value is higher and the topping has a sufficiently high yield stress. Other
toppings contain gelling agents, so thatjcan be smaller. Classical whipped
cream hasj(air)& 0 :5 andj(fat globules) is at most 0.2. Firmness is here
obtained in another way. Fat globules have become attached to the air
bubbles and are, moreover, clumped (partially coalesced), so that they make
a space-filling network; this is comparable to (part of) the structure of ice
cream, shown in Figure 9.1b.
Concentrated Starch Gels. Starch is discussed in Section 6.6.
Especially gelatinization (in Section 6.6.2) and retrogradation (in 6.6.3) are
aspects of importance.
When native starch grains are heated in an excess of water,
gelatinization occurs, which implies that amylose leaches from the granules
and that the latter greatly swell and eventually fall apart. If the system then
is cooled and the amylose concentration is above the chain overlap
concentrationc*, corresponding to 2–4%starch (depending on starch type),
a gel is formed. This is mainly due to the formation of microcrystallites of
FIGURE17.27 Concentrated foams or emulsions. Values of (a) the modulusGand
(b) the yield stresssy, each divided by the apparent Laplace pressure of the particles
pL,a, as a function of the particle volume fractionj. (After results given by H. M.
Princen. In: R. D. Bee et al., eds. Food Colloids. Roy. Soc. Chem., Cambridge, 1989,
p. 14.)