Meat fermentation

The meats are usually classified as either dry or semi-dry (Table 13.1). Dry sausages have an Aw of less than 0.9, tend not to be smoked or heat processed and are generally eaten without cooking. Semi-dry products have an Aw of 0.9-0.95 and are customarily heated at 60-68°C during smoking.

The fermentation temperature is normally below 22° C for dry and mould-ripened sausages, but 22-26°C for semi-dry sausages.

If a starter is used, then the pH reached is in the range of 4-4.5. Starter cultures are primarily the lactic acid bacteria lactobacilli and pediococci, such as Lactobacillus sakei, Pediococcus pentosaceus, Lactobacillus curvatus, Lactobacillus plantarum and Lactobacillus pentosus. Also of importance, especially when nitrate replaces nitrite, are the non-pathogenic catalase positive cocci Streptococcus carnosus and Micrococcus varians.

If no starter culture is used, then the pH reaches only 4.6-5. Fermentation here is dependent upon endogenous organisms such as Lactobacillus sakei and Lb. curvatus.

In the production of fermented sausages, the comminuted lean and fatty tissue is mixed with salt, spice, sugar, curing agent and starter cultures and put into casings. The Aw of a starting semi-dry sausage mix is achieved by employing some 30-35% of fatty tissue and 2.5-3% salt. Nitrite is added in the range of 100-150 mg kg-1, and ascorbic acid is also generally included at 300-500 mg kg-1. For dry sausages, nitrate may replace nitrite and the fermentation temperature is likely to be lower. Mixes incorporate 0.3% glucose to act as substrate for lactic acid bacteria. The oxygen is rapidly consumed by endogenous meat enzymes. The acid produced in fermentation promotes the reaction of nitrite with metmyoglobin to produce NO-myoglobin. Any residual nitrite is reduced by the microflora. The temperature is lowered to approximately 15°C and the relative humidity in the chamber is brought down to 75-80%. Much of the flavour and aroma that develops is due to the degradation of lipids, notably through autoxidation and the microbial transformation of the products generated by lipid degradation (Fig. 13.2). Additionally, pro-teinases produce peptides that are converted by the microflora to amino acids and volatile fatty acids.

The sausage may be aged (dried) and smoked. A surface growth may be allowed to develop and this comprises inter alia salt-tolerant yeasts (e.g. Debaromyces hansenii) and moulds. Where smoking is performed, surface microflora are eliminated. The flora may also be reinforced by starters of Penicillium nalgiovense or Penicillium chrysogenum. The surface moulds scavenge oxygen and assist the drying process.

Carbonyls

Fig. 13.2 The fundamental route for autoxidation of unsaturated fatty acids.

Carbonyls

Fig. 13.2 The fundamental route for autoxidation of unsaturated fatty acids.

The pH of unground meat must be below 5.8 to prevent the growth of undesirable organisms (pathogens). It is also important that the raw material should not be oxidised (i.e. it should have a low peroxide value). To this end, the meat may first be chilled or frozen to prevent oxidation. Furthermore, the access of oxygen to the meat will be minimised at all stages. To ferment unground meat, salt is first rubbed into the surface, or the meat is dipped in brine, or it is injected with the salt. The meat is then kept at 10°C to allow the salt to become evenly distributed throughout the piece. The meat is then shifted to 15-30°C to allow for water loss and the action of endogenous proteinases in the meat, which degrade the protein structure and increase tenderness and improve the flavour. During this time, a surface bloom of cocci, moulds and yeasts may develop. The meat may be smoked and then dried to the target Aw.

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