Raw meat quality and salt levels affect the bacterial species diversity and community dynamics during the fermentation of pork mince
Introduction
Fermented meats are produced based on the various metabolic actions of lactic acid bacteria (LAB; Ravyts et al., 2012) and catalase-positive cocci, in particular coagulase-negative staphylococci (CNS; Sánchez Mainar et al., 2017; Stavropoulou et al., 2018a). These valuable food products are part of a culinary legacy, offering nutritional value, gastronomic pleasure, and convenience (Leroy et al., 2015a; Leroy and Degreef, 2015). Within this framework of traditionality, there is a growing interest in some of the more artisan-type production features, such as a return to spontaneous fermentation set-ups rather than the use of starter cultures (Janssens et al., 2012; Leroy et al., 2013). As spontaneously fermented and often home-made fermented foods are becoming increasingly popular, the need for sufficient empirical know-how should not be underestimated (Pot and Leroy, 2017). Deviations from validated expertise and practice may indeed pose serious quality and safety concerns (Centers for Disease Control and Prevention, 2012).
As a first constraint, a satisfactory level of initial meat quality needs to be respected. Pork, for instance, comes in various gradings, ranging from pale-soft-exudative (PSE) to dark-firm-dry (DFD) quality. Long-term ante mortem stressors are culpable for the DFD defect (Faustman and Cassens, 1990; Adzitey and Huda, 2011), which is characterized by a high final pH (≥6.0) and, thus, an elevated risk of microbial spoilage (Newton and Gill, 1981; Lister, 1989; Adzitey and Huda, 2011; Prieto et al., 2014; Ponnampalam et al., 2017). Although the use of DFD meat for sausage fermentation is acceptable, the higher risk of spoilage may require the use of starter cultures for enhanced process control (Ruiz and Pérez Palacios, 2014).
Substandard processing with non-conform raw materials and procedures is particularly risky when coupled to misconceptions about the relevance of technologically important factors (Leroy et al., 2018). Contemporary meat processing is often seen as harmful to health and has generated a demand for milder production technologies (Leroy et al., 2015b). However, added sugar is mostly converted into lactic acid and helps to kickstart the fermentation by the LAB, whilst the presence of sufficient salt is required for proper gel formation and microbial stabilisation (Työppönen et al., 2003; Desmond, 2006; Hutkins, 2006; Ockerman and Basu, 2007). Too zealous reduction of added sugar and salt may thus affect both the microbial communities as well as the formation of texture and flavour in the end-products (Corral et al., 2013; Desmond, 2006).
Because the technological conditions, such as the pH and salt level, play a primordial role in the shaping of microbial consortia during meat fermentation (Janssens et al., 2013; Leroy et al., 2014, 2015b; Yıldırım Yalçın and Seker, 2015), any alteration in processing conditions needs to be carefully addressed. This concern is not only related to the potential growth of undesirable microorganisms, including pathogens and spoilage-inducing Enterobacterales, but also with respect to the technological microbiota (i.e., the LAB and CNS). On the level of the CNS communities for instance, Staphylococcus equorum and Staphylococcus xylosus overtake Staphylococcus saprophyticus, and especially Staphylococcus carnosus, in less acidic environments (Janssens et al., 2013; Stavropoulou et al., 2018b). Also, suboptimal meat fermentation may give rise to the pathogenic species Staphylococcus aureus (Messier et al., 1989; Stavropoulou et al., 2018a). Therefore, the purpose of the present study was to delineate between a safe operating space and malpractice when using different raw meat qualities (normal versus DFD) and salt levels (0–4%) in fermented meat models that were spontaneously fermented without added carbohydrates. With respect to the microbiota, focus was on LAB, catalase-positive cocci (in particular CNS), and Enterobacterales.
Section snippets
Meat model preparation and experimental set-up
Fermented meat models were used, as described previously (Stavropoulou et al., 2018a). An experimental set-up with two different raw meat qualities was performed (normal meat with a pH of 5.74 ± 0.01 and DFD meat with a pH of 6.02 ± 0.02). DFD meat was purchased from a local butcher based on colour and exudation level. Each sausage meat batter consisted of fresh pork mince [2 kg, 14% fat fraction; obtained from a local butcher (Brussels, Belgium) based on colour and exudation level], sodium
The effect of different salt levels on the bacterial community dynamics during the fermentation of pork of normal raw meat quality
In a first experiment, raw meat batters consisting of pork mince of a normal raw meat quality level were used. The initial aw of the unsalted batter was 0.979 ± 0.01 but decreased with increasing salt concentrations, and also as fermentation proceeded over time (Table S1). After 14 days of fermentation, the aw at 0%, 1%, 2%, 3%, and 4% of salt was 0.969 ± 0.01, 0.964 ± 0.01, 0.955 ± 0.01, 0.950 ± 0.01, and 0.948 ± 0.01, respectively (Table S1). The initial pH of the sausage meat batter was
Discussion
Meat fermentation is a well understood biotechnological process, carried out either spontaneously (artisan-scale) or starter culture-initiated with LAB and/or CNS (Hutkins, 2006). Yet, the species diversity of bacterial consortia within fermented meat matrices is known to vary according to the raw material used and the processing methods applied (Talon et al., 2007; Janssens et al., 2012; Ravyts et al., 2012; Stavropoulou et al., 2018a, 2018b). Consequently, more information on the ecological
Conclusions
Both the initial raw meat quality and the degree of salting can interfere with the growth of the microbiota during meat fermentation processes. This may not only affect the technologically important bacteria, in particular the LAB and CNS, but also cause outgrowth of potentially harmful ones, in particular members of the Enterobacterales group. The use of DFD meat specifically warrants for appropriate salting and sufficient control over the acidification. The latter may be especially
Declarations of competing interest
None.
Acknowledgements
Funding: This work was supported by the Research Council of the Vrije Universiteit Brussel (SRP7 and IOF342 projects), and in particular the Interdisciplinary Research Program IRP11 ‘Tradition and naturalness of animal products within a societal context of change’ and the Hercules Foundation (project UABR 09/004).
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