556 Part V: Fruits, Vegetables, and Cereals
be hydrolyzed into reducing sugars that can later
be involved in other reactions, for instance, in the
Maillard reaction (Linko et al. 1981, Camire et al.
1990). Noguchi et al. (1982) observed that 10% of
the initial sucrose molecules in a severely processed
biscuit mix were hydrolyzed into glucose and fruc-
tose, thus permitting the Maillard reaction.
The chemical changes that take place during the
technological processes used in the elaboration of
these types of cereal-based foods, contribute, to
some extent, to their typical organoleptic character-
istics. However, important losses of lysine due to the
formation of chemically stable and nutritionally
unavailable derivatives of protein-bound lysine can
be observed under Maillard reaction conditions
(McAuley et al. 1987).
The amount of lysine and its biological availabil-
ity are meaningful criteria for the protein nutritive
quality of cereals. Foods processed from cereal grains
are low in essential amino acids such as lysine and
methionine (Meredith and Caster, 1984). As this
deficiency can be further impaired by losses from
browning reactions during processing, a compro-
mise must be found in which the objectives of heat
treatment are reached with a minimal decrease in the
nutritional quality of the food. For that purpose,
indicators of heat-treated foods have proved to be
useful for the control of processes, allowing the pos-
sibility of optimizing conditions. Thus, many indica-
tors are available for evaluating the extent of non-
enzymatic browning in cereal-based foods. Although
not all the studies on cereal-based products reported
in this review are carried out directly on cookies,
crackers, or breakfast cereals, they have been con-
sidered since both the manufacture and composition
could be similar, and consequently the same chemi-
cal reactions may be involved.
AVAILABLE LYSINE
The determination of available lysine has been used
to evaluate the effect of heating on the protein quality
of the following cereal-based products: pasta (Nepal-
Sing and Chauhan 1989, Acquistucci and Quattrucci
1993), breads (Tsen et al. 1983, Ramírez-Jiménez et
al. 2001), infant cereals (Fernández-Artigas et al.
1999a), cookies fortified with oilseed flours (Mar-
tinkus et al. 1977), and biscuits (Singh et al. 2000).
Since extrusion cooking is a well-established
technology for the industrial elaboration of cereal-
based foods, several authors have studied the effect
of the initial composition and the different operating
conditions on lysine loss in extruded materials. Nog-
uchi et al. (1982), in samples of protein-enriched
biscuits, found that the loss of reactive lysine is sig-
nificant (up to 40% of the initial value) when the
extrusion cooking is carried out at a high tempera-
ture range (190–210°C) and relatively low water
content (13%). When the water content is increased
to 18%, the lysine loss is much less pronounced or
even negligible.
Noguchi et al. (1982) also studied the effect of the
decrease of pH on lysine loss in biscuits obtained by
extrusion; they observed that lysine loss increases
with low pH, since strong acidification markedly
increases starch or sucrose hydrolysis and conse-
quently the formation of reducing carbohydrates.
The formation of reducing sugars by starch hydroly-
sis was proposed to be the main cause of lysine loss
in extruded wheat flours (Bjorck et al. 1984).
Bjorck et al. (1983) measured the effect of extru-
sion cooking on available lysine in extruded biscuits
and found that the decrease in lysine was about
11%, and lysine retention was negatively influenced
by increasing the process temperature and positively
influenced by increasing the moisture content of the
mix of ingredients.
McAuley et al. (1987) observed an important de-
crease of available lysine content (40.9–69.2%) in
wheat grain processed by flaking and toasting, and
they attributed these results to the high temperature
reached during toasting.
In general, extrusion cooking of cereal-based
foods appears to cause lysine losses that do not ex-
ceed those for other methods of food processing. In
order to keep lysine losses low (10–15%), it is nec-
essary to avoid operating conditions above 180°C at
water contents below 15% (even if a subsequent dry-
ing step is then necessary; Cheftel 1986). Phillips
(1988) suggested that if the processing conditions
are controlled, the Maillard reaction is more likely
to occur in expanded snack foods in which nutrition-
al quality is not a major factor than in other extruded
foods with higher moisture content.
Horvatic ́ and Guterman (1997), in a study on
available lysine content during industrial cereal
(wheat, rye, barley, and oat) flake production, found
that the effects of particular processing phases can
result in a significant decrease of available lysine in
rye and oat flakes, whereas less influence can be
observed in the case of wheat and barley flakes.
Apart from the importance of processing conditions,