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30 Non-Enzymatic Browning in Cookies, Crackers and Breakfast Cereals 585Furthermore, other chemical reactions that might take place
during processing of these products may also affect the extent of
non-enzymatic browning. Thus, starch and non-reducing sugars
such as sucrose can be hydrolysed into reducing sugars that
can later be involved in other reactions, for instance, in the MR
(Camire et al. 1990).
The chemical changes that take place during the technologi-
cal process used in the elaboration of this type of cereal-based
foods contribute, to a certain extent, to their typical organolep-
tic characteristics. However, important losses of lysine due to
the formation of chemically stable and nutritionally unavailable
derivatives of protein-bound lysine can be observed under MR
conditions (Torbatinejad et al. 2005).
The amount of lysine and its biological availability are mean-
ingful criteria for the nutritive quality of cereals, as foods
processed from cereal grains are low in essential amino acids
such as lysine and methionine (Horvati ́c and Eres 2002). As
this deficiency can be further impaired by losses originated in
browning reactions during processing, a compromise must be
found where the objectives of heat treatment are reached with a
minimal decrease in the nutritional quality of the food. Different
indicators based on the evaluation of the extent of non-enzymatic
browning have proved to be useful, not only for processing con-
trol, but for optimisation of operating conditions in the manu-
facture of cereal-based foods (Singh et al. 2000, Gokmen et al. ̈
2008a, 2008b). Although some of the studies on cereal-based
products reported in this review have not been directly carried
out in cookies, crackers or breakfast cereals, they have been con-
sidered here since both the manufacture and composition could
be similar and, consequently, the same chemical reactions might
be involved.NON-ENZYMATIC BROWNING
INDICATORSAvailable LysineThe determination of available lysine has been used to evalu-
ate the effect of heating on the protein quality of the following
cereal-based products: pasta (Nepal-Sing and Chauhan 1989,
Acquistucci and Quattrucci 1993), bread (Tsen et al. 1983,
Ram ́ırez-Jim ́enez et al. 2001), cereals (Fernandez-Artigas et al. ́
1999a), cookies fortified with oilseed flours (Martinkus 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. In
samples of protein-enriched biscuits, Noguchi et al. (1982) found
that the loss of reactive lysine is significant (up to 40% of the
initial value) when the extrusion cooking is carried out at high
temperature (190–210◦C) with a relatively low water content
(13%). When moisture 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. These au-
thors observed a higher lysine loss at low pH, since strong acid-ification markedly increases starch or sucrose hydrolysis and,
consequently, the formation of reducing carbohydrates. Starch
hydrolysis and corresponding formation of reducing sugars was
also proposed as the main cause of lysine loss in extruded wheat
flours (Bjorck et al. 1984).
An important decrease of available lysine content (40.9–
69.2%) in wheat grain processed by flaking and toasting was
observed by McAuley et al. (1987), and they attributed these
results to the high temperature reached during toasting. In gen-
eral, extrusion cooking of cereal-based foods appears to cause
lysine losses that do not exceed those for other methods of
food processing. In order to keep lysine losses within low levels
(10–15%), it is necessary to avoid operating conditions above
180 ◦C at water contents below 15% (even if a subsequent drying
step is then necessary) (Cheftel 1986).
Phillips (1988) suggested that the MR is more likely to occur
in expanded snack foods in which nutritional quality is not a
major factor than in other extruded foods with higher moisture
contents, if the processing conditions are controlled. Horvati ́c
and Guterman (1997), in a study on the available lysine content
during industrial cereal (wheat, rye, barley and oat) flake pro-
duction, found that the effects of particular processing phases
can result in a significant decrease of available lysine amounts
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, the results obtained by these authors
seem to indicate that the decrease of lysine availability is higher
for cereals with greater available lysine contents in total proteins.
Other important consideration is to avoid the presence of re-
ducing sugars during extrusion. Lysine loss and browning are
more intense when reducing carbohydrates such as glucose, fruc-
tose or lactose (added as skim milk) are added to the food mix
above 2–5% level (Cheftel et al. 1981). In agreement with these
results, Singh et al. (2000) and Awasthi and Yadav (2000) found
higher lysine loss and browning in traditionally elaborated bis-
cuits enriched with whey or skim milk.
An investigation of the changes in available lysine content
during industrial production of dietetic biscuits was performed
by Horvati ́c and Eres (2002). Dough preparation did not signifi-
cantly affect the available lysine content. However, after baking,
a significant loss (27–47%) of available lysine was observed in
the studied biscuits. The loss of available lysine was found to be
significantly correlated with technological parameters, mainly
baking temperature/time conditions.
The influence of storage on the lysine loss in protein-enriched
biscuits was studied by Noguchi et al. (1982). Lysine loss was
observed to increase when samples were stored at room temper-
ature for a long period of time. Hozova et al. (1997) estimated,
by measurement of lysine, the nutritional quality of amaranth
biscuits and crackers stored during 4 months under laboratory
conditions (20◦C and 62% RH). Although a slight decrease in the
level of lysine was detected, this was not significant. However,
these authors suggested that lysine degradation can continue
with prolonged storage and it is necessary to consider this fact
in relation to consumers and the extension of storage time.
In a survey on 20 commercially available cereal-based break-
fast foods, Torbatinejad et al. (2005) stated that reactive lysine