318 Chapter 12
of strategies to achieve them is critical for
developing and using modifi ed protein ingre-
dients. Numerous reports on enhancing the
functionality of food proteins by limited pro-
teolysis have been published. Functionality
imparts a valuable dimension to food pro-
teins, complementing their established nutri-
tional value.
The functional properties of proteins are
those physicochemical properties that govern
their performance and behavior in food
systems during their preparation, processing,
storage, and consumption. These properties
are infl uenced by the nature and extent of
interaction of the proteins with themselves,
other components, and water in the food
system. The factors that affect the functional
properties of food proteins have been clas-
sifi ed as intrinsic factors, extrinsic factors,
and process treatments or conditions. The
food system and the processing dictate most
of the extrinsic factors and process treat-
ment/conditions and storage conditions, and
hence are amenable to a limited range of
maneuverability.
Endopeptidases cleave the peptide linkage
between two adjacent amino acid residues in
the primary sequence of a protein, yielding
two peptides. Factors affecting the enzymatic
hydrolysis of proteins include enzyme speci-
fi city, extent of protein denaturation, sub-
strate and enzyme concentrations, pH, ionic
strength, temperature, and absence or pres-
ence of inhibitory substances. The specifi city
of an enzyme is a key factor, infl uencing both
the number and location of the peptide link-
ages that are hydrolyzed. Proteolysis can
proceed either sequentially, releasing one
peptide at a time, or through the formation of
intermediates that are further hydrolyzed to
smaller peptides as proteolysis progresses,
which is often termed the zipper mechanism.
The extent of proteolysis is quantifi ed
as the degree of hydrolysis (DH), which
refers to the percentage of peptide bonds
cleaved. The DH is commonly measured
and monitored by the amount of base that islactose. The enzyme chymosin, or rennet, is
a key to the manufacture of cheeses and
gelled dairy products. Microbes that contain
enzymes transform milk to yogurt, and cream
to sour cream and cream cheese. In these
microbial transformations, the lactose content
of products is reduced and desirable textures
are created. Various chapters in this book
discuss microbially transformed dairy
products.
Enzyme - modifi ed proteins and lipids
are another category of dairy ingredients.
Although dairy ingredients are relatively
expensive to use in cost - sensitive formula-
tions, in some instances, modifi cation of
functionalities of ingredients is desirable and
enzymes are useful for this purpose. For
example, enzymes help to obtain the nutri-
tionally desirable fractions of macromole-
cules that have recently become popular
additions to foods.
This chapter discusses protein and lipid
modifi cations in dairy ingredients. Lactose
modifi cation is not discussed because it is
used only in fl uid milk products (beverage
milk).
Protein Modifi cation
Proteins are increasingly being used to
perform functional roles in food formula-
tions. Common food proteins have good
functional properties including solubility,
gelation, emulsifi cation, and foaming. The
functional properties of proteins are impaired
near their isoelectric point (pI), as is the
case in most acidic foods. Enzymatic modi-
fi cation of food proteins by controlled prote-
olysis can enhance their functional properties
over a wide pH range and other processing
conditions.
Choosing the right proteolytic enzyme,
environmental conditions for hydrolysis, and
degree of hydrolysis is crucial for enhancing
the functional properties of proteins. Under-
standing the molecular properties required
for protein functionality and the development