Fate of Escherichia coli and Kluyveromyces marxianus contaminants during storage of Greek-style yogurt produced by centrifugation or ultrafiltration

Evaluating the antagonistic effect of Lactobacillus acidophilus against Shiga toxigenic and non-toxigenic Escherichia coli strains in bioyogurt

This study was conducted to determine the effect of acidophilus yoghurt (yoghurt fortified with Lactobacillus acidophilus) in comparison to traditional plain yoghurt (St. thermophilus and L. bulgaricus starter cultures) on the survival of three pathogenic Escherichia coli strains; Shiga toxigenic O157 (STx O157), non-toxigenic O157 (Non-STx O157) and Shiga toxigenic non-O157 (STx O145). After six days of refrigerated storage of laboratory-manufactured yoghurt inoculated with the three strains of E. coli separately, all were eliminated in acidophilus yoghurt, while their survival extended in the traditional yoghurt along the storage period (17 d). Reduction percentages of the tested strains in acidophilus yoghurt were 99.93, 99.93 and 99.86%, with log reduction of 3.176, 3.176, and 2.865 cfu/g for Stx O157, Non-Stx O157, and Stx O145 E. coli, respectively, in comparison to 91.67, 93.33 and 93.33%, with log reduction of 1.079, 1.176 and 1.176 cfu/g in traditional yoghurt. Statistical analysis showed a significant effect of acidophilus yoghurt in reducing the count of Stx E. coli O157 (P = 0.001), Non-Stx E. coli O157 (P < 0.01) and Stx E. coli O145 (P < 0.01) compared to the traditional yoghurt. These findings emphasize the potential use of acidophilus yoghurt as a biocontrol alternative method for eliminating pathogenic E. coli, as well as other similar applications in the dairy industry.

Effect of Probiotic Lactic Acid Bacteria (LAB) on the Quality and Safety of Greek Yogurt

Greek yogurt is a strained yogurt with a high protein content that brings nutritional benefits. To enhance the functional benefits of Greek yogurt, Greek yogurt was prepared with various combinations of probiotic lactic acid bacteria (LAB) (Streptococcus thermophilus, Lactobacillus bulgaricus, Lactobacillus gasseri BNR17, and Lactobacillus plantarum HY7714). Effects of probiotic LAB on quality, sensory, and microbiological characteristics of Greek yogurt were then compared. Among samples, Greek yogurt fermented by S. thermophilus and L. bulgaricus showed the highest changes of pH and titratable acidity during 21 d of storage at 4 °C. Greek yogurt fermented with L. plantarum HY7714 had a higher viscosity than other samples. Greek yogurt fermented with S. thermophilus, L. bulgaricus, L. gasseri BNR17, and L. plantarum HY7714 showed superior physicochemical properties and received the highest preference score from sensory evaluation among samples. Overall, the population of enterohaemorrhagic Escherichia coli (EHEC) was more effectively reduced in Greek yogurt fermented with probiotic LAB than in commercial Greek yogurt during storage at 4, 10, and 25 °C. Thus, the addition of L. gasseri BNR17 and L. plantarum HY7714 as starter cultures could enhance the microbial safety of Greek yogurt and sensory acceptance by consumers.

Spoilage Potential of Contaminating Yeast Species Kluyveromyces marxianus, Pichia kudriavzevii and Torulaspora delbrueckii during Cold Storage of Skyr

This study investigated the spoilage potential of yeast strains Kluyveromyces marxianus (Km1, Km2 and Km3), Pichia kudriavzevii Pk1 and Torulaspora delbrueckii Td1 grown in skyr in cold storage. Yeast strains were isolated from skyr and identified by sequencing of the 26S rRNA gene. K. marxianus yeasts were grown in skyr to high numbers, generating large amounts of volatile organic compounds (VOC) associated with off-flavours, among them were alcohols (3-methyl-1-butanol, 2-methyl-1-propanol and 1-hexanol), esters (ethyl acetate and 3-methylbutyl acetate) and aldehydes (hexanal, methylbutanal and methylpropanal). Growth of P. kudriavzevii Pk1 led to moderate increases in several alcohols and esters (mostly, 3-methyl-1-butanol and ethyl acetate), whereas only minor shifts in VOCs were associated with T. delbrueckii Td2. The levels of the key aroma compounds, diacetyl and acetoin, were significantly decreased by all K. marxianus strains and P. kudriavzevii Pk1. In contrast to the other yeast species, K. marxianus was able to utilize lactose, producing ethanol and carbon dioxide. Based on the overall results, K. marxianus was characterised by the highest spoilage potential. The study revealed the differences between the yeast species in fermentative and spoilage activities, and clarified the role of yeast metabolites for off-flavour formation and quality defects in skyr during cold storage.

A review of factors influencing the quality and sensory evaluation techniques applied to Greek yogurt

Greek yogurt is one of the fastest growing products in the dairy industry. It is also known as strained yogurt, which is obtained after draining the whey. As a result of the draining process, Greek yogurt has higher total solids and lower lactose than regular yogurt. Since it is a concentrated yogurt, its sensory characteristics are different from regular yogurt. However, there is little information about factors influencing the quality of Greek yogurt and sensory evaluation techniques applied to Greek yogurt. This review aims to describe the effects of ingredients, starter cultures, processing techniques and other parameters on quality characteristics and sensory properties of Greek yogurt. In addition, advantages and limitations of novel sensory evaluation techniques applied to Greek yogurt products are discussed. In particular, we take a look at advanced techniques such as the electronic nose and electronic tongue and the benefits of these techniques with regard to Greek yogurt. This review should help the Greek yogurt industry to improve its current products and develop innovative products based on appropriate food evaluation techniques.

Quantitative Microbial Risk Assessment of Listeria monocytogenes and Enterohemorrhagic Escherichia coli in Yogurt

Listeria monocytogenes can survive in yogurt stored at a refrigeration temperature. Enterohemorrhagic Escherichia coli (EHEC) has a strong acid resistance that can survive in the yogurt with a low pH. We estimated the risk of L. monocytogenes and EHEC due to yogurt consumption with @Risk. Predictive survival models for L. monocytogenes and EHEC in drinking and regular yogurt were developed at 4, 10, 17, 25, and 36 °C, and the survival of both pathogens in yogurt was predicted during distribution and storage at home. The average initial contamination level in drinking and regular yogurt was calculated to be −3.941 log CFU/g and −3.608 log CFU/g, respectively, and the contamination level of both LM and EHEC decreased in yogurt from the market to home. Mean values of the possibility of illness caused by EHEC were higher (drinking: 1.44 × 10⁻⁸; regular: 5.09 × 10⁻⁹) than L. monocytogenes (drinking: 1.91 × 10⁻¹⁵; regular: 2.87 × 10⁻¹⁶) in the susceptible population. Both pathogens had a positive correlation with the initial contamination level and consumption. These results show that the foodborne illness risk from L. monocytogenes and EHEC due to yogurt consumption is very low. However, controlling the initial contamination level of EHEC during yogurt manufacture should be emphasized.