Food Biochemistry and Food Processing (2 edition)

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18 Biochemistry of Fermented Meat 337

as starter. The ratio between thel-andd-enantiomers depends

on the action ofl-andd-lactate dehydrogenase, respectively,

and the presence of lactate racemase. The rate of generation

and the final amount of lactic acid depend on the type of lac-

tic acid bacteria species used as starter, the type and content

of carbohydrates, the fermentation temperature, and other pro-

cessing parameters. The accumulation of lactic acid produces a

pH drop, more or less, intense depending on its generation rate.

Some secondary products such as acetic acid, acetoin, and others

may be formed through heterofermentative pathways (Demeyer

and Stahnke 2002). Acid pH favors protein coagulation, as it

approaches its isoelectric point, and thus also favors water re-

lease. Acid pH also contributes to safety by contributing to the

inhibition of undesirable pathogenic or spoilage bacteria. The

pH drop favors initial proteolysis and lipolysis by stimulating

the activity of muscle cathepsin D and lysosomal acid lipase,

both active at acid pH, but an excessive pH drop does not favor

later enzymatic reactions involved in the generation of flavor

compounds (Toldra and Verplaetse 1995). ́

PROCESSING STAGE 4: RIPENING

AND DRYING

Temperature, relative humidity, and air flow have to be carefully

controlled during fermentation and ripening to allow correct mi-

crobial growth and enzyme action while maintaining adequate

drying progress. The air velocity is kept at around 0.1 m/s, which

is enough for a good homogenization of the environment. Ripen-

ing and drying are important for enzymatic reactions related to

flavor development and obtaining the required water loss and

thus reduction inaw. The length of the ripening/drying period

takes from 7 to 90 days, depending on many factors, including

the kind of product, its diameter, dryness degree, fat content,

desired flavor intensity, and so on. The reduction inawis slower

in beef-containing sausages. The casing must remain attached

to the sausage when it shrinks during drying. In general, long-

ripened products tend to be drier and more flavorful.

Physical Changes

The most important physical changes during fermentation and

ripening/drying are summarized in Figure 18.6. The acidulation

produced during the fermentation stage induces protein coagu-

lation and thus some water release. The acidulation also reduces

the solubility of sarcoplasmic and myofibrillar proteins, and the

sausage starts to develop consistency. The drying process is a

delicate operation that must achieve an equilibrium between

two different mass transfer processes—diffusion and evapora-

tion (Baldini et al. 2000). Water inside the sausage must diffuse

to the outer surface and then evaporate to the environment. Both

rates must be in equilibrium because a very fast reduction in the

relative humidity of the chamber would cause excessive evap-

oration from the sausage surface that would reduce the water

content on the outer parts of the sausage, causing hardening.

This is typical of sausages of large diameter because of the slow

water diffusion rate. The cross section of these sausages shows

a darker, dry, hard outer ring. On the other hand, when the water

Acidulation

Protein coagulation

Water release

Water diffusion Evaporation

Figure 18.6.Scheme showing important physical changes during

the processing of fermented meats.

diffusion rate is much higher than the evaporation rate, water

accumulates on the surface of the sausage, causing a wrinkled

casing. This situation may happen in small-diameter sausages

being ripened in a chamber with high relative humidity. The

progress in drying reduces the water content, up to 20% weight

loss in semidry sausages and 30% in dry sausages (Table 18.1).

Theawdecreases according to the drying rate, reaching values

below 0.90 for long-ripened sausages.

Chemical Changes

There are different enzymes, of both muscle and microbial ori-

gin, involved in reactions related to color, texture, and flavor gen-

eration. These reactions, which are summarized in Figure 18.7,

are very important for the final sensory quality of the product.

One of the most important groups of reactions, mainly affect-

ing myofibrillar proteins and yielding small peptides and free

amino acids as final products, is known as proteolysis (Toldr ́a

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, producing an accumulation of polypeptides

that are further hydrolyzed to small peptides by muscle and mi-

crobial peptidyl peptidases and to free amino acids by muscle

and microbial aminopeptidases (Sanz et al. 2002). The genera-

tion 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

enzyme activity. Free amino acids may be further transformed

into other products, for example, volatile compounds through

Strecker degradations and Maillard reactions; ammonia through