Contribution of Catalase Positive Cocci on Flavour Formation in Fermented Sausages

Flavour is an essential quality of food and formed by large number of volatile (alcohol, ketones, aldehydes, esters and terpenes etc.) and non-volatile compounds (amino acids and peptides etc.). In fermented sausage, flavour develops from biochemical reactions and is influenced by several variables such as formulation, process conditions and starter culture. Microorganisms involved in fermentation play an important role in the formation of aroma. Lactic acid bacteria and catalase-positive cocci are the most important starter cultures for fermented sausage. Lactic acid bacteria are mainly responsible for lactic acid production in sausage. Catalase-positive cocci have an impact on catalase activity, colour stability, prevention of rancidity and flavour formation. These microorganisms enhance flavour through proteolytic and lipolytic activity. This study reviews the functions of catalase-positive cocci and their effects on flavour in fermented sausages.


INTRODUCTION
Fermentation is one of the oldest methods for extending the shelf life of food products. Biochemical and physical reactions that take place during fermentation change the product's initial properties. Fermented sausages are produced from a mixture of meat, fat, curing agents (nitrate/nitrite), spices and sugars through lactic acid fermentation [1]. Lactic acid bacteria and catalase-positive cocci are the most important microorganisms in the technological and sensorial properties of dry or semi-dry fermented sausages [2][3][4][5]. Lactic acid bacteria play a major role in acidification [6]. Therefore, they improve the product's safety and stability. Gram-positive catalase-positive cocci play an important role in colour stability, prevention of rancidity and flavour formation [7-9].
Catalase-positive cocci grow slowly or not at all during ripening, and the growth occurs mainly on the surface of unsmoked sausages. Because of this, high levels of catalase-positive cocci (10 6 -10 7 cfu/g) are added to sausage mixture. At this point, the specific enzyme activity of the strain is particularly important [10]. Slow ripening in fermented meat products provides an advantageous environment for catalase-positive cocci. These microorganisms are generally active until lactic acid formation results in low pH levels [11]. Adding small amounts of sugar at low ripening temperature causes slow acid formation, which encourages the growth of these microorganisms.
Moreover, in fermented sausages produced using nitrate and low levels of sugar, such as traditional Italian fermented sausages, catalase-positive cocci may be the dominant flora [12].
The typical flavour of dry fermented sausages is the result of a careful balance between volatile (alcohol, ketones, aldehydes, esters, terpenes, aliphatic hydrocarbons, aromatic hydracarbons and furans) and non-volatile compounds (amino acids, peptides, sugars and nucleotides), which come from the basic ingredients (meat, spices, nitrites and other additives) or generated by the breakdown of carbohydrates, lipids and proteins through the action of microbial and endogenous meat enzymes during ripening [13]. This study reviews the functions of catalase-positive cocci and their effects on flavour in fermented sausages.

THE ROLE OF CATALASE-POSITIVE COCCI IN FERMENTED SAUSAGES
The catalase-positive cocci identified from various types of traditional fermented sausage are given in Table 1. Staphylococcus xylosus is the most frequently isolated catalase-positive cocci in dry fermented sausages [14-16]. Strains of Kocuria and coagulase-negative Staphylococcus are used as starter cultures. These species grow in the presence of 10% NaCl, produce catalase and reduce nitrate to nitrite. However, they exhibit slow growth under anaerobic conditions. In sausages which are ripened normally and slowly, there are fewer catalase-positive cocci in the core than the periphery after a few days of ripening, due to Catalase-positive cocci play an important role in product colour, stabilization, the prevention of rancidity and the emergence of volatile compounds that are effective in flavour. Effects of catalase positive cocci as starter culture on fermented meat products are given in Fig. 1 51]. Also, the enzymes involved in the β-oxidation of fatty acids, which enhance the cured aroma in fermented sausages by increasing the concentration of methyl ketones, are purified from S. Carnosus [51]. On the other hand, Staphylococcus strains used as starter cultures have lipolytic activity [52], but it is also argued that lipase activity is limited under fermented sausage conditions [51].
For flavour development, proteolysis is the most important biochemical change. It involves the generation of amino acids, peptides, amines and aldehydes, which are precursors of volatile compounds [53][54][55]. It has been reported that S. xylosus, S. equorum, S. carnosus, S. simulans, S. warneri and S. cohnii strains that were isolated from dry cured meat products had proteolytic activity on sarcoplasmic and/or myofibrillar proteins [40, 53,56]. Aroma compounds originating from the degradation of amino acids are important for sausage flavour, especially the breakdown of leucine, isoleucine and valine into branchedchain methyl aldeydes and acids [7,57]. The initial step is the deamination of leucine catalyzed by branched-chain acid transferases, amino acid oxidases and dehydrogenases [58].
In one study, Stahnke [42] showed that the cell counts of S. carnosus and S. xylosus were correlated with methyl-branched aldehydes and ketones. Many factors can influence amino acid metabolism in these strains. Adding free amino acids to fermentation media did not affect total transaminase activity or volatile compounds. However, adding α-ketoglutarate increased the levels of methyl-branched alcohols, aldehydes, ethanol, 2-phenyl ethanol, phenylacetaldehyde, benzaldeyhde, diacetyl, 4-methyl, 2-pentanone and etyhl esters. α-ketoglutarate increased transaminase activity. The limiting factor in the transamination of branched-chain amino acids (BCAAs) was the amount of amino group acceptors [59].
Leucine catabolism of S. xylosus [64] and S. carnosus [60,61] were inhibited by nitrate. The formation of 3-methyl butanoic acid was significantly reduced by the presence of nitrate [60,61,64]. However, Stahnke [42] showed that nitrate influences the production of 3-methyl butanoic acid from leucine catabolism. Another study reported that nitrite content increased the level of free amino acids while nitrate decreased in dry fermented sausages [65]. In contrast, Olesen et al. [64] determined that nitrite decreased the levels of volatile compounds through BCAA degradation.
The volatile profile of dry-fermented sausage consists of a wide variety of compounds such as aromatic and aliphatic hydrocarbons, aldehydes, alcohols, esters, sulphur containing compounds and furanes, etc. [6,48, [66][67][68] and also most of those volatile compounds effect sausage flavour [57]. Aroma compounds generated from the degradation of amino acids play a major role in sausage flavour [42], especially the degradation of BCAAs into branched-chain alcohols, aldehydes, acids and esters [61,62].
It is thought that there is a correlation between amino acid and volatile compounds profiles. Waade and Stahnke [65] showed that amounts of valine, isoleucine and leucine are inversely correlated with 2-methyl propanal, 2-methyl butanal and 3-methyl butanal, respectively.
Masson et al. [60] showed that the use of nitrate has a strong effect on the formation of 3-methyl butanal in the presence of S. carnosus. However, it was also reported that adding nitrate decreased 3-methyl butanol and 3-methyl butanoic acid formation in the same strain.

CONCLUSION
Many factors affect the volatile compounds generated by catalase-positive cocci in sausage fermentation. Studies strongly agree that volatile compounds result from the degradation of amino acid products by catalase-positive cocci. Flavour can be enhanced by regulating enzyme activities. Further studies on optimizing enzyme activity of catalase-positive cocci would lead to a better understanding of flavour formation during sausage fermentation.