalline fat phase at refrigeration temperatures undergoes partial coalescence during the whipping and freezing stage, and a network of agglomerated fat develops, which partially surrounds the air bubbles and produces a solid-like structure (Hartel 1996, Goff 1997) Flavourings and colourings may be added either to the mix before freezing, or to the soft semi-frozen mix exiting the heat exchanger The mix typically exits the barrel of the freezer at –6◦ C, and is transferred immediately to a hardening chamber (–30◦ C or below) where the majority of the unfrozen water freezes Today, ice cream is available in a wide range of forms and shapes (e.g., stick, brick or tub, low- or full-fat varieties) PROTEIN HYDROLYSATES The bovine caseins contain several peptide sequences with specific biological activities when released by enzymatic hydrolysis (Table 25.5) Such enzymatic hydrolysis can occur either in vivo during the digestion of ingested food, or in vitro by treating the parent protein with appropriate enzymes under closely controlled conditions Casein-derived bioactive peptides have been the subject of considerable research for several years and the very extensive literature has been reviewed by Miesel (1998), PihlantoLappăalăa (2002), Gobbetti et al (2002) and FitzGerald & Meisel (2003) Several bioactive peptides are liberated during the digestion of bovine milk, as shown by studies of the intestinal contents of consumers, confirming that such peptides are liberated in vivo 483 Table 25.5 Range and Properties of Casein-Derived Peptides with Potential Biological Activity Peptides Putative Biological Activities Phosphopeptides Caseinomacropeptide Metal binding Anticancerogenic action; inhibition of viral and bacterial adhesion; bifidogenic action; immunomodulatory activity; suppression of gastric secretions Opioid agonist and ACE inhibitors (antihypertensive action) Immunomodulatory activity Inhibition of aggregation of platelets Casomorphins Immunomodulating peptides Blood platelet-modifying (antithrombic) peptides (e.g., casoplatelin) Angiotensin converting enzyme (ACE) Bacteriocidal peptides (casocidins) Anti-hypertension action; blood pressure inhibitors (casokinins) regulation; effects on immune and nervous systems Antibiotic-like activity Laboratory-scale processes for the production and purification (e.g., using chromatography, salt fractionation or UF) of many interesting peptides from the caseins have been developed; enzymes used for hydrolysis include chymotrypsin and pepsin (Pihlanto-Lappăalăa 2002) Bioactive peptides may also be produced on enzymatic hydrolysis of whey proteins; α- and β-lactorphins, derived from α-lactalbumin and β-lg, respectively, are opioid agonists and possess angiotensin-converting enzyme (ACE) inhibitory activity The whey proteins are also the source of lactokinins, which are probably ACE inhibitory Currently, few milk-derived biologically active peptides are produced commercially Perhaps the peptides most likely to be commercially viable in the short term are the caseinophosphopeptides, which contain clusters of phosphoserine residues and are claimed to promote the absorption of metals (Ca, Fe, Zn) through chelation, and acting as passive transport carriers for the metals across the distal small intestine, although evidence for this is equivocal (Miquel and Farr´e 2008, Phelan et al 2009) Caseinophosphopeptides are currently used in some dietary and pharmaceutical supplements, for example in the prevention of dental caries The CMP (κ-CN f106–169) is a product of the hydrolysis of the Phe105 –Met106 bond of κ-casein by rennet; during cheesemaking, it diffuses into the whey, while the N-terminal portion of κ-casein remains with the cheese curd CMP has several interesting biological properties; for example it has no aromatic amino acids and is thus suitable for individuals suffering from phenylketonuria; however, it lacks several essential amino acids P1: SFK/UKS BLBS102-c25 P2: SFK BLBS102-Simpson March 21, 2012 13:23 484 Trim: 276mm X 219mm Printer Name: Yet to Come Part 4: Milk It also inhibits viral and bacterial adhesion, acts as a bifidogenic factor, suppresses gastric secretions, modulates immune system responses and inhibits the binding of bacterial toxins (e.g., toxins produced by cholera and E coli) Of particular interest from the viewpoint of the commercial exploitation of CMP, relatively high levels of this peptide are present in whey (∼4% of total casein, 15–20% of protein in cheese whey, an estimated 180 × 103 tonnes per annum are available globally in whey), and can be recovered therefrom quite easily Overall, detailed information is lacking regarding the physiological efficacy and mechanism of action of many milk proteinderived peptides, and possible adverse effects Technological barriers also remain in terms of methods for industrial-scale production and purification of desired products CHOCOLATE Milk powder is a key ingredient in many chocolate products, to which they contribute flavour and texture, for example through the role of milk fat in retarding the undesirable appearance of a white discolouration called chocolate bloom The presence of milk powder (at levels up to 20% w/w) also influences processing characteristics of the molten chocolate during manufacture, for example flow properties Certain characteristics of WMP that may be undesirable in other applications (e.g., high free fat content, low vacuole volume) are actually advantageous in chocolate For this reason, roller-drying was often preferred to spray-drying for the production of milk powder for chocolate, although the latter process may be modified to yield powers with optimised properties for this application (Keogh et al 2003) In addition, new approaches for tailoring powder functionality for chocolate, such as extrusion, have been described (Franke and Heinzelmann 2008) INFANT FORMULAE Today, a high proportion of infants in the developed world receive some or all of their nutritional requirements during the first year of life from prepared infant formulae, as opposed to breast milk The raw material for such formulae is usually bovine milk or ingredients derived therefrom, but there are significant differences between the composition of bovine and human milk This fact has led to the development of specialised processing strategies for transforming its composition to a product more nutritionally suitable for the human neonate Today, most formulae are in fact prepared from isolated constituents of bovine milk (e.g., casein, whey proteins, lactose), blended with non-milk components This, combined with the requirement for high hygienic standards and the absence of potentially harmful agents, makes the manufacture of infant formulae a highly specialised branch of the dairy processing industry, with almost pharmaceutical-grade quality control Most infant formulae are formulated by blending dairy proteins, vegetable (e.g., soya) proteins, lactose and other sugars, with vegetable oils and fats, minerals, vitamins, emulsifiers and micronutrients (O’Callaghan et al 2011) The mixture of ingredients is then homogenised and heat treated to ensure microbio- logical safety Subsequent processing steps differ in the case of dry or liquid formulae The dairy ingredients used are generally demineralised, as the mineral balance in bovine milk is very different from that of human milk (Burling 2002), and desired minerals are added back to the formula as required Certain proteins (e.g., lactoferrin and α-lactalbumin) are present at higher levels in human than in bovine milk, and β-lg is absent from the former There is interest in fortifying infant formulae with α-lactalbumin and/or lactoferrin, although technological challenges exist in the economical production of such proteins at acceptable purity The exact formulation of infant formulae differs based on the age and special requirements of the infant Formulae for very young (