Food Biochemistry and Food Processing (2 edition)

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25 Biochemistry of Milk Processing 483

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 4 hours; this step is

called ageing, and facilitates the hydration of milk proteins and

stabilisers, and crystallisation of fat globules. During this pe-

riod, 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 scraped-

surface heat exchanger, cooled using a suitable refrigerant flow-

ing in the jacket, under high-shear conditions with the intro-

duction 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 par-

tial 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 spe-

cific biological activities when released by enzymatic hydrolysis

(Table 25.5). Such enzymatic hydrolysis can occur eitherin vivo

during the digestion of ingested food, orin vitroby treating

the parent protein with appropriate enzymes under closely con-

trolled conditions.

Casein-derived bioactive peptides have been the subject of

considerable research for several years and the very exten-

sive literature has been reviewed by Miesel (1998), Pihlanto-

Lappal ̈ ̈a (2002), Gobbetti et al. (2002) and FitzGerald & Meisel

(2003). Several bioactive peptides are liberated during the diges-

tion of bovine milk, as shown by studies of the intestinal contents

of consumers, confirming that such peptides are liberatedin vivo.

Table 25.5.Range and Properties of Casein-Derived

Peptides with Potential Biological Activity

Peptides Putative Biological Activities

Phosphopeptides Metal binding

Caseinomacropeptide Anticancerogenic action;

inhibition of viral and

bacterial adhesion;

bifidogenic action;

immunomodulatory activity;

suppression of gastric

secretions

Casomorphins Opioid agonist and ACE

inhibitors (antihypertensive

action)

Immunomodulating peptides Immunomodulatory activity

Blood platelet-modifying

(antithrombic) peptides

(e.g., casoplatelin)

Inhibition of aggregation of

platelets

Angiotensin converting

enzyme (ACE)

Anti-hypertension action;

blood pressure inhibitors

(casokinins) regulation;

effects on immune and

nervous systems

Bacteriocidal peptides

(casocidins)

Antibiotic-like activity

Laboratory-scale processes for the production and purifica-

tion (e.g., using chromatography, salt fractionation or UF) of

many interesting peptides from the caseins have been devel-

oped; enzymes used for hydrolysis include chymotrypsin and

pepsin (Pihlanto-Lapp ̈al ̈a 2002).

Bioactive peptides may also be produced on enzymatic hy-

drolysis of whey proteins;α-andβ-lactorphins, derived from

α-lactalbumin andβ-lg, respectively, are opioid agonists and

possess angiotensin-converting enzyme (ACE) inhibitory activ-

ity. 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 caseinophospho-

peptides, 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 Phe 105 –Met 106 bond ofκ-casein by rennet; during cheese-

making, it diffuses into the whey, while theN-terminal portion

ofκ-casein remains with the cheese curd. CMP has several in-

teresting 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.