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Handbook of Functional Lipids

Although the

crystal tends to form first in many fats, it typically transforms

to a more stable polymorph. This is related to, among other things, chemical com-
position. The diversity of FA chain length, TAG carbon number, and amount of
liquid oil present determine which polymorph is favored in a fat [13,41]. Fats tend
to prefer either the

β′

modification. For example, milk fat, beef tallow, and

palm fats are

β′

-tending. The majority of their solids remain in the

β′

modification

even after prolonged storage. In contrast, lard and fully hydrogenated soybean oil
are examples of

fats. Multicomponent fats tend to be more stable in metastable

states than those fats with narrow and symmetrical distributions of FAs [47,70]. For
example, when palm oil is introduced into hydrogenated canola oil, the increased
FA diversity stabilizes the

β′

polymorph [71]. Surfactants also delay polymorphic

transitions by interfering with the transformation kinetics [1,72].

Fat polymorphism is related to and complicated by phase behavior [73]. In blends

of fully hydrogenated canola oil and soybean oil, intersolubility and polymorphic changes
occurred simultaneously [74]. In milk fat, the

crystal has a relatively long lifetime in

milk fat, perhaps due to stabilization related to compound crystal formation [75].

6.2.5 F

RYSTAL

ETWORKS

Fats and fat-based foods consist of partially crystalline fat suspended in liquid oil.
Once crystals form, Brownian motion and van der Waals forces cause them to
aggregate into a three-dimensional network [76]. With as little as 10% crystalline
material, a semisolid network in which the oil is immobilized can be observed. The
strength of FCNs can increase for an extended period of time because of continued
crystallization, crystal rearrangement, and crystal aggregation [77–79]. Sintering
refers to the development of solid bridges in the narrow gaps of fat crystal networks
after crystallization [17,80,81]. Butter hardness can increase for months after man-
ufacturing, depending on temperature [82–84]. Indeed, thermodynamic equilibrium
may never be attained in a complex fat system. Segregation leading to crystals of
higher purity and rearrangement into more stable polymorphs can continue indefi-
nitely depending on several factors, including composition and storage temperature.

Although the ratio of solid to liquid fat is a big determinant of fat consistency, the

type of solids formed, the way in which these solids are arranged, and the interactions
between them are very important [78,85–88]. Fat microstructure refers to the level of
structure between roughly 0.25 and 200

m (Figure 6.1). The importance of fat micro-

structure on physical properties has been demonstrated [85,86,89]. At this level, crystal
number, shape, density, clustering, and distribution all affect the properties of a fat. These
parameters are themselves determined by chemical composition and processing condi-
tions [80,90,91]. For example, when milk fat fractions are processed at higher agitation
rates, nucleation is enhanced. The average crystal size is reduced and a more viscous
nature and lower elastic modulus was observed [92]. When blends of a high melting
milk fat fraction and sunflower oil were crystallized slowly, the crystals were larger and
more densely arranged, and had more regular boundaries than when fast cooling was
employed [93]. In turn, crystal properties can dramatically affect texture [39,41,96]. For
example, smaller crystals are generally associated with a harder fat. In contrast, platelet-
like crystals greater than 30

m give foods a grainy and sandy mouthfeel.

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