Microbial diversity and volatile odour-active compounds of barrelled ewes' cheese as an intermediate product that determines the quality of winter bryndza cheese

Highlights

• Barrelled ewes' cheese is a specific long-ripened product, which is not directly consumed.

• Microbial consortia were characterized by culture-based and culture-independent methods.

• Lactococci, lactobacilli and Geotrichum spp. were found dominant.

• 39 odour-active compounds or mixtures were identified by gas chromatography–olfactometry.

Abstract

Microbial diversity and volatile odour-active compounds were studied in barrelled ewes' cheese, which is a long-ripened intermediate product in the production of winter bryndza cheese, a type of PGI Slovakian bryndza cheese produced from unpasteurized ewes' milk. Microbial consortia were studied by both culture-based and culture-independent approaches. Microbiological analysis demonstrated that lactococci, lactobacilli and Geotrichum spp. were dominant, while levels of coliforms and coagulase-positive staphylococci were acceptably low. Culture-independent analysis facilitated identification of 26 prokaryotic taxa, with the majority of clones being Lactococcus lactis subsp. lactis and Streptococcus thermophilus, and 23 eukaryotic taxa, with the majority of clones being members of Galactomyces/Geotrichum group. Profiles of odour-active compounds were studied using gas chromatography – olfactometry supported by gas chromatography – mass spectrometry. Thirty-nine odour-active compounds, or mixtures in case of co-elution, were identified. Butanoic acid, ethyl butanoate, isovaleric acid, hexanoic acid, octanoic acid, decanoic acid, methyl octanoate, ethyl hexanoate, ethyl octanoate, p-cresol, and δ-decalactone were detected at high or moderate intensities in all samples. The data obtained demonstrate that barrelled ewes' cheese is a product with specific microflora and a very specific rich profile of odour-active volatile compounds.

Introduction

Long-ripened barrelled ewes' cheese is an intermediate product in the production of winter bryndza cheese, which is a type of PGI Slovakian bryndza cheese produced from unpasteurized ewes' milk. Traditional production starts at small mountain farms (salaš), where ewes' milk is processed immediately after milking by renneting at 29–31 °C for 30 min using chymosin or chymosin-identical rennet. No starter cultures are used. The curd is drained at 18–22 °C for 24 h, and then left to ripen for 3 days at 18–20 °C. After that, the curds are transported to a bryndza-producing dairy for processing. Here individual lumps are pressed, milled and loaded in layers mixed with salt in barrels, the NaCl content being 4%–6% (w/w) meaning up to 12% (w/v) in the water phase. The barrels are closed and the cheese is left to ripen at 2–10 °C for at least 2 months, with intermittent removal of foam formed on the top of the cheese. After ripening, the barrelled cheese is milled and mixed with lump cheese made from pasteurized cows' milk, at a ratio of 51:49%, forming winter bryndza, an aromatic cheese with a spreadable texture (Council Regulation, 2007, Görner, 1980, Palo and Kaláb, 1984).

Historically, the product was developed to utilize the excess ewes' lump cheese that could not be immediately processed and marketed in summer and, on the other hand, to provide a kind of ewes' cheese in the winter season. Mixing of barrelled ewes' cheese with cows' lump cheese was developed to partly neutralize a very strong odour of the former component. Recently, the product is gaining new interest as long-term ripening provides an apparently safer product, which is rich in specific odours that are absent in shorter-ripened ewes' cheeses. Long-term ripening in barrels in microaerobic conditions, at 4%–6% (w/w) NaCl and a temperature of 2–6 °C, provides conditions for the development of specific microbial consortia, producing specific profiles of volatile aroma compounds (Görner and Valík, 2004, Palo and Kaláb, 1984).

The aim of this study was to obtain, for the first time, parallel information on microbial consortia and principal odour-active compounds of barrelled ewes' cheese as an intermediate product in the production of winter bryndza cheese. For this purpose, we used effective methods developed in our previous research on May bryndza cheese, involving culture-based and culture-independent techniques to analyse the microflora (Pangallo et al., 2014), and gas chromatography – olfactometry supported by gas chromatography – mass spectrometry to analyse the odour-active compounds (Sádecká, Kolek, Pangallo, Valík, & Kuchta, 2014).