Quantitative Microbiological Analysis of Artisanal Stretched Cheese Manufacture
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applied sciences Article Quantitative Microbiological Analysis of Artisanal Stretched Cheese Manufacture Veronika Lehotová , Veronika Antálková, Alžbeta Medved’ová and L’ubomír Valík * Institute of Food Sciences and Nutrition, Faculty of Chemical and Food Technology, Slovak University of Technology in Bratislava, Radlinského 9, SK-812 37 Bratislava, Slovakia; [email protected] (V.L.); [email protected] (V.A.); [email protected] (A.M.) * Correspondence: [email protected]; Tel.: +421-2-59325-518 Abstract: To evaluate the behavior of the relevant microbial populations during stretched cheese production, quantitative microbiological analysis was performed during the critical steps of the preparation. The obtained data distributions proved statistically significant increases in all indicators, on average by 4.55 ± 0.64 log CFU/g of presumptive lactococci counts, 4.06 ± 0.61 of lactobacilli, 1.53 ± 0.57 log CFU/g of coliforms, 2.42 ± 0.67 log CFU/g of Escherichia coli, 1.53 ± 0.75 log CFU/g of yeasts and molds, and 0.99 ± 0.27 log CFU/g of presumptive Staphylococcus aureus, from the early stage of milk coagulation until curd ripening (0–24 h). The following steaming/stretching process caused reductions in viable counts with the most significant inactivation effect on coliform bacteria, including E. coli (−4.0 ± 1.0 log CFU/g). Total viable counts and yeasts and molds showed 2 and almost 3 log reduction (−2.2 ± 1.1 log CFU/g and −2.6 ± 0.9 log CFU/g), respectively. The lowest decreases in presumptive S. aureus counts were estimated at the level of −1.50 ± 0.64 log CFU/g. The counts of yeasts and molds showed the best indicatory function during the entire storage period ◦ Citation: Lehotová, V.; Antálková, V.; of vacuum-packaged cheeses at 6 C. Medved’ová, A.; Valík, L’. Quantitative Microbiological Keywords: raw milk stretched cheeses; lactic acid bacteria; coliform bacteria; Escherichia coli; Staphy- Analysis of Artisanal Stretched lococcus aureus; yeasts and molds Cheese Manufacture. Appl. Sci. 2021, 11, 2680. https://doi.org/10.3390/ app11062680 1. Introduction Academic Editor: The pasta filata cheeses share a unique processing step, when the curd is immersed in Francisco Artés-Hernández hot water or salt brine and mechanically stretched to a plastic consistency which can be formed or molded into a variety of shapes [1]. Some of this type of cheese are soft or semi- Received: 9 February 2021 soft and usually consumed fresh or after a brief period of ageing (e.g., fresh Mozzarella, Accepted: 12 March 2021 Published: 17 March 2021 low-moisture Mozzarella, and Scamorza). Others, e.g., Caciocavallo, Kashkaval, Provolone, and Ragusano, are hard or semi-hard ripened cheeses with a considerable ageing period Publisher’s Note: MDPI stays neutral before consumption [2]. The history of some Slovakian artisanal types of steamed and with regard to jurisdictional claims in stretched (pasta filata) cheeses can be observed in the Protections of Geographical Indica- published maps and institutional affil- tions (PGI) of the following original agriculture products: Slovenský oštiepok, Slovenská iations. parenica, Oravský korbáˇcik,Zázrivský korbáˇcik,and Zázrivké vojky. The records sub- mitted to the PGI designations showed cooking, molding, or stretching processes as an integral part of traditional artisanal cheese manufacture have been used probably since the 17th to mid-18th centuries in the area of present Slovakia [3–7]. Industrial pasta filata cheese production uses acid-coagulated or fermented rennet Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. curd from cow, sheep, goat, or buffalo pasteurized milk. The process technology includes This article is an open access article two essential steps: cooking and stretching. Cooking is the phase in which the curd is distributed under the terms and transferred to the hot water section of the cooker/stretcher. Typical water temperatures ◦ ◦ ◦ conditions of the Creative Commons vary between 60 C and 75 C, and cooked curd temperatures vary between 55 C and ◦ Attribution (CC BY) license (https:// 65 C. Stretching describes the mechanical treatment of the curd following cooking [2]. creativecommons.org/licenses/by/ On the other hand, traditional manufacture is based only on the artisanal cook- 4.0/). ing/stretching process of rennet curd made from raw milk [8]. Bacteria living in clusters Appl. Sci. 2021, 11, 2680. https://doi.org/10.3390/app11062680 https://www.mdpi.com/journal/applsci Appl. Sci. 2021, 11, 2680 2 of 16 and incorporated in the casein network mostly concentrate in the curd during its formation, and the rest of the microbiota leaves this step with the whey [9]. Stretching of the raw milk cheese curd in hot water substantially effects the distribution and viability of the present microbiota; however, it does not destroy it completely [10,11]. Generally, the duration of the heat treatment is controlled empirically by cheese-makers based on the curd’s melting and stretching properties [12]. In this context, many studies have pointed out the effect of the steaming and stretching phase on the microbial population of cheese curd, especially in re- lation to the presence of potentially pathogenic bacteria [13–16]. With current demands for milder food processing conditions, it is a challenge to apply a minimally required heat load while inactivating the target microorganisms sufficiently [17]. Thus, quantitative studies are needed to understand the behavior of the microbial population over the course of raw milk/cheese processing and to assess consumer exposure to undesirable microorganisms from this type of raw milk cheese. That is why this study is focused on quantitative microbiological analysis in each processing step of raw milk steamed and stretched cheeses. The other aim was to provide a set of data for comparison with similar data from practice in order to help producers to improve the overall quality of these type artisan cheeses. In an effort to support the activity of native lactic acid bacteria (LAB), the culture Fresco DVS 1010 (Christian Hansen, Hør- sholm, Denmark) consisting of Lactococcus lactis subsp. lactis, Lc. lactis subsp. cremoris and Streptococcus salivarius ssp. thermophilus was added prior to raw milk coagulation. 2. Materials and Methods 2.1. Cheese Preparation Twenty steamed and stretched cheeses were prepared under laboratory conditions following the production steps described in the PGI documents [18,19]. Eighteen cheeses were produced using raw cow’s milk (obtained from a vending machine in Bratislava, Slovak Republic) and two cheeses were produced using raw ewe’s milk (provided by a local dairy farmer). The procedure of cheese production is schematically represented in Figure1. Raw milk kept at 4–6 ◦C until the manufacture was heated to 30 ± 1 ◦C, inoculated with 1% (v/v) of overnight culture Fresco DVS 1010 (Christian Hansen, Hørsholm, Denmark), and coagulated using commercial rennet (FROMASE 220, DSM, Heerlen, Netherlands, strength 220 IMCU/mL) for 30–45 min. The small amount of the rennet (1.2 mL) enabled us to obtain the proper coagulate firmness to cut it and heat to 45 ◦C. The grains were collected with sterile cheese cloth and hung up to drip and ferment at ambient temperature (22–23 ◦C) until a pH of 5.1 (± 0.2) was reached. This step took 24 h, generally. Stretching of the curd was performed manually in hot water for 5 min with internal temperatures of 61–63 ◦C recorded using a data logger (Ebro EBI 100-T100, Ingolstadt, Germany). Temperature data has been processed with Winlog.pro 2.66 software (Xylem Analytics Germany Sales GmbH & Co., KG Ebro, Ingolstadt, Germany). For the storage tests, performed at 6 ◦C for five weeks, the final products were vacuum packed. Appl. Sci. 2021, 11, 2680 3 of 16 24 h [22]; presumptive counts of Staphylococcus aureus (pSTA) on selective Baird-Parker medium (Merck KGaA, Darmstadt, Germany) incubated aerobically at 37 ± 1 °C for 48 h [23]; total viable count (TVC) was determined on Plate Count Agar (Biokar Diagnostics, Beauvais, France) incubated aerobically at 30 ± 1 °C for 72 h [24]; and yeasts and moulds Appl. Sci. 2021, 11, 2680 3 of 16 (Y&M) on Yeast Glucose Chloramphenicol agar (Biokar Diagnostics, Beauvais, France) incubated aerobically at 25 ± 1 °C for 72–120 h [25]. Raw milk 1% of LAB culture, rennet Coagulation (30 °C/30–45 min) Cutting Stirring Cheese cloth Whey Curd Fermentation (22–23 °C/~24 h) Fermented curd (pH 4.9–5.3) Steaming and stretching (61–63 °C/5 min) Fresh stretched cheese Packaging and storage (in vacuum; 6 °C/5 weeks) Figure 1. FlowFigure diagram 1. Flow of thediagram artisanal of the manufacture artisanal manufacture process applied process in the applied preparation in the preparation of steamed andof steamed stretched cheeses. The Fresco DVSand stretched 1010 (Christian cheeses. Hansen, The Fresco Hørsholm, DVS 1010 Denmark) (Christi wasan Hansen, used as LABHørsholm, culture De usednmark) in this was work. used as LAB culture used in this work. 2.2. Microbiological Analysis 2.3. Additional Analysis Samples of raw milk, curd after coagulation, curd before and after the stretching During cheese-makingprocess, and cheesetrials, the at different pH and titratable storage times acidity (1, were 2, 3, 4, determined. and 5 weeks) The were pH taken, and 10- value of curd foldbefore dilutions the stretching according proc toess ISO was 6887-5:2010 measured [20 using] were a prepared.WTW 720 ThepH-meter following microbial (Inolab, Weilheim,groups Germany) were determined: with a Sen the Tix presumptive 81 glass electrode counts (WTW of lactococci GmbH, (pLC) Weilheim, were determined on Germany). TheM17 titratable agar (Biolife,acidity (°SH) Milan, was Italy) manu accordingally determined to EN ISO by 15214 visual [21 titration], with with aerobic incubation 0.25 M NaOH ataccording30 ± 0.5 to◦C Calamarifor 48 h; et lactobacilli al., 2016 [26].