Heat resistance of wild Lactococcus lactis strains under a thermal gradient of cooked cheese, in milk and in mini-cheeses

Abstract

-The purpose of this study was to show the advantages and limits of cultures in milk in order to predict lactococcal growth in the manufacture of cooked cheeses. Eighteen lactococci isolated from raw milk and natural starters were genetically identified as strains of Lactococcus lactis ssp. lactis or cremoris. They were characterized in pure and cocktail cultures by their growth and their acidifying and proteolytic activities, either in milk culture at 30°C and under a thermal gradient, or in cooked mini-cheeses. A wide diversity was found not only between pure strains, but also within a same strain cocktail depending on the conditions used. Growth in mini-cheese could not be exactly reproduced in milk culture, even when conditions were as close as possible to cheese manufacture (the same starters, milk and thermal gradient). However, milk cultures were extremely helpful in comparing the heat resistance of pure strains. Selection of mesophilic starters to be used for milk maturation during cooked cheese manufacture must consider the diversity of heat resistance among strains. This diversity may be useful in understanding the role of lactococci in the final quality of cooked cheeses.Lactococci / heat resistance / milk maturation / strain selection / cooked cheese Résumé -Résistance au chauffage de lactocoques sauvages pendant le gradient thermique d'une technologie à pâte pressée cuite -Comportements dans le lait et dans les mini-fromages.Ce travail a pour objectif de montrer les avantages et les limites des tests en culture lait pour prédire le comportement des lactocoques dans les fromages à pâte pressée cuite (PPC). Dix-huit lactocoques isolés de lait crus et de levains naturels ont été typés génétiquement comme des souches différentes de Lactococcus lactis ssp. lactis ou cremoris. Ils ont été caractérisés, en culture pure et en association de souches, par leur croissance et leurs activités acidifiantes et protéolytiques, soit en culture lait à 30°C et lors d'un gradient thermique type PPC, soit en mini-fromages de type PPC. Une grande diversité a été mise en évidence entre souches, mais aussi pour une même association de souches selon les conditions utilisées. Le comportement en mini-fromage n'a pu être reproduit exactement en culture sur lait, même dans des conditions mimant au plus près celles prévalant dans les fromages (mêmes levains, lait et gradient thermique). Cependant les cultures lait ont été * Corresponding author: jeanson@poligny.inra.fr † deceased 2 S. Jeanson et al.

Lactococcus lactis strains from raw ewe’s milk samples from the PDO Ossau-Iraty cheese area: levels, genotypic and technological diversity

“…At the end of the step 4, the Δ pH caused by the Prt + strains was three to four times greater than the decrease due to Prt − strains. Moreover, the decrease in pH during this thermal gradient cycle showed that a temperature of 40°C (step 2) did not prevent expression of the acidifying activity of the tested L. lactis strains, as also observed by Jeanson et al (2003). No significant differentiation was observed between the indigenous and commercial Prt + strains.…”

“…The increase in standard deviation throughout the cheesemaking steps demonstrates the diversity existing within each strain group, especially when the temperature decreased from 37 to 28°C (step 4). Under temperature conditions similar to those of non-cooked (Casalta et al 1995) and cooked (Jeanson et al 2003) cheese, the wild L. lactis strains also exhibited a wide diversity of acidifying activity, showing that they differed in their temperature sensitivity. The most acidifying strains belonged to the L. lactis subsp.…”

Behavior of artificial listerial contamination in model Greek Graviera cheeses manufactured with the indigenous nisin A‐producing strain Lactococcus lactis subsp. cremoris M104 as costarter culture

“…Also, the fresh cheese environment differs from that of the liquid milk because it contains rennin and is a solid‐state food matrix of reduced water activity following curd cutting and whey loss. Particularly the whey loss is enhanced upon reheating of the curd required for the production of cooked hard cheeses, like Graviera cheese (Jeanson, Berthier, Grappin, & Beuvier, ; Samelis et al, ; Schvartzman, Belessi, Butler, Skandamis, & Jordan, ; Wemmenhove, Stampelou, van Hooijdonk, Zwietering, & Wells‐Bennik, ). Hence, previous findings on the growth pattern and antilisterial activity of the NisA+ cremoris strains M104 and M78 in the milk environment cannot be directly extrapolated to the cheese environment.…”

Behavior of Staphylococcus aureus in Culture Broth, in Raw and Thermized Milk, and during Processing and Storage of Traditional Greek Graviera Cheese in the Presence or Absence of Lactococcus lactis subsp. cremoris M104, a Wild, Novel Nisin A–Producing Raw Milk Isolate

“…However, detection of high numbers of NisA-P colonies on the agar plates and/or the nisin gene in mini Graviera cheeses and their LAB consortia does not necessarily mean sufficient nisin expression and/or accu mulation of active nisin by the producer strain M l04 in the cheese matrix in situ. Nisin production depends on several interacting factors, including curd heating, pH, water activity, ripening temperature, and complex LAB and other microbial interactions (17,24,36). Further studies based on advanced quantitative real-time PCR (qrt-PCR) approaches are required to elucidate and quantify nisin expression by this L. lactis subsp.…”