650 Part VI: Fermented Foods
sausages (Table 28.1). The water activity decreases
according to the drying rate, reaching values below
0.90 for long-ripened sausages.
CHEMICALCHANGES
There are different enzymes, of both muscle and
microbial origin, involved in reactions related to col-
or, texture, and flavor generation. These reactions,
which are summarized in Figure 28.7, are very
important for the final sensory quality of the prod-
uct. One of the most important group of reactions,
mainly affecting myofibrillar proteins and yielding
small peptides and free amino acids as final prod-
ucts, is known as proteolysis (Toldrá 1998). An
intense proteolysis during fermentation and ripening
is mainly carried out by endogenous cathepsin D, an
acid muscle proteinase that is very active at acid pH.
This enzyme hydrolyses myosin and actin, produc-
ing an accumulation of polypeptides that are further
hydrolyzed to small peptides by muscle and micro-
bial peptidylpeptidases and to free amino acids by
muscle and microbial aminopeptidases (Sanz et al.
2002). The generation of small peptides and free
amino acids increases with the length of processing,
although the generation rate is reduced at acid pH
values because the conditions are far from optimal
for the enzyme activity. Free amino acids may be
further transformed into other products, for example,
volatile compounds through Strecker degradations
and Maillard reactions; ammonia through deamina-
tion and/or deamidation reactions by deaminases
and deamidases, respectively, present in yeasts and
molds; or amines by microbial decarboxylases.
Another important group of enzymatic reactions,
affecting muscle and adipose tissue lipids, is known
as lipolysis (Toldrá 1998). Thus, a large amount of
free fatty acids (between 0.5 and 7%) is generated
through the enzymatic hydrolysis of triacylglycerols
and phospholipids. Most of the observed lipolysis is
attributed, after extensive studies on model sterile
systems and sausages with added antibiotics, to
endogenous lipases present in muscle and adipose
tissue (e.g., lysosomal acid lipase, present in the
lysosomes and very active at acid pH; Toldrá 1992,
Hierro et al. 1997, Molly et al. 1997).
Catalases are mainly present in microorganisms
such as Kocuriaand Staphylococcus;they are re-
sponsible for peroxide reduction and thus contribute
to color and flavor stabilization. Nitrate reductase,
also present in these microorganisms, is also impor-
tant for reducing nitrate to nitrite in slow-ripened
sausages with an initial addition of nitrate. Recently,
two strains of Lactobacillus fermentumhave proved
to be able to generate nitric oxide and give an ac-
ceptable color in sausages without nitrate/nitrite.Figure 28.7.Scheme showing the most important reactions by muscle and microbial enzymes involved in chemical
and biochemical changes affecting the sensory quality of fermented meats.