474 Part IV: Milk
ICECREAM
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 par-
ticularly complex, with several phases (e.g., ice cry-
stals, fat globules and air bubbles, freeze-concentrated
aqueous phase) coexisting in a single product.
The production technology for ice cream was
reviewed by Goff (2002), and will be summarized
hereafter. The base for production of ice cream is milk
blended with sources of milk solids-not-fat (e.g.,
skim milk powder) and fat (e.g., cream) and added
sugars or other sweeteners, emulsifying agents, and
hydrocolloid stabilizers. The exact formulation
depends on the characteristics of the final product,
and once blended, the mix is pasteurized, in either
batch (e.g., 69°C for 30 minutes) or continuous (e.g.,
80°C for 25 seconds) processes, and homogenized
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 it facil-
itates the hydration of milk proteins and stabilizers
and the crystallization 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
scraped-surface heat exchanger, cooled using a suit-
able 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 incorporation of air bubbles, resulting in a sig-
nificant increase in volume (overrun) 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).
Flavorings and colorings may be added either to
the mix before freezing, or to the soft semifrozen
mix exiting the heat exchanger. The mix typically
exits the barrel of the freezer at 6°C, and is trans-
ferred 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 se-
quences that have specific biological activities when
released by enzymatic hydrolysis (Table 20.5). Such
enzymatic hydrolysis can occur either in vivo, dur-
ing 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,Table 20.5.Range and Properties of Casein-Derived Peptides with Potential Biological ActivityPeptides 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) Inhibition of aggregation of platelets
peptides (e.g., casoplatelin)
Angiotensin converting enzyme (ACE) Antihypertension action; blood pressure regulation; effects
inhibitors (casokinins) on immune and nervous systems
Bacteriocidal peptides (casocidins) Antibiotic-like activity