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aData from Ref. 53: starters used were fermented sausage, e.g. back slopping (S1) or a lyophilyzed culture of bacteria (S2). b Data from Ref. 13.

c Data from Ref. 63: results of two experiments are shown involving longer chopping with salt (1) or pork back fat (2) respectively.

aData from Ref. 53: starters used were fermented sausage, e.g. back slopping (S1) or a lyophilyzed culture of bacteria (S2). b Data from Ref. 13.

c Data from Ref. 63: results of two experiments are shown involving longer chopping with salt (1) or pork back fat (2) respectively.

Inhibitory effects on deamination activities of both rates and extent of acidulation, more dramatic with larger diameters, associated with lower rates of drying have been confirmed in recent work (Demeyer http://fltbwww.rug.ac.be/animalproduction). Protein fermentation seems to be associated with the importance of acetate in the recovery of pyruvate equivalents. The most striking finding, however, is the large variability in respired substrate: between 7% and 43% of pyruvate equivalents metabolized. These calculated values obviously reflect an enormous variability in oxygen consumption, which may involve chemical oxidations apart from respiration. Factors determining variation are probably related to processing characteristics such as sausage diameter and chopping (Table 2), but variability in factors such as vacuum stuffing, gaseous exchanges during ripening, and respiratory activity of bacteria and molds may be more important.

2. Sausage Metabolism and Sensory Quality

Estimates of the nature of sausage metabolism, as discussed above, allow better comprehension of the mechanisms determining both rate and extent of acidulation. It is however, clear, that the generalized pathways used in such estimates cover and/or accompany a set of detailed (bio)chemical reactions and end products affecting the development of sensory quality. Flavor is considered to be the more important sensory characteristic, determining repeat purchase by the consumer, whereas texture and color determine initial purchase and rejection (64).

a. Texture. Texture, which can be measured as the force necessary to penetrate the sausage surface or interior (sausage slice) under standardized conditions (e.g., hardness) is determined by two processes, occurring consecutively in NP and simultaneously in MP:

Gel formation due to acidulation. During chopping, myofibrillar structures are degraded (65) and myofibrillar proteins are solubilized into a sol—a network of filamentous aggregations of myosine molecules, whose dimensions and for mation depend on factors such as pH and NaCl concentration that determine the relative rates of filament formation and aggregation (66). Acidulation induces coagulation—the conversion of a sol into a gel by intensification of aggregations, associated with the release of water and the formation of a matrix surrounding fat and connective tissue particles. The pH necessary for coagulation increases with increasing salt concentration and is 5.3 for salt concentrations between 2% and 3% (67)

Drying. After gel formation, hardness is further increased because of loss of water, determined by diffusion-limited water transport.

The rate and extent of pH decline in the sausage, itself a reflection of overall sausage metabolism, determines both processes. During fermentation, muscle cathepsin D is activated by the decrease in pH and degrades sausage myosin (68,69). It is known that such damage lowers the strength of heat-coagulated myosin gels (16), and negative effects on texture of sausage proteolytic activity (70) and of increased myosin degradation because of added proteases (71) have indeed been reported. It is therefore clear that acidulation during fermentation induces two opposing effects on texture development: coagulation of the myosin sol into a gel as well as accelerating proteolytic cleaving of myosin molecules, lowering their contribution to gel strength, and, possibly, to water retention within the gel. Differences in gel structure and its water retention because of different relative rates of acid-induced coagulation and proteolysis may explain the positive relationship found between initial rates of acidulation and texture development during drying. The use of PSE pork (72), spices (54), starter organisms (13), and soy protein (40) increase rates of acidulation and drying and thus of texture development. For obvious reasons, an increase in sausage diameter decreases rate of drying and, thus, rate of hardness development (73). Also, however, the rate of pH decline is lowered because of an increasing contribution of proteolytic processes to metabolism (14,74).

b. Color. The stable red color of fermented sausage is due to nitrosylation and subsequent acid-induced denaturation of myoglobin. In MP, formation of the nitrosylating NO occurs after bacterial reduction of added nitrate to nitrite, a process generally attributed to Micrococceae. Its inhibition by pH values below 5.2 hampers the use of lactic acid bacterial starters to ensure rapid acidulation. In NP, characterized by rapid acidulation, nitrite is the additive ensuring color development. However, upon addition it acts as a very reactive oxidant for myoglobin: it is reduced to NO during chopping with an immediate gray discoloration of the batter, due to (nitrosylated) metmyoglobin formation. The rates of both this initial oxidation and the subsequent reduction and denaturation to the red nitrosylated myochromogen during ripening, as well as the stability to subsequent oxidation of the color formed, are determined by a complex set of factors, including the amounts of nitrite used, the rate of pH drop during fermentation, the use of antioxidant additives and the antioxidant activities of the meat and starter bacteria used. It would seem that for the Northern ripening process, the use of sodium ascorbate (e.g., 600 ppm) with minimal amounts of sodium nitrite (e.g., 150 ppm) is sufficient to obtain an acceptable color stability also reflected in a low redox potential (30) and minimal lipid oxidation (75). These conditions are promoted by the use of starter organisms with antioxidant activities (catalase, superoxide dismutase, and/or nitrate reductase activities in Micrococcaceae) also contributing to flavor development (76) and/or low hydrogen peroxide-producing activity (lactic acid bacteria). Net peroxide production is low at lower rates of acidulation (4), and minimal oxidation during sausage metabolism because of a low redox potential may be reflected in its high potential of oxygen removal/ consumption (Torfs and Demeyer, in preparation).

c. Flavor. The simultaneous confrontation of the consumer with the texture, taste, and smell of the product during chewing creates his or her impression of flavor. Aroma (smell, odor), determined by volatile compounds, is considered to be the most important component because of the very high sensitivity of the nasal receptors. It is often considered separately from taste, which is determined by nonvolatiles sensed by the receptors predominantly situated on the tongue. One should, however, be conscious of a ''taste-olfaction integration'' of senses (77), also apparent from the aroma enhancement due to the glutamate-umami taste. Peptides in fermented sausages may have a similar effect. In this respect, it may be significant that fermented sausages were found to be better distinguished by taste than by odor for the descriptor ''dry sausage'' (78).

Volatile aroma compounds. More than 200 chemical compounds have been identified by gas chromatography-mass spectrometry or gas chromatography-olfactometry in volatiles present in the sausage ''head space'' or isolated by steam distillation (5,19). Not all compounds in such ''spectra'' are of sensory relevance, and the majority are derived from spices and smoking (NP). A limited group of compounds, thought to be responsible for the specific ''fermented sausage'' flavor, however, consist of:

Compounds considered to be derived from carbohydrate metabolism represented by acetic, propionic and butyric acids, acetaldehyde, diacetyl, and acetoin.

Compounds considered to be derived from protein metabolism, mainly represented by branched aldehydes and the corresponding acids and alcohols.

Compounds derived from lipid degradation, mainly represented by methyl ketones, produced by microbial h-oxidation. Chemical autooxidation of unsaturated fatty acids produces a whole range of volatile carbonyl compounds, such as hexanal, contributing to the rancid notes and as such important for the overall flavor.

These compounds have been clearly associated with sensory descriptors such as maturity and salami and their relative importance, as well as that of esters, is increased by the Mediterranean low-temperature and long-time-ripening process with use of staphylococci as starter organisms (79). Higher temperatures and the use of pediococci as lactic acid-producing starters promote the production of dairy-related volatiles such as diacetyl and accelerate the rate of pH drop, inhibiting Staphylococci (5).

Nonvolatile taste compounds. In contrast to MP, an ''acid'' taste is often sought for in NP and is positively correlated with the contents of D-lactate and acetate (61,80). The extent of proteolysis, as reflected in the levels of low-molecular-weight peptides and free amino acids is clearly correlated with sensory analysis (14), specifically in relation to mold growth (10), and it is known that peptides affect taste rather than aroma, as shown for raw ham (81). Preliminary data (15) have associated small peptide (<500 D) and amino acid containing fractions isolated by gel permeation chromatography with sensory descriptors such as salami and bouillon, in analogy with the known importance of such fractions for raw ham flavor (82). Also, the nonprotein nitrogen fraction will affect sausage pH (59) and may thus affect liberation of aroma determining acid compounds during chewing (83). ATP metabolites such as IMP and hypoxanthine contribute to taste, whereas free higher fatty acids are generally considered of less importance (64).

The relative importance of the different flavor compounds is determined by interactions between muscle and microbial metabolism as well as chemical reactions. The use of antibiotics and paucibacterial meat incubations has clearly established that initial proteo-lytic changes mainly involve myosin and actin degradation through the action of cathepsin D-like enzymes. The contribution of bacteria in further endo- and, mainly, exoproteolytic changes increases down to ammonia production, the end of the proteolytic chain. Mediterranean, low-temperature ripening lowers rate of pH drop and thus cathepsin D activity and initial protein degradation, but further proteolysis is not affected. Paucibacterial meat incubations demonstrate free amino acid production by meat enzymes.

In similar experiments, it was clearly demonstrated that endogenous lipases are by far the mainly responsible enzymes for the liberation of free fatty acids during ripening, with preferential release of polyunsaturated fatty acids, both because of the more important phopholipase activity on muscle membrane phospholipids and the specificity of fat cell lipases. The importance of lipolysis for lipid oxidation, and thus flavor, remains unclear but a promoting effect is often assumed (15). Our laboratory has participated in studies on the impact of processing and of bacteria on the production of volatiles important for flavor in meat and meat products (6,15,84), using standardized methodology, and our results suggest that the major contribution of bacteria to dry sausage ripening may be related more to lowering of lipid oxidation than to amino acid fermentation. Anyway, both amino acid fermentation and antioxidant characteristics are probably more important selection criteria for flavor-enhancing starter bacteria than their lipolytic and proteolytic properties. It is indeed now generally accepted that initial proteolytic and lipolytic changes during dry sausage ripening are brought about by enzymatic activity of the raw materials, rather than from microorganisms (10). It is known that such activity in muscle shows considerable variation, related to anatomical location, gender, animal age, and postmortem rate of pH drop (26,27,85). The effects of such metabolic variability on flavor development in relation to bacterial activity should be further investigated.

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