Characterization of Lactococcus strains isolated from artisanal Oaxaca cheese
Introduction
Oaxaca cheese is one of the most popular Mexican cheeses with a production of about 14,700 tons (SIAP, 2016); it has becoming increasingly produced in the United States and other countries as well. It is considered a soft pasta filata cheese (Caro et al., 2014) and its making process involves curd acidification (until pH of 5.3) kneading in hot water (72 °C) and stretching, forming long and thin strips of curd, which are cooled in chilled water, salted, cut into 0.2–2 kg segments and moulded with a ball shape (Caro et al., 2011; De Oca-Flores, Castelán-Ortega, Estrada-Flores, & Espinoza-Ortega, 2009). These authors described the main quality attributes of this cheese, such as a fibrous structure, acidic taste, mild flavour, high creaminess, and good meltability. Two types of Oaxaca cheeses are recognized: Those produced on medium or large factories using pasteurized milk acidified either with starters –not specifically designed for this cheese– or organic acids (Colín-Cruz, Dublán-García, Espinoza-Ostega, & Domínguez Lópéz, 2012), and those manufactured in small factories with naturally fermented raw milk (Caro et al., 2011).
Soft cheeses should be produced with pasteurized milk for health reasons. In order to maintain the sensorial properties of artisanal cheese, a suggested approach in pasteurized milk cheeses is to select indigenous microorganisms for the design of specific starter cultures (Cogan et al., 1997). Lactococcus strains have been widely used as starters; selected indigenous strains intended to be used as starters should produce acid quickly and specific flavour and texture (Leroy & De Vuyst, 2004). Moreover, they should not carry virulence factors or other risk factors such as antibiotic resistance, high amino acid-decarboxylase activity, etc. Furthermore, they must be identified and characterized for their technological properties (Randazzo, Caggia, & Neviani, 2009).
The aim of this study was to identify and characterize Lactococcus spp. strains from artisanal raw-milk Oaxaca cheeses in order to select potential candidate strains for the design of a suitable starter culture to be used in pasteurized-milk Oaxaca cheese production.
Section snippets
Sampling and LAB isolation
Samples of fresh milk (FM) at arrival to the cheese factory, acidified milk (AM) before renneting, acidified curd (AC) before kneading, and fresh cheese (CH) after salting were collected from two artisanal raw-milk cheese factories (Tulancingo Valley, Mexico) on three working days. FM and AM (250 ml), AC (500 g), and CH (500 g) samples were transported into sterile screw-capped flasks or sterile containers at 4 °C to the laboratory and analysed within 4 h after sampling. Representative portions
LAB population
M17 and MRS agar LAB counts are shown in Table 2. FM presented relatively high counts, which could be attributed to temperature abuse of milk before processing. De Oca-Flores, CastelÁn-Ortega, Estrada-Flores, and Espinoza-Ortega (2009) have reported temperature and acidity of milk at arrival to artisanal Oaxaca dairy factories of 18–28 °C and 17-22°D. The highest LAB counts (p < 0.05) were found in both AM and AC for both M17 and MRS media. CH showed lower LAB mean counts than AM, although
Conclusions
Lactococcus lactis subsp. lactis is the predominant species in raw-milk Oaxaca cheese. Significant genotypic and phenotypic differences among the studied L. lactis strains suggest high interspecies variability. Six strains are proposed as potential starter culture for pasteurized milk Oaxaca cheese mainly due to their high acidifying activity and antibiotic susceptibility. Among them, 1002 strain, due to its higher production of 2-methylpropanal and 3-methylbutanal, would be recommended to
CRediT authorship contribution statement
I. Caro: Conceptualization, Methodology, Investigation, Writing - review & editing, Funding acquisition. E.J. Quinto: Writing - original draft, Writing - review & editing. L. Fuentes: Investigation. V. Alessandria: Methodology, Resources. L.S. Cocolin: Methodology, Resources. M.P. Redondo-del-Río: Writing - original draft. B. Mayo: Methodology, Resources. A.B. Flórez: Methodology, Resources. J. Mateo: Conceptualization, Resources, Writing - original draft, Writing - review & editing, Funding
Declaration of competing interest
None.
Acknowledgements
Lucía Fuentes thanks the Secretaría de Educación Pública, México, for the fellowship granted. The authors thank Mr. Julio Armando Claro for his technical assistance.
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