Meat Starter Cultures

Once microbiologists began to study and identify the microorganisms present in fermented sausages in the 1940s, it became clear that lactic acid bacteria were the primary organisms responsible for the fermentation.This conclusion was based on the fact that the predominant organisms isolated from naturally fermented sausages were species of Lacto-bacillus .When the isolates were propagated and re-inoculated into fresh meat, a well-fermented sausage could be produced with all the expected characteristics.

Patents, based on using these bacteria as meat starter cultures, were assigned in Europe and the United States. However, application of this technology was initially unsuccessful. This was because the Lactobacillus strains that had been identified and used successfully in trial situations were difficult to mass produce in a form convenient for sausage manufacturers. For any organism to function well as a starter culture, it must not only satisfy the performance criteria, but it must also be present in high numbers and be viable at the time of use. In the case of the Lactobacillus cultures, cell viability following lyophilization (or freeze drying, the main form of starter culture preservation) was poor, leading to slow and unacceptable fermentation rates.

Demand for a culture that could be used for fast, consistent, large-scale production of fermented sausage eventually led to the discovery of other lactic acid bacteria that not only had the relevant performance characteristics, but also the durability required for commercial applications. One organism, classified as Pedio-coccus cerevisae, was found to have these properties. Even though pediococci are not normally found in fermented sausage (they are, however, involved in vegetable fermentations), this organism was introduced in the United States in the late 1950s as the first meat starter culture. Strains of this species, which were later re-classified as either Pediococcus acidilactici or Pediococcus pentosaceus, are still widely used today.

In addition, improvements in starter culture technology and the development of frozen concentrated cultures (see Chapter 3) enhanced culture viability such that Lactobacil-lus starter cultures are also now available. Initially, the Lactobacillus strains that were isolated from fermented sausage and used as starter cultures were classified as Lactobacil-lus plantarum. Other strains of Lactobacillus that had different physiological properties and that performed well in sausage manufacture were subsequently isolated. Thus, the closely related species, Lactobacillus sake and Lacto-bacillus curvatus, are also used in starter culture preparations.

Although the different Pediococcus and Lac-tobacillus strains used as starter cultures perform the same basic role—to ferment sugars and produce organic acid—they vary with respect to several important physiological and biochemical properties (Table 6-3).These differences influence how they are used as starter cultures. First, different species have different temperature optima and different thermal tol erances. For example, L. sake and L. curvatus are considered psychrotrophic, meaning they are capable of growth, albeit slowly, at temperatures as low as 4°C.Thus,they are suitable for fermentations conducted at cool ambient temperatures.

As discussed in Box 6-1, many of the European sausages are fermented at low temperatures (20°C) to control the fermentation rate and to provide sufficient time for nitrate-reduction and subsequent color and flavor development. In contrast, P. acidilactici has a growth optimum near 40°C and is preferred by manufacturers interested in fast, high temperature fermentations. Metabolic differences also exist between these organisms.Whereas all pediococci and lac-tobacilli ferment glucose, other sugars are fermented only by specific strains. Pediococci, for example, do not ferment lactose. Sausage formulations must, therefore, account for the metabolic capacity of the culture.

There are occasions when starter culture metabolism can lead to problems and defects in the finished product.All pediococci are considered to be homofermentative, producing only lactic acid from glucose. However, under some circumstances, such as during sugar limitation, heterofermentative products such as ethanol, carbon dioxide, and acetic acid can be formed. Similarly, small amounts of these end products can also be produced by homofer-mentative L. plantarum during sugar limitation or aerobic growth. Of the heterofermenta-tive end products that are formed, acetic acid is especially undesirable in fermented sausage because it imparts a sour, vinegar-like flavor rather than the tart, tangy flavor contributed by lactic acid.

The ability of some strains to produce peroxides also is a serious problem. Depending on the starter culture strain and the level of oxygen in the environment, hydrogen peroxide can be formed directly in the fermenting sausage. Hydrogen peroxide can react with heme proteins in the muscle tissue to form undesirable, green pigments. In addition, hydrogen peroxide and peroxide radicals promote lipid oxidation, a serious flavor defect.

Table 6.3. Properties of bacteria used in meat start cultures1.

Organism

Minimum Temperature

Temperature Optimum

Acid from Glucose2

Nitrate Reductase

Primary Function

Lactobacillus sakei

4°C

32°C — 35°C

+

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