Nonvolatile compounds contribute to the taste of meat products. They mainly consist of inorganic salts, nucleotide metabolites, sugars, acids, amino acids, and peptides and are often present in relatively high quantity (61).

1. Acids

The acid taste is an important component of the overall taste of fermented meat products; it is sought in the northern process but may be rejected in the southern process. It is positively correlated with lactate and acetate contents (5,67,68). Too much acid production has been claimed to lead to undesirable ''sour'', ''prickly,'' and ''astringent" off-flavors (69). Ramihone et al. (70) associate acidity with excessive amounts of D-lactic acid.

Lactic and acetic acids are produced mainly by carbohydrate fermentation by lactic acid bacteria. Their production depends on the type and concentration of carbohydrates added to the meat batter, on technological factors, and particularly on the strains of lactic acid bacteria (71). In fact, acidification rate constitutes a major criterion of selection for lactic acid bacteria.

Other short-chain organic acids have been reported in fermented sausages; their concentrations are 100- to 1000-fold lower than those of acetic acid (68,72,73). Given the low odor and taste levels (low ppm range) of many of these acids (74), the possibility that these compounds play a role in flavor development cannot be discarded (68).

The level of short-chain organic acids including acetic acid but not lactic acid is very different in southern and northern products (73). It varies from 8.9 to 22 mg/kg DM in French, Italian, and Spanish sausages versus 79 mg/kg DM in German salami (73). The levels of lactate and acetate in northern products are higher than in southern products, as mentioned in Sec. II.

2. Peptides and Amino Acids

Peptides and amino acids belong to the nonvolatile fraction of dry sausages. Their contribution to the taste of dry sausages has not yet been clearly elucidated. Free amino acids and some peptides have some sweetness, bitterness, sourness, saltiness, and umami taste with generally high threshold (>50 mg/L for glutamate, up to 2000 mg/L for leucine) (61,75). Van

Hoye et al. (76) and Dierickx (77) show a clear correlation between free amino acid and peptide concentrations in sausage and taste descriptors such as spicy, beefy, sweet, bitter, and astringent. Henricksen and Stahnke (78) have identified amino acids and peptides that could contribute to the taste. However, a strong and rapid increase in amino acid concentration following proteinase addition has no marked impact on the final flavor but accelerates maturation (43,79-81). Similarly, products that have a different amino acid composition after inoculation of Pediococcus pentosaceus or Pediococcus acidilactici in association with Kocuria varians have identical sensory characteristics (82).

In dry sausages, myofibrillar proteins are the most degraded proteins. Their breakdown into peptides is due to endogenous enzymes because the use of antibiotics and paucimicrobial meat incubations does not reduce degradation of actin, myosin, or troponin (83,84). Bacterial starters have a low proteolytic activity on myofibrillar proteins (85) but they contribute to the hydrolysis of sarcoplasmic proteins (86-90). Lactic acid bacteria indirectly contribute to proteolysis by reducing the pH, which increases cathepsin D activity (83,84). Proteolysis of myofibrillar proteins is greatly controlled by pH.

In dry sausages, breakdown of polypeptides into smaller peptides and free amino acids is of either endogenous (60%) or microbial origin (40%) (83). Thereby, sausages containing antibiotics have lower amino acid concentrations than those inoculated with Staphylococcus and Kocuria (91,92). Peptidase activities are reported in lactic acid bacteria (82,88-90,93,94). However, at pH values below 6, such peptidase activities are low (93).

Proteolysis is very different in northern and southern processes because pH is a very important parameter in proteolysis. This means that acidic sausages produced according to the northern process are characterized by a clear degradation of actin and myosin and consequently a high peptide formation but low ammonia production. Nonacidic sausages produced in southern countries show no or a weak breakdown of myosin and actin and thus a low production of peptides and free amino acids. These different processes clearly have an impact on proteinase as well as peptidase activity (6,68,83).

3. Fatty Acids

Lipolysis in dry sausages releases free fatty acids with chain length between 16 and 18 carbon atoms (95). Their direct role in flavor has not been demonstrated yet. Short chain fatty acids have sour tastes but their sensory characteristics diminish with increasing chain length (74). In fact, fatty acids with chain length greater than 12 carbon atoms have no taste or smell (96). Addition of exogenous lipases leads to a faster and more intense lipolytic process during ripening of dry fermented sausages; however, sensorial improvement of these products is not as obvious as expected (97,98). What is considered to be of greatest significance for flavor is the role of these fatty acids and of unsaturated fatty acids as precursors of smaller-molecular-weight compounds with highly characteristic flavor properties (68).

It is now well established that lipolysis in dry sausages is, to a great extent, of endogenous origin (84,99). There are two types of endogenous lipases: triglyceride lipases present in the muscle and in the adipose tissue, which act on triglycerides, and phospho-lipases, which hydrolyze phospholipids and whose optimal pH of 5.5 is close to that of sausages (100-102). The importance of endogenous lipolysis has been demonstrated by manufacturing aseptically or in paucimicrobial sausages inoculated by micrococci or staphylococci and comparing the increase in the level of fatty acids (101,102) and also by manufacturing sausages using antibiotics (99). According to Hierro et al (102), lipolysis by endogenous enzymes accounts for more than 60% of total free fatty acid release. Similarly, Johansson (103) considers that about 30% of lipolysis come from the lipolytic strain of

Staphylococcus xylosus inoculated in a sterile mixture of fat and pork lean. Some lipolytic bacteria such as Staphylococcus and Kocuria are described, nonetheless, their activity is very low at pH 5.0, which is the pH of northern sausages, whereas at a pH near 6.0 (final pH of some Mediterranean sausages, some lipolytic activities are measured (104-106).

Free fatty acid content is obviously determined by both length of ripening period and raw materials used. A higher level of total free fatty acids is noticed in the southern process: 37 mg/g versus 27 mg/g in the northern process (6).

E. Aroma

Odor or aroma is by far the most important component of flavor because of the high sensitivity of nasal receptors to the numerous volatile components released during chewing and ingestion. The number of aroma compounds derived from spices and smoking (northern-type sausages) exceeds that of compounds derived from metabolism (107). It is clear that northern products are characterized by a smoked flavor in relation to numerous compounds such as phenols, methoxyphenols, and cyclic ketones (108,109). Garlic addition to sausages result in high levels of sulfur compounds with strong aroma. Similarly, addition of pepper leads to the presence of many terpenes but does not contribute greatly to the overall odor (107). Beside the volatile compounds originating from spices or smoke, compounds derived from the further metabolism of carbohydrates, amino acids, and fatty acids are considered to be very important for the specific flavor (110-112). The level of some catabolites varies with the process.

1. Volatile Compounds Arising from Carbohydrates

Carbohydrate fermentation mainly results in the production of lactic and acetic acids as mentioned above. Acetic acid is involved in the acidic taste but also in the vinegar odor of sausages. This odor note is higher in northern products (107,109). Sugar fermentation also results in the production of polyfunctional ketones with a strong buttery smell (diacetyl, acetoin). The level of diacetyl and acetoin can reach high values in northern products, 13001400 ng/g of sausages (108) and from 3052 to 4246 nmol/kg of sausages (6,109), whereas it is about 500 ng/g of sausages (108) and varies from 500 to 681 nmol/kg (6,109) in southern sausages. These ketones are produced by lactic acid bacteria (113) and staphylococci, particularly Staphylococcus saprophyticus and Staphylococcus warneri (110,114).

2. Volatile Compounds Arising from Amino Acids

Amino acids can be broken down into amines, ammonia, or aromatic compounds.

Volatile amines have been little studied in fermented products, although biogenic amines are often detected and have to be avoided because of their toxicity (see Sec. III.B). Among these amines, only putrescine and cadaverine have unpleasant odors with high threshold values, and it is doubtful whether the amounts produced are sufficient to influence taste or smell. Given its contribution to the increase in pH observed during drying, ammonia could also influence the sensory properties of compounds with ionizable groups (68). Ammonia production is particularly evident in sausages with a long drying phase, such as southern-type sausages (6,115).

The breakdown of branched-chain amino acids (leucine, isoleucine, valine) or phenyl-alanine results into aldehydes, alcohols, and acids with odors detected at very low threshold values (112). Thus methyl alcohols with fermented fruity odors, methyl aldehydes with malty and bit fruity odors, and methyl acids with cheesy and sweaty socks odors contribute to the dry sausage aroma (110,111,114,116). Phenylacetaldehyde with floral odor and benzaldehyde with almond and acre odors are also detected in sausage (112). The degradation of methionine, which leads to methanethiol and dimethylsulfide, with cabbage and putrid odors, must be avoided (68,109).

Catabolites of branched chain amino acids are found in both types of sausages. The relative amounts of these catabolites are highly variable. According to van Opstaele and Dirinck (108), the levels of aldehydes and branched alcohols are higher in French and Spanish sausages than in Belgian sausages. The level of methyl acids is higher in French products than in Belgian ones, and acids are absent in Spanish products (108). According to Schmidt and Berger (107), methyl acids are present in much higher levels in German sausages compared to Mediterranean sausages (France, Spain, and Italy).

The catabolism of amino acids could involve the Strecker reaction (117,118) as well as bacterial catabolism. The production of volatile compounds arising from amino acids in sausages is modulated by the inoculated flora and, in particular, by staphylococci (110,114). In fact, in sausages inoculated with Staphylococcus carnosus, the desorption of 3-methyl butanol, 3-methyl butanal, and 3-methyl butanoic acid is larger than in those inoculated with Staphylococcus warneri and Staphylococcus saprophyticus. The 3-methyl butanal plays an important role in the cured meat aroma of sausages inoculated with S. xylosus (111). Hinrichsen et al. (119) associate the detection of 3-methyl butanal with the presence of S. warneri, S. saprophyticus, and S. xylosus in bacon. Under laboratory conditions, S. carnosus produces large amounts of 3-methyl butanoic acid but also 3-methyl butanal and 3-methyl butanol from leucine (120,121). Lactic acid bacteria have a restricted aromatic potential. Under laboratory conditions, Lactobacillus sakei, Lactobacillus plantarum, Lactobacillus curvatus, and Pediococcus acidilactici only weakly degrade leucine, mainly into a-ketoiso-caproate, a molecule that is not very odorous (121).

3. Volatile Compounds Arising from Fatty Acids

Fatty acid oxidation results in numerous compounds, which belong to six families: alkanes, alkenes, aldehydes, alcohols, ketones, and acids. Although the production of these compounds in sausages is low, on the order of parts per million, their low sensory threshold (except for alkanes and alkenes, which are odorless) means that they have a real effect (72, 122). In southern sausages without spices, they represent 60% of the volatile fraction (110).

Results of aldehyde analysis show that Mediterranean sausages obviously contain higher amounts of 6-10 carbon straight-chain aldehydes, with a marked difference for hexanal, a typical product of linoleic acid oxidation (6,107,108). They have flavor thresholds of 5 to 60 ppb, and flavors are variously described as green, metallic, fruity, fatty, and rancid (68). Ketones from 2-pentanone to 2-nonanone with fruity, musty, cheesy odours and alcohols (saturated and unsaturated) from 6 to 8 carbon atoms with fruity, green, and mushroom odors are found in higher amounts in southern products than in northern ones (107109).

Oxidation of fatty acids can be chemical (peroxidation) or enzymatic (h-oxidation) (122). Peroxidation of mainly unsaturated fatty acids is a radical process that leads to the synthesis of hydroperoxides, which are oxidation products but also substrates for the formation of other radicals (RO, ROO, etc.) that propagate the chain reactions. The condensation of two free radicals halts the process and generates secondary oxidation products (aldehydes, alkanes, etc.) (61,123). Oxidation reactions are affected by many factors, such as oxygen content (diameter of sausages), the presence of pro-oxidative compounds (NaCl, metals) or anti-oxidative compounds (nitrite, spices), and the amount of unsaturated lipids

(122,124). In sausages, microorganisms play a vital role in the regulation of oxidation. Thereby, model sausages inoculated with S. carnosus and S. xylosus have lower alkane and aldehyde desorption levels than those inoculated with S. warneri and S. saprophyticus (110,114). The latter are characterized by a rancid aroma (114). Under laboratory conditions, S. saprophyticus and S. warneri, but above all S. xylosus and S. carnosus, limit the oxidation of linoleic and linolenic unsaturated fatty acids (125). The catalase and superoxide dismutase (SOD) activities of S. carnosus have been characterized (126). In S. xylosus, Barriere et al. (127,128) used mutants deficient in SOD or catalase activity to show that these enzymes contribute to limiting the oxidation of unsaturated fatty acids.

The presence of ketones in sausages is associated with the inoculation of S. carnosus or S. xylosus (110,114). These ketones may be the result of incomplete fatty acid h-oxidation. Usually, h-oxidation degrades saturated fatty acids into acetic acid by successively eliminating acetyl CoA groups. However, intermediate CoA esters can be freed; they are successively converted into h-ketoacid via thioesterase activity and then into methyl-ketone and secondary alcohol via decarboxylase and dehydrogenase action. The mechanism of ketone formation is described for molds, and because molds are found in southern products, this can partly explain the higher level of ketones in these products (129). Concerning S. carnosus, Engelvin et al. (130) have shown that intermediates were freed during the h-oxidation cycle. They also mentioned the thioesterase activity of this strain. Fadda et al. (129,131) have shown that S. carnosus has h-decarboxylase activity. All these results suggest that S. carnosus may produce ketones via this pathway.

4. Esters

Esters are present in fermented meat products and their aromatic characteristics contribute to the fruity note of the products (114,132). Most of the esters in sausages are ethyl esters, their production depending on the presence of ethanol and different acids (from two to eight carbon atoms) as well as on technological factors and microorganisms.

According to van Opstaele and Dirinck (108), sausages of the southern type are characterized by a higher proportion of ethyl esters. For Schmidt and Berger (107), it is also true for two types of Mediterranean products, the French and the Italian ones, whereas Spanish and German products have the same level of esters.

Esters could be of chemical origin because they are found in dry raw ham with a low bacterial count. But they can also be of bacterial origin. In laboratory media, some staphylococci have esterase activities capable of hydrolyzing and forming ethyl esters (133,134). In sausages, esters are associated with the presence of S. xylosus or S. carnosus strains (114,132).

Finally, the large analytical differences between the two types of sausages are reflected in the results of sensory analyses. German sausages exhibit a strong buttery and sour odour, but low levels of spicy and fruity notes (107). Mediterranean sausages (French, Spanish, Italian) are characterized by fruity, sweet odors with medium scores for buttery, sour, and pungent attributes (107). In another study, the panel characterized northern products as more acid and southern products as more mature (6,109).

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