4.2. Role of heat in soymilk production
Heat by itself plays a central role in soymilk production because
it directly determines the structure and characteristics of soymilk
particles. All thermal treatments, whether pre- or post-extraction
heating, alter the native structure of soy proteins. The hydrophobic
regions of native proteins are buried inside, whereas the hydrophilic
groups are located on the protein surface (Nishinari et al.,
2014). With the increase in heat, a larger number of hydrophobic
regions are exposed to the surface, thereby significantly increasing
the surface hydrophobicity of a particle. Heat rate also affects the
denaturation and aggregation of proteins. Tang (2007) investigated
the influence of heat rates (1.6e23.8 _C/min) and reported that
soymilk viscosity is considerably high (higher by ~40 times) and
protein aggregation is low when the lowest heat rate is employed.
In addition, the content of free eSH is another important attribute
of protein particles that relating to heat treatment. Disulfide bond
and hydrophobic interaction are both important driving forces of
protein particle formation (shown in Fig. 3).
Moreover, heat is an “easy-to-handle” parameter that contributes
to the physicochemical and sensory quality of soymilk. Simple
adjustment in heating (e.g., pre- or post-extraction heating, temperature,
two-step heating) varies the particle size, viscosity, and
dispersion stability of the final product. In addition, the particle size
of off-flavor soymilk is significantly larger than that of traditional
soymilk (as discussed in Section 4.1) resulting from pre-extraction
heat treatment, indicating the substantially larger degree of protein
aggregation in off-flavor soymilk. As a result, “chalkiness” caused
by these large protein particles becomes a major defect in soymilk
produced through the Western methods because such fine and
grainy particles can fill the pores in mucous membranes of the
mouth after intake (Rosenthal et al., 2003). Furthermore, either
flavor or off-flavor compounds (e.g., aldehydes and ketones) show
different degrees of affinity to soy proteins (Gremli, 1974).
Damodaran and Kinsella (1981) reported that small flavor molecules
can hydrophobically interact with soy protein and demonstrate
a stronger affinity to 7S than to 11S. Aside from subunit
aggregation, local conformation of polypeptide chains also changes
during heat-induced denaturation. The exposed hydrophobic region
on the surface of heat-denatured protein particle provides
active binding sites for flavor molecules, which in turn affects the
sensory quality of soymilk.