1Institute of Food Commodity Sciences, West Pomeranian University of Technology, Szczecin, Poland
Yoghurts are one of the most popular dairy products on the food market due to their biological and nutritional value [Brodziak and Król 2016Brodziak, A., Król, J. (2016). Mleczne napoje fermentowane – właściwości prozdrowotne [Milk fermented beverages – health-promoting properties]. Przem. Spoż., 10, 22–28 [in Polish].]. According to the definition adopted by FAO/WHO, yogurt is a fermented milk product obtained through acidifying milk with yoghurt bacteria cultures (Lactobacillus delbruecckii ssp., bulgaricus and Streptococcus salivarius ssp. thermophilus). In case of new generation yoghurts, also Lactobacillus acidophilus and Bifidobacterium strains are added [Nowak et al. 2010Nowak, A., Śliżewska, K., Libudzisz, Z., Socha, J. (2010). Probiotki – Efekty Zdrowotne [Probiotics – Health effects]. Żyw. Nauka. Tech. Jakość, 4(71), 20–36 [in Polish].].
At the moment of consumption, 1cm3 of yogurt should approximately contain 1×107 of live microorganisms [Kapka-Skrzypczak et al. 2012Kapka-Skrzypczak, L., Niedźwiedzka, J., Wojtyła, A., Kruszewski, M. (2012). Probiotyki i prebiotyki jako aktywny składnik żywności funkcjonalnej [Probiotics and prebiotics as an active ingredient in functional foods]. Pediatriv Endoc. Diab. and Met. 18(2), 79–83 [in Polish].].
Health-related qualities and organoleptic features of natural yoghurts like palatability, consistency or aroma reflect their quality and play a leading role in the choice of product by a customer.
The consumers find yoghurts to be a significant part of the diet, as the annual consumption of yoghurts is estimated at 7–8 kg per individual. Fruit yoghurts have the highest share in consumption, while the faster growth rate of consumption is observed in segment of potable and natural yoghurts, which is related to the changing requirements and expectations of consumers [Rój and Przybyłowski 2012Rój, A., Przybyłowski, P. (2012). Ocena barwy jogurtów naturalnych [Evaluation of the color of natural yoghurts]. Bromat. Chem. Toksykol., XLV, (3), 813–816 [in Polish].]. Natural yoghurts are identified as a “healthier” alternative to cream [Wichrowska and Wojdyła 2014Wichrowska, D., Wojdyła, T. (2014). Ocena sensoryczna i fizykochemiczna wybranych jogurtów naturalnych i ekologicznych [Sensory and physicochemical evaluation of selected natural and organic yoghurts]. Inż. Ap. Chem., 53(6), 421–423 [in Polish].].
Production of fermented milk beverages is a complex process and even slight inadvertences can cause product defects. Proper selection of raw material and lactic acid bacteria, use of top-quality additives, adherence to the technological regime and ensuring the highest hygiene standards are crucial requirements that guarantee a high organoleptic and physicochemical characteristics of fermented milk beverages [Baranowska et al. 2010Baranowska, M., Bohdziewicz, K., Staniewski, B. (2010). Jogurt z dodatkiem preparatu enzymatycznego transglutaminazy Saprona TG1Y [Yogurt with the addition of Sapron TG1Y transglutaminase enzyme preparation]. Przeg. Mlecz., 5, 12–18 [in Polish].]. Therefore, it is necessary to understand and control the factors that determine yoghurt quality, vital to meet the expectations, needs and requirements of consumers.
For this reason, the aim of our study was to assess the sensory, physicochemical and microbiological quality of natural yoghurts offered by the three largest producers located in Szczecin city market.
The research material included natural yoghurts purchased in retail trade in Szczecin. The yoghurts tested in our research were produced by the three largest producers of fermented beverages located in the Polish market. The research material consisted of 90 yoghurts samples (30 separate packages of each producer) with a net weight of 150 to 180 g: brand I (1.5% fat), brand II (3.0%), brand III (3% fat). All yogurts were purchased with a 14-day shelf-life date. Yoghurts were stored in 4°C, in a refrigeration cabinet. From of the each purchased batch of yoghurts of a specific brand we took 10 packages for physicochemical assessment, 10 packages for sensory tests and 10 packages for microbiological analysis. Microbiological and physicochemical tests were performed the next day after purchasing.
The following physiochemical parameters were determined in yogurt samples: pH, titratable acidity, colour and basic chemical composition. The colour assessment was carried out using the MiniScan XE Plus 45/0 camera, with a 31.8 mm diam. measuring port. We referred to CIE L* a* b* scale, D65 illuminate and standard 10°C observer [CIE 1976CIE (1976). Colourimetry: Official recommendations of the international commission on illumination. Publication CIE 15 (E-1.3.1) Boreau Central de la Commission Internationale De L’Eclairage, Paris.] were used. The pH measurement was performed using a combined glass electrode and a Cyber Scan pH meter. Evaluation of total acidity in Soxhlet-Henkel grades was performed using the standard method [PN-75/A-86130 1975PN-75/A-86130 (1975). Mleko i przetwory mleczarskie. Napoje mleczne. Metody badań. [Polish standard. Milk and dairy products. Milk drinks. Test methods]. [in Polish].]. The acidity of examined products was determined at room temperature, and the final result was the arithmetic mean of two parallel measurements.
Additionally, the chemical composition of yoghurts was determined: dry matter content, sugars, total protein and fat [Krełowska-Kułas 1993Krełowska-Kułas, M. (1993). Badanie jakości produktów spożywczych [Research on the quality of food products]. PWE, Warszawa [in Polish].]. The Kjeldahl method was used to determine the total protein content, and the fat content was measured with the Röse-Gottlieb method. The Bertrand method was used to determine the content of sugars (lactose). The dry matter content was determined by drying at 130°C.
Our study was also focused on microbiological evaluation of natural yoghurts. For the microbiological tests, 1 gram of yogurt was transferred to 9 ml of sterile saline. The suspension was shaken, and decimal dilutions were made. One ml of suspension from each of the dilutions was inoculated into the following media: MacConkey Agar, XLD Medium (to identify Gram-negative Enterobacteriaceae bacteria), Baird-Parker Agar (to identify Staphylococcus bacteria), Edwards Medium (to identify streptococci) and MRS Agar (to isolate Lactobacillus spp.). The cultures were incubated under aerobic conditions at 37°C for 24 hours. After that time the colonies were counted with consideration to the previously made decimal dilutions (PN-93/A-860334-08 1993PN-93/A-860334-08 (1993). Mleko i przetwory mleczarskie. Badania mikrobiologiczne. Wykrywanie bakterii z grupy coli i oznaczanie NPL [Polish standard. Milk and dairy products. Microbiological testing. Detection of coliforms and NPL determination]. [in Polish]., PN-93/A-860334-15 1993PN-93/A-860334-15 (1993). Mleko i przetwory mleczarskie. Badania mikrobiologiczne. Oznaczanie liczby charakterystycznych drobnoustrojów [Milk and dairy products. Microbiological testing. Determination of the number of characteristic microorganisms]. [in Polish].). The number of yeast and mould cells was determined in reference to the Polish Standard PN-93/A-860334-07 [1993PN-93/A-860334-07 (1993). Mleko i przetwory mleczarskie. Badania mikrobiologiczne. Pleśnie i drożdże – oznaczanie metodą płytkową w temperaturze 25°C [Milk and dairy products. Microbiological testing. Molds and yeasts – lamellar determination at 25°C]. [in Polish].]. The results of all microbiological determinations were expressed as CFU·g–1.
Sensory analysis was performed with a five-point scale according to Pieczonka [1995Pieczonka, W. (1995). Mleko i przetwory mleczne. Standaryzacja jakości i metody badań [Milk and milk products. Standardization of quality and test methods]. Skrypt. Akademia Rolnicza, Kraków [in Polish].]: 1 point-disqualifying rate, 5 points-very good rate. The appearance, colour, consistency, smell and taste of natural yoghurts were evaluated. Sensory analysis was carried out by a five-person team verified in terms of sensory sensitivity, according to PN-ISO-4121 [1998PN-ISO-4121 (1998). Analiza sensoryczna – Metodologia – Ocena produktów żywnościowych przy użyciu metod skalowania [Polish standard – Sensory analysis – Methodology – Evaluation of food stuffs by using calibration methods]. [in Polish].].
Statistical analysis of the results was performed with the Statistica 10 software. We calculated the mean values and standard deviation and carried out a one-way analysis of variance. The Duncan test was used to estimate the differences between the experimental groups.
The results of organoleptic evaluation did not show statistically significant influence of the tested yogurt brand on appearance, colour and consistency. In all the three cases, the tested beverages were rated with the highest score: 5.0 points (Table 1). All yoghurts were characterized by white colour, and their consistency was uniform and dense, which proves a high quality of evaluated yoghurts. The results stay in line with opinion of Shukla and Jain [1991Shukla, F.C., Jain, S.C. (1991). Effect of additives on quality of yoghurt. Ind. J. Dairy Sci., 44, 130–139.] and Fortuna et al. [2001Fortuna, T., Gołębiowska-Gajda, A., Juszczak, L. (2001). Sensoryczne i reologiczne właściwości jogurtów naturalnych [Sensory and rheological properties of natural yoghurts]. Techn. Żyw., 13, 5–16 [in Polish].] and, for whom a good yogurt is characterized by a smooth surface, a firm clot and a consistency with no clumps felt in mouth.
The tested yoghurts differed in taste and aroma (Table 1). Significantly the best taste (P ≤ 0.01) was found in brand II (4.83 points) as compared to brand III (4.65 points) and brand I (3.92). Fat content was related to the taste – the more fat there was, the higher was the taste rating (Tables 1 and 3). Nevertheless, taste of all yoghurt samples was described as pure, aromatic and slightly sour. In fragrance, the highest note (P ≤ 0.01) was given to brand III (4.65 pts), while the other two yoghurts (brand I and II) were rated identically (4.33 pts). It should be emphasized that the examined beverages were characterized by an aromatic and characteristic fragrance.
Yoghurts owe their characteristic taste and smell primarily to the products of lactic fermentation such as lactic acid, acetaldehyde and acetoin [Biliaderis et al. 1992Biliaderis, C.G., Khan, M.M., Blank, G. (1992). Rheological and sensory properties of yoghurt from skim milk and ultrafiltrated retentates. Inter. Dairy J., 2, 311–323., Kowal and Libudzisz 2000Kowal, K., Libudzisz, Z. (2000). Mikrobiologia techniczna [Technical microbiology]. Wyd. Pol. Łódzkiej, Łódź [in Polish].].
The examined yoghurts showed no variations in taste and smell, and these parameters in all the tested samples were appropriate.
Titratable acidity of the examined fermented beverages ranged from 46.56 to 52.68°SH and there were no statistically significant differences between the samples (Table 2). The values we measured were slightly higher than those required by the Polish Standard PN-83/86061 [2006PN-A-86061 (2006). Mleko i przetwory mleczne. Mleko fermentowane. [Milk and milk products. Fermented milk]. [in Polish].] (from 35 to 48°SH). In similar studies performed by Wichrowska and Wojdyła [2014Wichrowska, D., Wojdyła, T. (2014). Ocena sensoryczna i fizykochemiczna wybranych jogurtów naturalnych i ekologicznych [Sensory and physicochemical evaluation of selected natural and organic yoghurts]. Inż. Ap. Chem., 53(6), 421–423 [in Polish].], the authors assessed physicochemical quality of selected natural yoghurts of various producers and found that all the tested yoghurts represented a correct titratable acidity (from 38.1 to 44.25°SH) after 7 days of storage.
The lowest titratable acidity was found in the product with 1.5% fat content (brand I), and the highest in products with 3% fat content (brand II and III) (Table 2 and 3). Similar results were obtained by Bonczar and Wszołek [2002Bonczar, G., Wszołek, M. (2002). Charakterystyka jogurtów z mleka owczego o normalizowanej zawartości tłuszczu [Characteristics of yoghurt from sheep's milk with normalized fat content]. Żyw. Nauka. Techn. Jakość, 1(30), 109–115 [in Polish].], who showed that after fourteen days of storage the highest titratable acidity characterized the fat-free yoghurts. Change in acidity of yogurt results from the presence of yoghurt bacteria in the final product, which show diversified acidifying, proteolytic and lipolytic activity. During the maturing and storage of yoghurts, the bacteria use milk compounds, however, the rates and ranges differ. As the shelf life of tested products was only two weeks, we expected that the acidity of yogurt will increase during the storage, which would be consistent with the literature reports [Kisza et al. 1993Kisza, J., Domagała, J., Wszołek, M, Kołczak, T. (1993). Yoghurts from sheep milk. Acta Acad. Agric. et Tech. Olsten., 25, 75 [in Polish]., Imhof et al. 1994Imhof, R., Glättli, H., Bosset, J.O. (1994). Volatile organic aroma compounds produced by thermophilic and mesophilic mixed strain dairy starters cultures. Lebensm. – Wiss. Technol., 27, 442–449., Bonczar and Wszołek 1997Bonczar, G., Wszołek, M. (1997). Jakość i trwałość kefiru i jogurtu produkowanego z owczego mleka [The quality and durability of kefir and yoghurt produced from sheep's milk]. Żyw. Techn. Jakość, 1(10), 61–68 [in Polish]., Sady et al. 2007Sady, M., Domagała, J., Grega, T., Kalicka, D. (2007). Wpływ czasu przechowywania na mikroflorę jogurtów z dodatkiem nasion amarantusa i ziaren owsa [The influence of storage time on the yogurt microflora with the addition of amaranth seeds and oat grains]. Żyw. Nauka. Techn. Jakość, 6(55), 242–250 [in Polish]., Cais-Sokolińska et al. 2009Cais-Sokolińska, D., Danków, R., Pikul, J. (2009). Dynamika zmian kwasowości jogurtu z dodatkiem produktów zbożowych podczas chłodniczego przechowywania [The dynamics of yogurt acidity changes with the addition of cereal products during refrigerated storage]. Nauka Przyr. Tech., 3(4), 1–5 [in Polish]., Mituniewicz-Małek et al. 2010Mituniewicz-Małek, A., Dmytrów, I., Pilarczyk, R., Brajer, A. (2010). Ocena wybranych cech jakościowych jogurtów o podwyższonej zawartości tłuszczu w czasie chłodniczego przechowywania [Evaluation of selected quality traits of yoghurts with increased fat content during refrigerated storage]. Przegl. Mlecz., 6, 12–18 [in Polish]., 2013Mituniewicz-Małek, A., Dmytrów, I., Balejko, J., Ziarno, M. (2013). Komercyjne kultury probiotyczne Lactobacillus sp. (Lb. paracasei, Lb. casei i Lb. acidophilus) w napojach fermentowanych z mleka koziego [Commercial probiotic cultures of Lactobacillus sp. (Lb. paracasei, Lb. casei and Lb. acidophilus) in beverages fermented from goat's milk]. Żyw. Nauka. Tech. Jakość, 3(88), 99-110 [in Polish]., Wichrowska and Wojdyła 2014Wichrowska, D., Wojdyła, T. (2014). Ocena sensoryczna i fizykochemiczna wybranych jogurtów naturalnych i ekologicznych [Sensory and physicochemical evaluation of selected natural and organic yoghurts]. Inż. Ap. Chem., 53(6), 421–423 [in Polish].]. According to Mituniewicz-Małek et al. [2013Mituniewicz-Małek, A., Dmytrów, I., Balejko, J., Ziarno, M. (2013). Komercyjne kultury probiotyczne Lactobacillus sp. (Lb. paracasei, Lb. casei i Lb. acidophilus) w napojach fermentowanych z mleka koziego [Commercial probiotic cultures of Lactobacillus sp. (Lb. paracasei, Lb. casei and Lb. acidophilus) in beverages fermented from goat's milk]. Żyw. Nauka. Tech. Jakość, 3(88), 99-110 [in Polish].] changes in the acidity of acidified milk during refrigerated storage are caused by fermentative activity of microorganisms included in inoculants, which in 4°C still digest lactose, although much slower than in the optimal temperature recommended for the growth of lactic acid bacteria.
The observed pH values were in accordance with the standard. Brand I yoghurt had significantly (P ≤ 0.01) higher pH (4.43) than yoghurts of the other two brands (4.48 and 4.49 for II and III, respectively) (Table 2). The pH values measured in our study were similar to those observed by Wichrowska and Wojdyła  in yoghurts from the retail trade after a weekly storage period.
The brightness of colour (L*) in tested natural yogurt samples ranged from 93.77 to 94.34, and our statistical analysis showed no significant differences in this parameter. According to Rój and Przybyłowski [2012Rój, A., Przybyłowski, P. (2012). Ocena barwy jogurtów naturalnych [Evaluation of the color of natural yoghurts]. Bromat. Chem. Toksykol., XLV, (3), 813–816 [in Polish].], natural yoghurts show the differences in brightness parameter (L*) depending on the declared fat content, which however has not been confirmed in this study. The a* parameter (change in colour from green to magenta) took negative values for all the examined yogurt samples, ranging from –1.50 to –2.21. The b* parameter (change in colour from blue to yellow) took positive values for all the tested samples of yoghurts in range from 9.76 to 11.03.
The brand I yoghurt was characterized by the highest protein content (5.13%) as compared to brand II (4.72%) and brand III (4.65%) (Table 3). Our measures of protein content did not differ from those given by yoghurt producers (Table 4), and were in accordance with the Polish Standard (PN-A-86061: 2002/Az1).
The percentage of fat in evaluated yoghurts was close to that declared by yoghurt producers on the product labels (Table 4). The largest amount of fat was found in brand II yogurt (2.65%), and the smallest fat content was found in brand I sample (1.43%) which most probably was the reason to the fact that the taste of this yogurt was worse when compared to the other two. According to Żbikowska and Żbikowski [1995Żbikowska, A., Żbikowski, Z. (1995). Znaczenie i właściwości białek w mlecznych napojach fermentowanych [Importance and properties of proteins in fermented milk beverages]. Przeg. Mlecz., 11, 304–311 [in Polish].], the higher fat content increases the final clot volume and limits syneresis, which results in the consistency of yogurt being more attractive to the consumer. In the tested yoghurts, the texture was evaluated very highly regardless of the fat content (5 points). We found a decrease in sugar content in the tested yoghurts (Table 3 and 4). The highest sugar content was measured in brand I and III yoghurts (4.57% and 4.26% respectively), in comparison to brand II yoghurt (2.40%). Fluctuations in sugar content in the analysed yoghurts can be explained by the fermentative activity of microorganisms included in the inoculants, which at 4°C still digest lactose [Cais-Sokolińska et al. 2009Cais-Sokolińska, D., Danków, R., Pikul, J. (2009). Dynamika zmian kwasowości jogurtu z dodatkiem produktów zbożowych podczas chłodniczego przechowywania [The dynamics of yogurt acidity changes with the addition of cereal products during refrigerated storage]. Nauka Przyr. Tech., 3(4), 1–5 [in Polish].]. The content of dry matter in the tested yoghurts ranged from 12.9% to 14.5% (Table 3).
Microbiological analyses were made the next day after the purchase, and the yogurts were about two weeks of shelf life. The highest content of Lactobacillus bacteria was recorded in brand III yoghurt: 9.8×105 CFU · g–1. In the other two brands, the content of Lactobacillus was similar and ranged from 9.3×105 CFU · g–1 to 9.4×105 CFU · g–1 (Table 5). The number of Streptococcus bacteria was also determined in the examined yogurts. The number of these bacteria present in the samples right after the purchase was from 4.48×103 CFU · g–1 to 5.8×103 CFU · g–1. No infections with moulds, yeasts and Enterobacteriaceae bacteria were observed.
Fermented milk beverages should be characterized with an appropriate content of live typical (and possibly additional) cells in the final shelf-life. The requirements of FAO/WHO and FIL/IDF expect their minimal concentration at 107 CFU · g–1 [Kudełka 2005Kudełka, W. (2005). Charakterystyka mlecznych napojów fermentowanych w Unii Europejskiej oraz w Polsce [Characteristics of milk fermented beverages in the European Union and in Poland]. Zesz. Naukowe, 678, 149–160 [in Polish].]. In all the tested brands, there was no normative number of live cells noted two weeks before the expiration date. This result is not consistent with the studies on the quality of natural yoghurts in the Warsaw market by Orzechowska et al. [2002Orzechowska, K., Molska, I., Frelik, I. (2002). Jakość mikrobiologiczna jogurtu i biojogurtu na rynku warszawskim w latach 2000/2001 [Microbiological quality of yoghurt and bio-yoghurt on the Warsaw market in 2000/2001]. Przegl. Mlecz., 9, 404-406 [in Polish].]. These authors report that the number of Lactobacillus bacteria below the normative value of 107 was found in 40% of the tested natural yoghurts, and in Streptococcus bacteria, only in 10% of the products. Sosnowska and Kurek [2012Sosnowska, B., Kurek, M. (2012). Analiza jakościowa wybranych jogurtów naturalnych dostępnych na rynku lubelskim [Analysis of Quality of Chosen Natural Yoghurts Available at the Market of Lublin]. Towaroznawcze problemy jakości, 2, 54–56 [in Polish].], analysing the quality of natural yoghurts in the Lublin market, noted that all the tested products contained the required number of live cells typical for the yogurt microflora, and in many products this number exceeded the required quantity by 2–3 orders of magnitude. Also Pluta et al. [2001Pluta, A., Berthold, A., Ziarno, M., Molska, I., Frelik, I., Nowosielska, R., Orzechowska, K. (2001). Jakość mikrobiologiczna jogurtów krajowych [Microbiological quality of domestic yoghurts]. Przem. Spoż., 4, 37–39 [in Polish].] analysing the quality of natural yoghurts on the domestic market in 1995–2001 showed that in 97% of the tested samples the characteristic microflora occured in the number of 107. Kycia and Krysiński [2014Kycia, K., Krysiński, C. (2014). Jakość mikrobiologiczna i higieniczna rynkowych jogurtów z mleka koziego w kontekście ich właściwości terapeutycznych [Microbiological and hygienic quality of market yoghurts from goat's milk in the context of their therapeutic properties]. Probl. Hig. Epidemiol., 95(1), 186–191 [in Polish].], who assessed the microbiological and hygienic quality of goat milk yogurts also showed that 84.3% of the tested samples met the requirements for the number of typical yoghurt microorganisms, specified in the Codex Alimentarius standard, throughout the shelf life of the products.
The viability of lactic bacteria in fermented and cool-stored milk depends on many factors, including the time and temperature of fermentation, storage conditions of the final product, its acidity, content of dry matter and sugars, access to bacterial nutrients, presence of oxygen, and the type and species and strain of lactic acid bacteria used in fermentation Mituniewicz-Małek et al. [2013Mituniewicz-Małek, A., Dmytrów, I., Balejko, J., Ziarno, M. (2013). Komercyjne kultury probiotyczne Lactobacillus sp. (Lb. paracasei, Lb. casei i Lb. acidophilus) w napojach fermentowanych z mleka koziego [Commercial probiotic cultures of Lactobacillus sp. (Lb. paracasei, Lb. casei and Lb. acidophilus) in beverages fermented from goat's milk]. Żyw. Nauka. Tech. Jakość, 3(88), 99-110 [in Polish].].
Studies performed by many researchers clearly define that storage time affects the number of microflora typical for yoghurts. Sady et al. [2007Sady, M., Domagała, J., Grega, T., Kalicka, D. (2007). Wpływ czasu przechowywania na mikroflorę jogurtów z dodatkiem nasion amarantusa i ziaren owsa [The influence of storage time on the yogurt microflora with the addition of amaranth seeds and oat grains]. Żyw. Nauka. Techn. Jakość, 6(55), 242–250 [in Polish].] showed that levels of L. bulgaricus and S. thermophilus increased slightly up to the 3rd day of storage, and then decreased to the lowest level on day 14th. In addition, they observed that the number of bacilli was decreasing slower than streptococci. In fresh products, the ratio of L. bulgaricus to L. thermophilus was 1:1.46, while after 14 days it changed into 1.23:1. This change is related to the advancing acidification that happens during the cool storage (S. thermophilus is less tolerant to high acidity). Similar results were obtained by Laye et al. [1993Laye, I., Karleskind, D., Morr, C.V., (1993). Chemical, microbiological and sensory properties of plain non – fat yoghurt. J. Food Sci., 58, 991–995.] and Beal et al. [\hyperref[Beal1999Com]], whereas Kneifel et al. [1993Kneifel, W., Jaros, D., Erhard, F. (1993). Microflora and acidification properties of yogurt and yogurt-related products fermented with commercially available starter cultures. Int. J. Food Microbiol., 18, 179–189.] and Barrantes et al. [1994Barrantes, E., Tamime, A.Y., Muir, D.D., Sword, A.M. (1994). The effects of substitution of fat by microparticulate whey protein on the quality of set-up natural yoghurt. J. Soc. Dairy Technol., 47, 61–68. https://doi.org/10.1111/j.1471-0307.1994.tb01274.x.] did not observe any significant changes in the number of bacteria during yoghurt storage. Moreover, Cais-Sokolińska and Pikul [2001Cais-Sokolińska, D., Pikul, J. (2001). Wpływ chłodniczych temperatur przechowywania na jakość i trwałość jogurtu naturalnego [The effect of refrigeration storage temperatures on the quality and durability of natural yoghurt]. Chłodnictwo, 8–9, 84–88 [in Polish].] reported the effect of storage temperature on a decrease in the number of lactic acid microorganisms. The number of cells was decreasing slower in yogurt stored at 6 ±1°C in comparison to when it was stored at a higher temperature (10 ±1°C).
Jakubowska, M., Matusevičius, P. (1995). Quality assessment of natural yoghurts offered in retail trade. Acta Sci. Pol. Zootechnica, 17(4), 5–12. DOI:10.21005/asp.2018.17.4.01.