Background Citric acid, an intermediate product of the Krebs cycle, has a wide usage area in the food industry since it has some functions such as acidulant, flavouring agent, preservative and antioxidant. Although molds are the most commonly used microorganisms in the citric acid production, it is known that there are significant advantages of using yeasts. Purpose and scope The microbial citric acid production mechanism needs to be well understood to make production more efficient. In this study, the yeasts used in the production, fermentation types and the factors affecting production were reviewed with studies. Methodology Although production of citric acid can be produced by chemical synthesis, the fermentation is preferred because of its low cost and ease of use. More than 90% of citric acid produced in the world is obtained by fermentation. Results Yarrowia lipolytica, Candida zeylanoides and Candida oleophila are evaluated for citric acid production with substrates such as molasses, glucose, sucrose and glycerol. On the other hand, there is great interest in developing processes with new substrates and/ or microorganisms. Conclusion Although the microbial strain is an important factor, the factors such as carbon, phosphorus and nitrogen sources, aeration, the presence of trace elements and pH are also parameters affecting the production.

Summary The study aims to evaluate the technological properties of autochthonous strains (Lactobacillus sakei S15, Lactobacillus plantarum S24, L. plantarum S91, Pediococcus pentosaceus S128b and Staphylococcus carnosus G109) in Turkish dry fermented sausage (sucuk). After 24 h of fermentaPtion, all lactic acid bacteria strains reduced the pH to below 5.0, while the pH in the control group was above 5.3. The number of lactic acid bacteria strains reached 108–109 cfu g−1 during fermentation. Staphylococcus carnosus G109 remained at the inoculation level of 106 cfu g−1 during ripening. Lactobacillus sakei S15 as mono‐culture showed higher TBARS values compared to other strains. The control group had the lowest L* value and autochthonous strains caused no significant difference for a* value. According to principal component analysis results, most volatile compounds were positively correlated with the group containing only L. sakei S15.

Salted-dried goose is a traditional Turkish product with specific flavor that is produced by dry salting, post-salting, and subsequently air-drying of the goose carcass. In this study, the leg and breast parts of salted-dried goose carcasses were analyzed in terms of microbiological properties and volatile compounds. Lactic acid bacteria and Micrococcus-Staphylococcus bacteria constituted a significant part of microbiota in both leg and breast samples. The Enterobacteriaceae count was below the detectable level (<2 log cfu g -1 ) in 60% of the leg samples and in 47% of the breast samples. The yeast–mold count was less than 5 log cfu g -1 in 80% of both leg and breast samples. Many volatile compounds belonging to different chemical groups, including aldehydes, aliphatic and aromatic hydrocarbons, esters, alcohols, terpenes, ketones, sulfur compounds, and furans, were identified from samples. The breast samples showed a higher mean amount of hexanal than the leg samples. No significant difference was found between the breast and leg samples in terms of ketones and sulfur compounds. It was also determined that a considerable part of volatile compounds is formed by lipid oxidation.

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In this study, three Yarrowia lipolytica strains (Y. lipolytica NRRL Y-1094, Y. lipolytica NRRL YB-423 and Y. lipolytica IFP29) were screened for acid-production capacity and the maximum zone-area was formed by Y. lipolytica NRRL Y-1094. The strain was then selected as a potential citric-acid (CA) producer for further studies. The CA production by Y. lipolytica NRRL Y-1094 was optimized using response surface methodology (RSM) and considering three factors, comprising initial pH-value, fermentation time, and initial glucose-concentration. The highest CA-concentration was 30.31 g/L under optimum conditions (pH 5.5, 6 days, and 125 g/L glucose) in shake flasks. It has been reported that this result gives better results than many productions with shake flasks. According to estimated regression-coefficients for CA concentration, the fermentation time had the greatest impact on CA production, followed by the substrate concentration and initial pH-level, respectively. On the other hand, this study is a fundamental step in solving and optimizing the production mechanism of Y. lipolytica NRRL Y-1094, a microorganism that has not yet been used in CA production with a glucose-based medium. The results suggest that future studies can perform higher yields by optimizing other medium constituents and environmental factors.

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