1. Meat and Fat

The meat to fat ratio is 2 to 1 in most industrial sausage mixes, which contain 50-70% frozen and ''tempered'' (« — 4°C) lean meat, consisting of pork and beef in equal amounts for NP and exclusively pork for the more valued MP. For pork, ham trimmings, shoulders, and bellies are used, as well as jowls and throats, in proportions determined by the sausage quality level as reflected in more than twofold price differences (F. Vandendriessche, personal communication, 2002). Pork necks are avoided as well as beef head meat, because of risks of antibiotic and Bovine Spongiform Encepalopathy (BSE) contamination, respectively. Only frozen (V —18°C) pork back and belly fat (excluding less firm sites from the groin and teat line) after removal of rind (lard) and classified according to a visual estimation of fat content (50% vs. 30%, respectively) are used as fat source. Pork rind may be added after cooking. Lard and ''sausage meat'' account for about 10% and 20% of the pig carcass, respectively (17), and classic selection criteria for these raw materials are mainly based on bacteriological quality and pH (<5.8) and on oxidation status and unsaturation of fat (<12% of polyunsaturated fatty acids, minimal peroxide value) (18, 19). Recent developments, however, report the use of differently specified alternative sources of "meat" and fat as well as the need for more specification for actual meat and fat to improve flavor in the more valued products. Technology for producing fermented sausage from poultry, including ostrich (20), from carp (21), and using olive oil (22) has been proposed. Lyophilized meat can be incorporated to shorten drying time (23), but this may require the lowering of salt addition, unless used after rehydration (24). For the more valued MP, muscle protease and muscle and fat lipase activities are probably important for (spices dependent) sausage flavor development (25) as described for raw ham (3). Relations of pig muscle enzyme activities with both carcass and meat quality have been demonstrated (26,27). Analogous to suggestions made for raw ham production, enzymes and corresponding genes might be considered for specification of raw materials (3) and selection of animals (28) in the production of fermented sausages.

2. Additives

Generally used additives and their ranges of incorporation level include salt (2-4%) containing NaNO2 (80-240 mg/kg added as curing salt containing 0.4-0.6% NaNO2), glucose (0.5-1%), sodium ascorbate or ascorbic acid (0.5-1%), and spices. The use of nitrite is considered essential because of its antibacterial, color forming, antioxidant, and flavoring properties. The use of lower nitrite levels, imposed because of health considerations, requires adaptation of technology—including, for example, minimal fermentation temperatures and oxygen-free chopping (29). For MP, part or all of the nitrite is substituted for by KNO3, although there is little technological and microbiological necessity for the use of nitrate, and its reported positive effects on color and flavor development may in fact be more related to the accompanying lower nitrite levels (4,30). Both the amounts and the nature of carbohydrates have been related to the rate and extent of acidulation (31), but only the use of lactose clearly results in lowering both rate and extent of acidulation (32), whereas its residual presence may improve sensory quality (33). Optimal levels of glucose (dextrose) are reported to be 0.3% and 0.7% for MP and NP, respectively (34). Both ascorbate and ascorbic acid are used to improve stability of the red nitrosylated pigment, an effect closely associated with the prevention of lipid oxidation. Both effects may also associated with the presence of antioxidants such as BHT and vitamin E in the raw materials, due to their use in animal feeding. However, although protective effects on lipid oxidation and color stability of supranutritional dosing of animal feeds with g-tocopherol are clear for beef and, less so, for fresh and cured pork (35), it would seem that it cannot prevent sausage color deterioration due to incorporation of polyunsaturated fat (36). Ground pepper (0.2-0.3%) is usually present in all types of sausages and, especially, MP may contain higher levels (1-3%) of other spices such as paprika and garlic that were shown to be effective antioxidants, comparable to ascorbate (37). Besides yielding flavor compounds, spices may stimulate lactic acid bacterial activity (e.g., by supplying manganese [54]) and decontamination should be considered (38). Apart from these ''classic'' additives, additional aditives are sometimes used for NP: phosphates (0.5%) to improve stability against oxidation (39), glucono-8-lactone (GdL, 0.5%) to ascertain fast but chemical acidulation with generally negative effects on flavor development, and vegetable proteins (mainly soya isolate) that may also accelerate fermentation (40).

3. Starter Cultures

Starter bacteria were first introduced in the United States to ensure rapid fermentation. In Europe, lower fermentation temperatures were/are more common and, certainly in MP, rapid fermentation results in color and flavor defects. In order to avoid the latter, micrococci were introduced as starters by Ni'vaara (41). Although excellent fermented sausages can be produced without the addition of starter cultures or re-inoculation with finished sausages, the majority of fermented sausage produced in Europe nowadays makes use of ''combined'' starter inoculation (1-2.106/g) as frozen cultures of both lactobacilli and Micrococcaceae to ensure rapid acidulation and optimal flavor development, respectively (4). Their desired properties have been discussed at length [e.g., (42)] and obviously include lag time and rate of lactic acid production for lactobacilli and the resistance of Micrococcaceae to an acid environment. Inhibition of the latter by rapid acidulation was indeed found to impair the typical flavor of MP (43). For flavor development, lipolytic and proteolytic activities have long been emphasized. It is now realized, however, that muscle and fat tissue enzymes are by far the more important actors in this respect and more important selection criteria for flavor producing starters such as S. carnosus may be their potential for leucine degradation to 3-methyl butanal and for protection of poly-unsaturated fatty acids against oxidation (5,8). Attention has also been directed recently to biogenic amine (44) and bacteriocin (45) production by starter bacteria. These aspects are considered of less importance, because raw material quality and processing appear to be the main factor controlling amine production (46), and bacteriocin effects are bound to be limited (10). Bacterocin production by L. sakei strains, isolated from fermented sausages, may however contribute to safety, without inhibition of Micrococcaceae and, thus, of flavor development (46,47). Besides bacteria, yeasts and fungi are used to a limited extent in the production of MP. Dabaryomyces hansenii and Penicillum nalgiovense are the major species sold as as starters for MP production (19). They are both aerobic organisms and thus situated at the periphery and/or surface of the sausage, where they oxidize lactic acid and produce ammonia. Their contribution to flavor development is less clear; it may involve fatty acid oxidation to methyl-ketones, and the strongest argument for the use of fungal starters may be the prevention of growth of mycotoxin-producing fungi (19).

B. Processing

1. Comminution or Chopping

Industrial processing generally uses the cutter, involving a rapidly rotating set of knives (1-3.103 rpm) producing a batter in a slowly rotating bowl (10-20 rpm) within less than 5 min. The relative speeds of rotation of bowl and knives as well the sequence of addition of raw materials and additives determine fat particle size (1-25 mm2) and are optimized to minimize both damage to the fat tissue added and increase of the batter temperature (< — 2°C). Because air bubbles in the batter and oxygen may interfere with drying and color development, respectively, chopping is best carried out under vacuum. The use of blunt rather than sharp knives is to be preferred for good texture (48). For traditional preparation of MP, often characterized by larger particle sizes, a meat grinder rather than a cutter may be used.

2. Stuffing

Traditional methods using nitrate may still incorporate a ''pan curing'' phase (49), leaving the batter 24 hr at low temperatures before stuffing to allow for optimal color development. In most industrial processes, however, vacuum-filling devices are used to immediately stuff the batter into natural or man-made collagen or cellulose-based casings, permeable to water and air. The size of sausage diameter (e.g., 2-15 cm) is related positively to the relative importance of fermentation (pH) versus drying (aw) for stability, and a small diameter of collagen casing is required to ensure sufficient oxygen supply for full development of the mold aroma in MP (4,50).

3. Fermentation and Drying

For industrial production of NP, fermentation and drying are usually carried out in separate rooms. Representative temperature/time/relative humidity (RH) combinations for NP and MP, respectively, are shown in Table 1. It is recommended to have air RH values not more than 0.10 points below the associated aw values of the sausage, to prevent case hardening. When pH is less than 5.3, a steeper RH gradient may be applied. Recommended air speeds are approximately 0.1 m/sec, and a back-and-forth shifting of RH between 80% and 88% during drying is recommended (34). Controlled fermentation and ripening in air-conditioned surroundings consumes considerable amounts of energy and alternative methods, involving the use of fresh air, have been proposed, inspired by the traditional methods for MP, adapted to local climatic conditions (51).

4. Smoking

At the end of the fermentation period, NP are subjected to smoke, generated by controlled combustion of wood (300-600 °C) to minimize the production of polycyclic hydrocarbons. Smoke contributes to antimicrobial and antioxidant effects, besides generating specific flavor and color components. Smoking is not used in the production of MP, except for Hungarian and Romanian products, where a light smoking period precedes fermentation (34).

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