Lacticaseibacillus rhamnosus Impedes Growth of Listeria spp. in Cottage Cheese through Manganese Limitation

Acidification and nutrient depletion by dairy starter cultures is often sufficient to prevent outgrowth of pathogens during post-processing of cultured dairy products. In the case of cottage cheese, however, the addition of cream dressing to the curd and subsequent cooling procedures can create environments that may be hospitable for the growth of Listeria monocytogenes. We report on a non-bacterio-cinogenic Lacticaseibacillus rhamnosus strain that severely limits the growth potential of L. monocytogenes in creamed cottage cheese. The main mechanism underlying Listeria spp. inhibition was found to be caused by depletion of manganese (Mn), thus through competitive exclusion of a trace element essential for the growth of many microorganisms. Growth of Streptococcus thermophilus and Lactococcus lactis that constitute the starter culture, on the other hand, were not influenced by reduced Mn levels. Addition of L. rhamnosus with Mn-based bioprotective properties during cottage cheese production therefore offers a solution to inhibit undesired bacteria in a bacteriocin-independent fashion.

Dairy lactococcal and streptococcal phage–host interactions: an industrial perspective in an evolving phage landscape

ABSTRACT Almost a century has elapsed since the discovery of bacteriophages (phages), and 85 years have passed since the emergence of evidence that phages can infect starter cultures, thereby impacting dairy fermentations. Soon afterward, research efforts were undertaken to investigate phage interactions regarding starter strains. Investigations into phage biology and morphology and phage–host relationships have been aimed at mitigating the negative impact phages have on the fermented dairy industry. From the viewpoint of a supplier of dairy starter cultures, this review examines the composition of an industrial phage collection, providing insight into the development of starter strains and cultures and the evolution of phages in the industry. Research advances in the diversity of phages and structural bases for phage–host recognition and an overview of the perpetual arms race between phage virulence and host defense are presented, with a perspective toward the development of improved phage-resistant starter culture systems.

High-throughput pH monitoring method for application in dairy fermentations

Optimization of dairy fermentation processes often requires multiplexed pH measurements over several hours. The method developed here measures up to 90 samples simultaneously, where traditional electrode-based methods require a lot more time for handing the same number of samples. Moreover, the new method employs commonly used materials and can be used with a wider range of fluorescence readers than commercial 96-well plates with optical pH sensors. For this application, a milk-like transparent medium is developed that shows acidification properties with dairy starters that are similar to milk. Combination of this milk-like medium and 3 fluorescent indicators allow precise measurements of pH in a range of 4·0–7·0. The new method showed much higher throughput compared to the benchmark electrode systems while being as accurate, as shown by successful application for a comparison of various dairy starter cultures and for optimizing the inoculation rate.

Chemical composition and nutritional quality of short-ripened rennet cheeses produced by traditional methods

The aim of the study was to determine the variability in the chemical composition and nutritive value parameters of smoked and unsmoked short-ripened rennet cheeses and unsmoked long-ripened rennet cheeses produced by traditional methods. The raw material for the production of short-ripened cheeses was pasteurized cows’ milk obtained from a dairy, whereas the long-ripened cheeses were manufactured from raw cows’ milk obtained from the producer’s farm. All three varieties of cheese examined were produced with commercial dairy starter cultures. The material for the study was collected in winter, directly at the producers’ retail outlets in southern and eastern Poland. The basic chemical composition was determined according to the Polish Standards, whereas the amino acid profiles of proteins from the cheeses were determined by ion-exchange chromatography. The result analysis revealed significant differences between the different varieties of cheese in terms of their water content, ranging from 32.2% to 42.1%, as well as protein content, which varied from 25.6% to 31.6%. Fat levels ranged between 22.2% and 24%, whereas total ash content amounted to 5.1-5.8%. The significantly highest salt content was found in unsmoked short-ripened cheeses. In all three cheese varieties, the total exogenous amino acid content was comparable, ranging from 46.17 g to 47.36 g/100 g protein, and that of endogenous amino acids varied from 52 g to 53 g/100 g protein. The biological value of proteins was determined by calculating to the chemical score (CS), as described by Mitchell and Block, and the essential amino acid index (EAAI), as described by Oser. A comparison of the results with the standard chicken egg white proteins showed that the limiting amino acids for all varieties of cheese were methionine and cysteine. On the other hand, a comparison with the FAO/WHO-suggested pattern of amino acid requirements (1991) for all age groups over 1 year of age showed that the limiting amino acids were methionine and cysteine in smoked and unsmoked short-ripened cheeses, and treonine in long-ripened cheeses. Considering the chemical indices, such as CS and EAAI, it may be concluded that the traditional rennet cheeses produced in southern and eastern Poland have a favourable amino acid composition of proteins and a high nutritive value..

Aerobic Respiration in Lactic Acid Bacteria

Lactic acid bacteria (LAB) are used as food-grade microorganisms for production of a variety of fermented milk products. They are also the most common probiotic organisms used for making functional foods. Lactic acid bacteria are well known for their fermentative metabolism wherein they convert simple carbohydrates to organic acids and other end products. Fermentation helps the bacteria to generate ATP required for various cellular activities via substrate level phosphorylation reaction. Fermentation results in incomplete oxidation of substrate and hence is an inefficient process with a low ATP yield. However, some LAB are genetically capable of activating an auxiliary respiratory metabolism in which a quinol oxidase serves as the final electron acceptor and high ATP production is achieved due to oxidative phosphorylation. The respiratory process is associated with high biomass production of LAB and more robust bacterial cells, which are essentially required for manufacture of high viability starter culture. This chapter explores LAB's current and future applications in dairy starter cultures.

Lactococcus lactisDiversity in Undefined Mixed Dairy Starter Cultures as Revealed by Comparative Genome Analyses and Targeted Amplicon Sequencing ofepsD

ABSTRACTUndefined mesophilic mixed (DL) starter cultures are used in the production of continental cheeses and contain unknown strain mixtures ofLactococcus lactisand leuconostocs. The choice of starter culture affects the taste, aroma, and quality of the final product. To gain insight into the diversity ofLactococcus lactisstrains in starter cultures, we whole-genome sequenced 95 isolates from three different starter cultures. Pan-genomic analyses, which included 30 publically available complete genomes, grouped the strains into 21L. lactissubsp. lactisand 28L. lactissubsp.cremorislineages. Only one of the 95 isolates grouped with previously sequenced strains, and the three starter cultures showed no overlap in lineage distributions. The culture diversity was assessed by targeted amplicon sequencing usingpurR, a core gene, andepsD, present in 93 of the 95 starter culture isolates but absent in most of the reference strains. This enabled an unprecedented discrimination of starter cultureLactococcus lactisand revealed substantial differences between the three starter cultures and compositional shifts during the cultivation of cultures in milk.IMPORTANCEIn contemporary cheese production, standardized frozen seed stock starter cultures are used to ensure production stability, reproducibility, and quality control of the product. The dairy industry experiences significant disruptions of cheese production due to phage attacks, and one commonly used countermeasure to phage attack is to employ a starter rotation strategy, in which two or more starters with minimal overlap in phage sensitivity are used alternately. A culture-independent analysis of the lactococcal diversity in complex undefined starter cultures revealed large differences between the three starter cultures and temporal shifts in lactococcal composition during the production of bulk starters. A better understanding of the lactococcal diversity in starter cultures will enable the development of more robust starter cultures and assist in maintaining the efficiency and stability of the production process by ensuring the presence of key bacteria that are important to the characteristics of the product.