Research Article

The effect of storage time and temperature on the quality of natural yoghurt

Małgorzata Jakubowska  , Tadeusz Karamucki

Laboratory of Commodity Science of Food Products, Department of Microbiology and Biotechnology West Pomeranian University of Technology in Szczecin, al. Piastów 45, 70-311 Szczecin, Poland

Abstract. Cooling temperature and storage time have a significant effect on the quality of natural yogurts and their durability in retail. The aim of the study was to perform a physicochemical, microbiological and organoleptic assessment of commercial yogurt, immediately after purchase and during 3 weeks of storage at 2°C and 8°C. The following physicochemical parameters were measured: pH, titratable acidity, color (CIEL a*, b*) and basic chemical composition. The organoleptic assessment included the general appearance and color, consistency, aroma and taste and desirability of yogurt. Microbiological tests have determined the amount of Lactobacillus, Streptococcus, Enterobacteriaceae bacteria, and checked the presence of mold and fungi. We have observed a significant effect of storage time on titratable and active acidity in the tested samples. No statistically significant effect of storage time on color of yogurt was observed. As the storage time increased, the number of yogurt microorganisms was decreasing. The minimum normative total amount of lactic acid bacteria was recorded on the day of purchase and after one week of storage at 8°C. As the storage time and the storage temperature increased, the taste and aroma of yogurt was decreasing significantly.

Keywords: natural yoghurt,temperature, storage time, quality: physicochemical, sensory, microbiological

INTRODUCTION

In the recent years, a dynamic increase in production of fermented milk drinks belonging to a “functional food” category has been observed. In Poland, the production of fermented milk drinks in 2017 reached 700 thousand tons, of which 380 thousand tons was yoghurt. Its annual consumption in a typical household since 2014 is around 6 kg per person [Brodziak et al. 2018Brodziak, A., Król, J., Wolanciuk, A., Barłowska, J., Kędzierska-Matysek, M. (2018). Quality assessment of fermented milk beverages produced with milk of Simmental cows kept on organic farms [Ocena jakości mlecznych napojów fermentowanych produkowanych na bazie mleka krów rasy simentalskiej utrzymywanych w gospodarstwach ekologicznych]. XIII Konferencja Naukowa z cyklu Żywność XXI Wieku ,,Żywność a składniki bioaktywne'', 24–25 września 2018, Kraków, Monografia, 44–51 [in Polish]. Google Scholar]. The factors that caused the interest in consumption of yogurt include its organoleptic characteristics, nutritional, dietary and medical values, as well as its wide assortment at the market [Zaręba and Ziarno, 2014, Brodziak and Król 2016Brodziak, A., Król, J. (2016). Milk fermented beverages – health-promoting properties [Mleczne napoje fermentowane – właściwości prozdrowotne]. Przem. Spoż., 70(10), 22–28 [in Polish]. Google Scholar, Jakubowska and Matusevicius 2018Jakubowska, M., Matusevicius, P. (2018). Quality assessment of natural yoghurts offered in retail trade. Acta Sci. Pol. Zootechnica, 17(4), 5–12. https://doi.org/10.21005/asp.2018.17.4.01].

Natural yogurt is a fermented dairy product that is obtained by acidifying milk with specific yogurt bacteria (Lactobacillus delbruecckii ssp. bulgaricus and Streptococcus salivarius ssp. thermophilus). In case of so-called new generation yoghurts, the products are also supplemented with Lactobacillus acidophilus and Bifidobacterium strains [Zaręba and Ziarno 2013Zaręba, D., Ziarno, M. (2013). Viability of lactic acid bacteria and bifidobacteria in the commercially-available yoghurts [Przeżywalność bakterii kwasu mlekowego i bifidobakterii w jogurtach handlowych]. Prz. Mlecz., 9, 14–20 [in Polish]. Google Scholar].

An appropriate selection of raw materials and the method of production as well as storage conditions significantly affect physicochemical, sensory and microbiological characteristics of yogurt.

Yoghurt belongs to the group of dairy products with a short shelf life, which is normally is about 3 weeks or less [Mojka 2013Mojka, K. (2013). Charakterystyka mlecznych napojów fermentowanych. Characteristics of fermented milk drinks. Probl. Hig. Epidemiol., 94(4), 722–729 [in Polish]. Google Scholar]. Improper storage conditions can lead to a decrease in nutritional value and deterioration of sensory characteristics of yoghurts, as well as to a reduction in content of yogurt bacteria that carry out the process of lactic fermentation [Cais-Sokolińska and Pikul 2001Cais-Sokolińska, D., Pikul, J. (2001). The effect of refrigeration storage temperatures on the quality and durability of natural yoghurt [Wpływ chłodniczych temperatur przechowywania na jakość i trwałość jogurtu naturalnego]. Chłodnictwo, 8–9, 84–88 [in Polish]. Google Scholar, Zaręba and Ziarno 2013Zaręba, D., Ziarno, M. (2013). Viability of lactic acid bacteria and bifidobacteria in the commercially-available yoghurts [Przeżywalność bakterii kwasu mlekowego i bifidobakterii w jogurtach handlowych]. Prz. Mlecz., 9, 14–20 [in Polish]. Google Scholar, Wojtczak et al. 2018Wojtczak, A., Ziarno, M., Czarniak, K. (2018). The influence of technological factors on the survivability\emph{ Lb. acidophilus} and\emph{ Lb. casei} [Wpływ czynników technologicznych na przeżywalności \emph{Lb. acidophilus i Lb.} \emph{Casei}]\emph{.} Przem. Spoż., 72(8), 42–45 [in Polish]. Google Scholar]. The storage temperature of dairy products also significantly determines their quality and durability. Improper storage temperature in yogurt coolers can lead to a change in the number and mutual proportions of lactic acid bacteria composition typical for the product, and in consequence, to its acidification, lowering pH and deterioration of its sensory quality [Cais-Sokolińska and Pikul 2001Cais-Sokolińska, D., Pikul, J. (2001). The effect of refrigeration storage temperatures on the quality and durability of natural yoghurt [Wpływ chłodniczych temperatur przechowywania na jakość i trwałość jogurtu naturalnego]. Chłodnictwo, 8–9, 84–88 [in Polish]. Google Scholar]. The recommended storage temperature of yoghurt ranges from 1°C to 8°C, but its durability may be enlarged by storing at 4°C or less during the whole shelf life [Pikul 2004Pikul, J. (2004). Factors affecting the shelf life and milk product. Part II. Dairy products with short, intermediate and long shelf life [Czynniki wpływające na trwałość mleka i produktów mlecznych. Część II. Produkty mleczne o krótkim, średnim orz długim okresie trwałości]. Chłodnictwo XXXIX, 10, 41–47 [in Polish]. Google Scholar]. The studies on the impact of storage time and conditions of fermented milk products are aimed at finding a method of handling yogurt that would maximally limit the changes in quality parameters (e.g. color, taste, aroma, pH, appropriate concentration of LAB bacteria) during retail storage. Recent reports underline the very good level of domestic fermented milk drinks, but still indicate their diversified quality [Molska et al. 2003Molska, I., Nowosielska, R., Frelik, I. (2003). Zmiany jakości mikrobiologicznej kefiru i jogurtu rynku warszawskiego w latach 1995-2001 [The changes in microbiological quality of kefir and yoghurt on the Warsaw market in the years 1995-2001]. Roczn. PZH 54(2), 145–152 [in Polish]. Google Scholar, Zaręba and Ziarno 2013Zaręba, D., Ziarno, M. (2013). Viability of lactic acid bacteria and bifidobacteria in the commercially-available yoghurts [Przeżywalność bakterii kwasu mlekowego i bifidobakterii w jogurtach handlowych]. Prz. Mlecz., 9, 14–20 [in Polish]. Google Scholar, Zaręba and Ziarno 2014Zaręba, D., Ziarno, M. (2014). Yoghurts and their consumers [Jogurty i ich konsumenci]. Prz. Mlecz., 10, 39–47 [in Polish]. Google Scholar, Wichrowska and Wojdyła 2014Wichrowska, D., Wojdyła, T. (2014). Sensory and physicochemical assessment of the selected natural and ecological yoghurts [Ocena sensoryczna i fizykochemiczna wybranych jogurtw naturalnych i ekologicznych]. Inż. Ap. Chem., 53(6), 421–423 [in Polish]. Google Scholar, Jakubowska and Matusevicius 2018Jakubowska, M., Matusevicius, P. (2018). Quality assessment of natural yoghurts offered in retail trade. Acta Sci. Pol. Zootechnica, 17(4), 5–12. https://doi.org/10.21005/asp.2018.17.4.01].

Therefore, in our study we aimed at assessing the physicochemical, microbiological and organoleptic features of commercial natural yogurt, immediately after purchase and during 3 weeks of storage at various cooling temperatures: 2°C and 8°C.

MATERIAL AND METHODS

Our research material included the samples of natural yoghurt produced by one of the Dairy Cooperatives in Podlaskie, available in retail chains in West Pomerania.

105 unit packages of yogurt were purchased at the local dairy wholesaler’s. From the whole purchased batch, 10 packages were randomly selected for physicochemical and organoleptic tests, and this group was set as the control. Additionally, 5 randomly selected yoghurts were subjected to microbiological tests. The rest of the yoghurts was divided into two groups and cooled in refrigerators at 8°C and 2°C. The analyzed shelf life of yogurt was 3 weeks and included the proper shelf life and the week after the expiration date. The plan of our experiment included testing of yogurt immediately after purchase and after each week of storage.

After each week of storage, from each of the experimental group, we have randomly selected 10 yoghurts for physicochemical and organoleptic examinations and 5 yoghurts for microbiological tests.

The yogurt composition, according to the manufacturer's data was as follows: pasteurized milk, milk proteins, sugar, live yogurt bacteria cultures and bifidobacteria. Nutritional value in 100g of product: energy proteins 275 kJ/61 kcal, proteins 3.6 g, carbohydrates 6.1 g, fat 2.5 g.

Tests and measures

The following physiochemical parameters were analyzed in our yogurt samples: pH, titratable acidity and color. The color assessment was carried out with the MiniScan XE Plus 45/0 camera, with a 31.8 mm diam. measuring port. We used the CIE L* a* b* scale, D 65 illuminate and used the standard 10°C observer [CIE 1976CIE (1976). Colourimetry: Official recommendations of the international commission on illumination. Publication CIE 15 (E-1.3.1). Bureau Central de la Commission Internationale De L'Eclairage, Paris. Google Scholar].

The pH measurement was taken using a combined glass electrode and a Cyber Scan pH meter. Evaluation of the total acidity in Soxhlet-Henkel grades was performed according to the standard method [PN-75 1975PN-75 (1975). Polish standard PN-75/A-86130/1975: Milk and dairy products. Milk drinks. Test methods [Mleko i przetwory mleczarskie. Napoje mleczne. Metody badań]. [in Polish]. Google Scholar]. The acidity of tested products was measured at room temperature, and the final result was the arithmetic mean of the two parallel measurements.

We have also analyzed the content of sugars, total protein and fat [Krełowska-Kułas 1993Krełowska-Kułas, M. (1993). Research on the quality of food products [Badanie jakości produktów spożywczych]. PWE, Warszawa [in Polish]. Google Scholar]. 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).

Our study included also microbiological testing of the natural yoghurts. For the microbiological tests, 1 gram of yogurt was transferred to 9 ml of sterile saline. The suspension was stirred and then the 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 the 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 a consideration to the previously made decimal dilutions [PN-93 1993bPN-93 (1993b). Polish standard: PN-93/A-860334-08/1993: Milk and dairy products. Microbiological testing. Detection of coliforms and NPL determination [Mleko i przetwory mleczarskie. Badania mikrobiologiczne. Wykrywanie bakterii z grupy coli i oznaczanie NPL]. [in Polish]. Google Scholar, PN-93 1993cPN-93 (1993c). Polish Standard PN-93/A-860334-15/1993: Milk and dairy products. Microbiological testing. Determination of the number of characteristic microorganisms [Mleko i przetwory mleczarskie. Badania mikrobiologiczne. Oznaczanie liczby charakterystycznych drobnoustrojów]. [in Polish]. Google Scholar]. The number of yeast and mold cells was determined in reference to the Polish Standard [PN-93 1993aPN-93 (1993a). Polish Standard PN-93/A-860334-07/1993: Milk and dairy products. Microbiological testing. Molds and yeasts – lamellar determination at 25\degree C [Mleko i przetwory mleczarskie. Badania mikrobiologiczne. Pleśnie i drożdże – oznaczanie metodą płytkową w temperaturze 25\degree C]. [in Polish]. Google Scholar]. The results of all microbiological determinations were expressed as CFU/g.

Sensory analysis was performed with a five-point scale according to Pieczonka [1995]Pieczonka, W. (1995). Milk and milk products. Standardization of quality and test methods [Mleko i przetwory mleczne. Standaryzacja jakości i metody badań]. Akademia Rolnicza, Kraków [in Polish]. Google Scholar: 1 point – disqualifying rate, 5 points – very good rate. The appearance, color, consistency, aroma 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-98 [1998]PN-98 (1998). Polish standard PN-ISO-4121/1998: Sensory analysis. Methodology. Evaluation of food stuffs by using calibration methods [Analiza sensoryczna – Metodologia – Ocena produktów żywnościowych przy użyciu metod skalowania]. [in Polish]. Google Scholar.

Statistical analysis of the results was performed with the Statistica 10 software. We calculated the mean values and standard deviations, and carried out a one-way analysis of variance, with regard to the effect of storage at 2°C and 8°C on the quality features of examined samples. The Duncan test was used to estimate the differences between the experimental groups.

RESULTS AND DISCUSSION

Effect of storage on physicochemical features

The tests showed a significant effect (P ≤ 0.05) of storage time at both tested temperature ranges (2°C and 8°C), on the active acidity of examined samples. The acidity of yoghurts on the day of purchase was 4.17. During 14 days of storage of yoghurts at 2°C, the pH dropped to 4.10, and later a slight increase was noted on the 21st day of storage to the initial value of 4.17 that was measured on the day of purchase. A similar relationship was observed in case of storage at 8°C; on days the 7th and 14th, the pH dropped to 4.13 and then slightly increased to 4.16 on the 21st day of storage (Table 1). The results obtained by other researchers differ from the above. Cais-Sokolińska and Pikul [2001]Cais-Sokolińska, D., Pikul, J. (2001). The effect of refrigeration storage temperatures on the quality and durability of natural yoghurt [Wpływ chłodniczych temperatur przechowywania na jakość i trwałość jogurtu naturalnego]. Chłodnictwo, 8–9, 84–88 [in Polish]. Google Scholar noted a regular increase in curd acidity (from 0.1 to 0.2 units) after each week of storage of natural yogurt. They have also showed that the decrease in active acidity is significantly affected by storage temperature. At higher temperatures, pH of dairy products decreases faster than in the same products, but stored at lower temperatures. Also Kneifel et al. [1993]Kneifel, 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. https://doi.org/10.1016/0168-1605(93)90043-G, Imhof et al. [1994]Imhof, 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. https://doi.org/10.1006/fstl.1994.1090, Sady et al. [2007]Sady, M., Domagała, J., Grega, T., Kalicka, D. (2007). The influence of storage time on the yogurt microflora with the addition of amaranth seeds and oat grains [Wpływ czasu przechowywania na mikroflorę jogurtów z dodatkiem nasion amarantusa i ziaren owsa]. Żywn. Nauka. Technol. Jakość. 6(55), 242–250 [in Polish]. Google Scholar and Eissa et al. [2010]Eissa, E.A., Mohamed Ahmed, I.A., Yagoub, A.E.A., Babiker, E.E. (2010). Physicochemical, microbiological and sensory characteristics of yoghurt produced from goat milk. Livestock Res. Rural Dev., 22(8). Google Scholar reported in their publications a steady decrease in pH associated with a prolonged storage time.

Table 1. Effect of storage on changes in active and potential acidity of yoghurts
Tabela 1. Wpływ przechowywania na zmiany kwasowości jogurtów

Days
Dni

pH

Acidity, °SH
Kwasowość, °SH

Temp. 2°C

Temp. 8°C

Temp. 2°C

Temp. 8°C

s

s

s

s

0

4.17a

0.03

4.17a

0.03

37.76a

0.01

37.76a

0.01

7

4.20a

0.02

4.13b

0.01

37.60a

3.61

40.76a

0.96

14

4.10b

0.01

4.13b

0.01

46.08b

0.95

46.80b

1.94

21

4.17a

0.01

4.16a

0.01

42.92b

1.15

45.60b

1.11

a, b, c – Means marked by different lower-case letters differ significantly (P ≤ 0.05).
a, b, c – Średnie oznaczone różnymi małymi literami różnią się od siebie istotnie (P ≤ 0,05).

An increase in the acidity of yogurt during storage is a result of the fermentation activity of microorganisms that make up the yogurt vaccine, and that even under refrigerated conditions are still able to digest lactose, although the process is slower [Jankowska and Reps 2013Jankowska, A., Reps, A. (2013). Factors affecting the shelf-life of yoghurt during storage [Czynniki decydujące o trwałości jogurtu podczas przechowywania]. Prz. Mlecz. 11, 2–5 [in Polish]. Google Scholar].

The pH values measured in this study turned out to be lower than those recorded by Wichrowska and Wojdyła [2014]Wichrowska, D., Wojdyła, T. (2014). Sensory and physicochemical assessment of the selected natural and ecological yoghurts [Ocena sensoryczna i fizykochemiczna wybranych jogurtw naturalnych i ekologicznych]. Inż. Ap. Chem., 53(6), 421–423 [in Polish]. Google Scholar and Jakubowska and Matusevicius [2018]Jakubowska, M., Matusevicius, P. (2018). Quality assessment of natural yoghurts offered in retail trade. Acta Sci. Pol. Zootechnica, 17(4), 5–12. https://doi.org/10.21005/asp.2018.17.4.01 in yoghurts obtained from the retail chain (4.38 and 4.48, respectively) after a week of storage, while similar to those recorded by Sady et al. [2007]Sady, M., Domagała, J., Grega, T., Kalicka, D. (2007). The influence of storage time on the yogurt microflora with the addition of amaranth seeds and oat grains [Wpływ czasu przechowywania na mikroflorę jogurtów z dodatkiem nasion amarantusa i ziaren owsa]. Żywn. Nauka. Technol. Jakość. 6(55), 242–250 [in Polish]. Google Scholar in natural yogurts after 14 days of storage (4.15). The reported pH values confirm the correct acidity of the tested samples; according to Jurczak [2003]Jurczak, M.E. (2003). Milk: Production, research and processing [Mleko: produkcja, badanie, przerób]. Wydawnictwo SGGW, Warszawa [in Polish]. Google Scholar, the active acidity of yoghurts should range from 4.0 to 4.5. However, basing on the observations by Jankowska and Reps [2013]Jankowska, A., Reps, A. (2013). Factors affecting the shelf-life of yoghurt during storage [Czynniki decydujące o trwałości jogurtu podczas przechowywania]. Prz. Mlecz. 11, 2–5 [in Polish]. Google Scholar, the acidity of yoghurts that responds to the consumer requirements is narrower and remains in the range of 4.2–4.5.

On the day of purchase, we have demonstrated that the titration acidity of natural yogurt was 37.76°SH (Table 1). During 14 days of storage, there was a slight increase in titratable acidity in both analyzed temperature ranges (up to 46.08°SH at 2°C and up to 46.80°SH at 8°C). On the other hand, on the 21st day of storage there was a slight decrease in titratable acidity, compared to the 14th day of storage. A statistically significant effect of storage on the titratable acidity was recorded only on days 14th and 21st. At the same time, we have observed that changes in titratable acidity of the samples proceeded more slowly during 14 days of storage for products stored at 2°C than at 8°C (Table 1).

Kneifel et al. [1993]Kneifel, 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. https://doi.org/10.1016/0168-1605(93)90043-G, Imhof et al. [1994]Imhof, 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. https://doi.org/10.1006/fstl.1994.1090, Sady et al. [2007]Sady, M., Domagała, J., Grega, T., Kalicka, D. (2007). The influence of storage time on the yogurt microflora with the addition of amaranth seeds and oat grains [Wpływ czasu przechowywania na mikroflorę jogurtów z dodatkiem nasion amarantusa i ziaren owsa]. Żywn. Nauka. Technol. Jakość. 6(55), 242–250 [in Polish]. Google Scholar and Eissa et al. [2010]Eissa, E.A., Mohamed Ahmed, I.A., Yagoub, A.E.A., Babiker, E.E. (2010). Physicochemical, microbiological and sensory characteristics of yoghurt produced from goat milk. Livestock Res. Rural Dev., 22(8). Google Scholar observed an increase in titratable acidity with longer storage time. In studies of Cais-Sokolińska and Pikula [2001], the titratable acidity of yogurt at the end of storage period compared to the acidity measured immediately after its production was on average about 32% higher. These authors also report that changes in titratable acidity occurred more slowly in yoghurts stored at 6 ±1°C than at 10 ±1°C.

A decrease in acidity of yogurt is affected by its microflora, which is diversified in its activity in acidifying, proteolytic and lipolytic processes. During the maturation and storage of milk drinks, the bacteria use milk ingredients at various rates and ranges. First, the lactose is converted into lactic acid, which causes a increase of yogurt acidity [Cais-Sokolińska et al. 2009Cais-Sokolińska, D., Danków, R., Pikul, J. (2009). The dynamics of yogurt acidity changes with the addition of cereal products during refrigerated storage [Dynamika zmian kwasowości jogurtu z dodatkiem produktów zbożowych podczas chłodniczego przechowywania]. Nauka Przyr. Technol., 3(4), 1–5 [in Polish]. Google Scholar].

We have demonstrated that the L* parameter in the tested samples ranged from 93.78 to 91.11. There was no significant effect of storage time on the brightness of yogurt samples (Table 2). As the storage time increased, a slight decrease in yogurt brightness was observed only on the 21st day of storage at 2°C and 8°C (Table 2). However, Cais-Sokolińska and Pikul [2008]Cais-Sokolińska, D., Pikul, J. (2008). Evaluation of yoghurt quality changes using colour measurements [Ocena jakości jogurtu na podstawie instrumentalnego pomiaru barwy]. Aparatura Badawcza i Dydaktyczna. Tom 13, 3, 8–12 [in Polish]. Google Scholar noted a significant deterioration in brightness of color during the subsequent weeks of storage. Teichert et al. [2015]Teichert, J., Danków, R., Pikul, J., Osten-Sacken, N. (2015). Properties of fermented beverages prepared from goat milk with the use of thickening starter cultures [Właściwości napojów fermentowanych wytworzonych z mleka koziego z udziałem kultur zagęszczających]. Nauka Przyr. Technol., 9(2), 1–13 [in Polish]. https://doi.org/10.17306/J.NPT.2015.2.28 noted a significant increase in brightness of color in goat yoghurts on the 21st day of cool storage.

The a* parameter of yoghurts (change of color in range from green to red) was negative for all the tested samples in range from –2.29 to –1.85 (Table 2). In yoghurts stored at 2°C, on the day of purchase and on the 7th day of storage, the values of a* were from –2.07 to –2.10, and on the 14th day there was a decrease in a* to –1.88, and then in turn an increase on the 21st day to –2.29 occured. In yoghurts stored at 8°C, a significantly (P ≤ 0.05) lower share of green color (a*) along with the extension of the storage time (from –2.07 on the day of purchase to –1.85 on the 21st day of storage) was noted.

Table 2. Effect of storage on changes in yoghurt color
Tabela 2. Wpływ przechowywania na zmiany barwy jogurtów

Days
Dni

L*

a*

b*

Temp. 2°C

Temp. 8°C

Temp. 2°C

Temp. 8°C

Temp. 2°C

Temp. 8°C

s

s

s

s

s

s

0

93.78

0.13

93.78

0.13

–2.07

0.02

–2.07b

0.02

11.24

0.13

11.24

0.12

7

93.39

0.10

93.76

0.41

–2.10

0.02

–2.05b

0.02

11.03

0.12

11.16

0.05

14

93.68

0.47

93.59

0.10

–1.88

0.03

–1.98b

0.05

10.77

0.07

11.27

0.05

21

91.11

4.82

92.56

1.47

–2.29

0.95

–1.85a

0.05

11.81

2.07

11.00

0.43

a, b – Means marked by different lower-case letters differ significantly (P ≤ 0.05).
a, b – Średnie oznaczone różnymi małymi literami różnią się od siebie istotnie (P ≤ 0,05).

On the other hand, the parameter b* (change of color in range from blue to yellow) was positive for all the tested yogurt samples in range from 11.24 (on the day of purchase) to 11.81–11.00 (on the 21st day of storage). There was no statistically significant effect of storage time on this feature in any of the tested temperature ranges (Table 2).

The use of instrumental measurement of yogurt color may reflect the changes that occur in yogurt as a result of activity of microflora that is intentionally introduced during the production process [Cais Sokolińska and Pikul 2008]. As a result of the tests, no significant color changes in the stored yogurt were found, while only a slight lightening was observed on the 21st day of storage. The measured color parameters were stable and did not change over time except for the change of the a* parameter in samples stored at 8°C. Meanwhile, studies conducted by Sofu and Ekinci [2007]Sofu, A., Ekinci, F.Y. (2007). Estimation of storage time of yoghurt with artificial neural network modeling. J. Dairy Sci., 7, 3118–3125. https://doi.org/10.3168/jds.2006-591 revealed some changes in yogurt color during storage. Grayish-greenish-yellow tones predominated in yoghurts at the end of their shelf life. According to Cais Sokolińska and Pikula [2008], the microbiological activity of yogurt microflora leads to the changes in potential acidity of the clot, and thus cause physical changes in the clot, resulting in a difference in the color brightness and the degree of color saturation. According to the research by Cais-Sokolińska and Majcher [2009]Cais-Sokolińska, D., Majcher, M. (2009). Zależność pomiędzy parametrami barwy skali CIE L*, a*, b* a podstawowym składem chemicznym permeatu i koncentratu mleka poddanego mikro- i ultrafiltracji [Relationship between color scale parameters CIE L*, a*, b* and basic chemical composition of permeate and concentrate of milk subjected to micro- and ultrafiltration]. Apar. Bad. Dydakt., 14(1), 92–96 [in Polish]. Google Scholar and after Teichert et al. [2015]Teichert, J., Danków, R., Pikul, J., Osten-Sacken, N. (2015). Properties of fermented beverages prepared from goat milk with the use of thickening starter cultures [Właściwości napojów fermentowanych wytworzonych z mleka koziego z udziałem kultur zagęszczających]. Nauka Przyr. Technol., 9(2), 1–13 [in Polish]. https://doi.org/10.17306/J.NPT.2015.2.28, the source of changes in color parameters during storage lays in the fact that the casein complex passes from the micelar state into a dispersed state. The colloidal casein-calcium complex disperses light and make the milk look white.

Effect of storage time on changes in yogurt microflora

On the day of purchase, the concentration of Lactobacillus bacteria in yoghurts was 9.5 · 105 CFU · g–1. On the 14th day of their shelf life and storage at 2°C, their quantity was 3.17 · 102 and at 8°C – 4.10 · 103. These results show a decrease in concentration of Lactobacillus bacteria over the entire storage period. Faster (from 105 to 100 CFU · g–1), the number of bacteria decreased in yoghurts stored at 2°C than when stored at 8°C (102 CFU · g–1) (Table 3). The growth of these bacteria depends primarily on the storage temperature and the type of strains included in the sourdough [Sady et al. 2007Sady, M., Domagała, J., Grega, T., Kalicka, D. (2007). The influence of storage time on the yogurt microflora with the addition of amaranth seeds and oat grains [Wpływ czasu przechowywania na mikroflorę jogurtów z dodatkiem nasion amarantusa i ziaren owsa]. Żywn. Nauka. Technol. Jakość. 6(55), 242–250 [in Polish]. Google Scholar]. Lactobacillus bacteria grow better at warmer temperatures.

Table 3. Effect of storage time on microbiological changes in yoghurts
Tabela 3. Wpływ czasu przechowywania na zmiany mikrobiologiczne jogurtów

Dni – Days

Lactobacillus

Streptococcus

Temp. 2°C

Temp. 8°C

Temp. 2°C

Temp. 8°C

0

9.50 ∙ 105

9.50 ∙ 105

5.17 ∙ 103

5.17 ∙ 103

7

1.67 ∙ 103

1.34 ∙ 104

3.21 ∙ 103

2.59 ∙ 103

14

3.17 ∙ 102

4.10 ∙ 103

2.93 ∙ 102

1.14 ∙ 102

21

4.67 ∙ 100

1.27 ∙ 102

0

2.67 ∙ 101

The number of Streptococcus bacteria was also examined in the tested samples. Also in the case of these bacteria, their number was decreasing along with the storage time. The number of bacteria found in yoghurts immediately after purchase was 5.17 · 103 CFU · g–1, while on the 14th day of the study, their number fluctuated regardless of the temperature from 2.93 · 102 to 1.14 · 102. However, on the 21st day of the study after the shelf-life, in yoghurts stored at 2°C, the number of Streptococcus bacteria dropped to zero, and in yoghurts stored at 8°C to 101 CFU · g–1 (Table 3).

In our study, the number of lactic acid bacteria decreased faster at the lower (2°C) than higher (8°C) temperature, inversely than in the Cais-Sokolińska and Pikula [2001] studies, which showed that the decrease in number of lactic acid microorganisms proceeds more slowly in yogurts stored at 6 ±1°C than when stored at higher temperature (10 ±1°C). The storage temperature of fermented beverages is very important, as it determines the size of population at an appropriate level, and should be kept at below 6°C [Zaręba and Ziarno 2013Zaręba, D., Ziarno, M. (2013). Viability of lactic acid bacteria and bifidobacteria in the commercially-available yoghurts [Przeżywalność bakterii kwasu mlekowego i bifidobakterii w jogurtach handlowych]. Prz. Mlecz., 9, 14–20 [in Polish]. Google Scholar].

Many studies clearly state that the storage time of dairy products is very important and has a decisive impact on the number of characteristic microflora of yogurt. Sady et al. [2007]Sady, M., Domagała, J., Grega, T., Kalicka, D. (2007). The influence of storage time on the yogurt microflora with the addition of amaranth seeds and oat grains [Wpływ czasu przechowywania na mikroflorę jogurtów z dodatkiem nasion amarantusa i ziaren owsa]. Żywn. Nauka. Technol. Jakość. 6(55), 242–250 [in Polish]. Google Scholar showed that the level of L. bulgaricus and S. thermophilus increased slightly until the 3rd day of storage, after which it decreased to the lowest level on the 14th day. In addition, they observed that the number of bacilli was decreasing more slowly than streptococci. In fresh products, the ratio of L. bulgaricus : S. thermophilus was 1:1.46, while after 14 days it changed into 1.23:1. This change is associated with the process of acidification during cool storage, as S. thermophilus is less tolerant to high acidity. Similar results were obtained by Laye et al. [1993]Laye, I., Karleskind, D., Morr, C.V. (1993). Chemical, microbiological and sensory properties of plain non-fat yogurt. J. Food Sci., 58, 991–995. https://doi.org/10.1111/j.1365-2621.1993.tb06096.x and Beal et al. [1999]Beal, C., Skokanova, J., Latrille, E., Martin, N., Corrieu, G. (1999). Combined effects of culture conditions and storage time on acidification and viscosity of stirred yoghurt., J. Dairy Sc., 82, 673–681. https://doi.org/10.3168/jds.S0022-0302(99)75283-5, while Kneifel et al. [1993]Kneifel, 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. https://doi.org/10.1016/0168-1605(93)90043-G and Barrantes et al. [1994]Barrantes, 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 yogurt storage. Numerous studies confirm a decrease in the number of bacteria during cool storage [Beal et al. 1999Beal, C., Skokanova, J., Latrille, E., Martin, N., Corrieu, G. (1999). Combined effects of culture conditions and storage time on acidification and viscosity of stirred yoghurt., J. Dairy Sc., 82, 673–681. https://doi.org/10.3168/jds.S0022-0302(99)75283-5, Cais Sokolińska and Pikul 2001, Sady et al. 2007Sady, M., Domagała, J., Grega, T., Kalicka, D. (2007). The influence of storage time on the yogurt microflora with the addition of amaranth seeds and oat grains [Wpływ czasu przechowywania na mikroflorę jogurtów z dodatkiem nasion amarantusa i ziaren owsa]. Żywn. Nauka. Technol. Jakość. 6(55), 242–250 [in Polish]. Google Scholar, Zaręba et al. 2008Zaręba, D., Ziarno, M., Obiedziński, M. (2008). Viability of yoghurt bacteria and probiotic strains in models of fermented and non fermented milk [Przeżywalność bakterii jogurtowych i probiotycznych w układach modelowych mleka fermentowanego i niefermentowanego]. Med. Weter. 64(8), 1007–1011 [in Polish]. Google Scholar, Zaręba and Ziarno 2013Zaręba, D., Ziarno, M. (2013). Viability of lactic acid bacteria and bifidobacteria in the commercially-available yoghurts [Przeżywalność bakterii kwasu mlekowego i bifidobakterii w jogurtach handlowych]. Prz. Mlecz., 9, 14–20 [in Polish]. Google Scholar].

In our research, Lactobacilluss (105) was the dominant species, compared to Streptococcus (103). Lactobacilluss tolerate much lower pH levels and are able to survive in an environment with pH below 4.0, in contrast to the other yogurt bacteria [Jankowska and Reps 2013Jankowska, A., Reps, A. (2013). Factors affecting the shelf-life of yoghurt during storage [Czynniki decydujące o trwałości jogurtu podczas przechowywania]. Prz. Mlecz. 11, 2–5 [in Polish]. Google Scholar. According to Zaręba and Ziarno [2013]Zaręba, D., Ziarno, M. (2013). Viability of lactic acid bacteria and bifidobacteria in the commercially-available yoghurts [Przeżywalność bakterii kwasu mlekowego i bifidobakterii w jogurtach handlowych]. Prz. Mlecz., 9, 14–20 [in Polish]. Google Scholar during the yoghurt production process, the growth of Streptococcus thermophilus is inhibited at pH 4.2–4.4, whereas Lb. bulgaricus continue to grow by acidifying the environment and produce more lactic acid (lactobacilli tolerate pH 3.5–3.8). However, in this study the acidity of yoghurts ranged from 4.16 to 4.10 throughout the entire cool storage period, which may be the reason for a lower concentration of Streptococcus bacteria in the tested yogurt samples (Table 1). According to the research made by Ziarno and Zareby [2013], the pH of commercial yogurt can be from 3.5–3.7 to 4.3–4.4.

Analyzing the compliance of our results with the requirements of FAO-WHO [2000]FAO-WHO (2000). Proposed Draft Standard A-11 for Fermented Milks. Alinorm 01/11/2000. Google Scholar and IDF/FIL which define the minimum amount of yogurt bacteria in total at 107 CFU · g–1, the analyzed yogurt met the recommended criteria on the day of purchase, where total number of CFU was 108 (Table 3). However, with proceeding storage time, the concentration of bacteria was decreasing. After 7 days of storage, the minimum content of lactic acid bacteria (107 CFU · g–1) was met only by yoghurts stored at 8°C. In the next periods of cool storage and a week after the shelf life, there was no minimal normative number of live cells of lactic acid bacteria (Table 3). A decrease in lactic acid bacteria levels in commercial yoghurts was also noted by Zaręba and Ziarno [2013]Zaręba, D., Ziarno, M. (2013). Viability of lactic acid bacteria and bifidobacteria in the commercially-available yoghurts [Przeżywalność bakterii kwasu mlekowego i bifidobakterii w jogurtach handlowych]. Prz. Mlecz., 9, 14–20 [in Polish]. Google Scholar, but not to such the extent as in our analyzes. The authors analyzed the survivability of lactic acid bacteria and bifidobacteria in 70 commercial yogurt samples during their shelf life and 2 weeks after the shelf life. They found that the population of streptococci was decreasing regardless of their initial number and the percentage of samples containing less than the normative 107 CFU · g–1 of bacilli increased. In the discussed studies, on the last day of shelf life, only in 4.3% of yogurt samples the number of Streptococcus thermophilus cells was below 106 CFU · g–1, and the number of Lactobacillus cells was below 107 CFU · g–1 in 10% of the examined samples. According to Zaręba and Ziarno [2014]Zaręba, D., Ziarno, M. (2014). Yoghurts and their consumers [Jogurty i ich konsumenci]. Prz. Mlecz., 10, 39–47 [in Polish]. Google Scholar, the highest reduction of bacterial populations in fermented milk drinks occurs immediately after their expiration date.

A diversified survivability of yogurt bacteria in this study may result from the different sensitivity of starter cultures used by the manufacturer, the technological treatments, and the conditions of storage of product in the warehouse before the purchase. Also some other factors like sugar concentration, dry matter content and access to nutrients are important for the amount of yoghurt bacteria in the final product. Level of pH of yogurt will play a very important role as well. In this study, the acidity of yogurt was from 4.16 to 4.10 throughout the entire cool storage period, hence perhaps the concentration of Lactobacilluss bacteria in the tested samples was higher in comparison to Streptococcus (Table 1). Bolin et al. [1998]Bolin, Z., Libudzisz, Z., Moneta, J. (1998). Viability of Lactobacillus acidophilus in fermented milk products during refrigerated storage. Pol., J. Food. Nutr. Sci., 3, 466–471. Google Scholar indicate that the yogurt fermentation process should end at pH 4.9–5.0, which ensures a correct bacterial survivability and normal metabolic processes, and guarantee the appropriate aroma and taste desirability.

The hygienic quality of the examined yogurts was very good. In the tested samples, no infections with mold and yeast or bacteria from the Enterobacteriaceae family were noted.

Effect of storage time on sensory features

Natural yogurt should be characterized by a semi-smooth smooth consistency with a dense homogeneous curd, without protein lumps and air bubbles. The yogurt taste should be fresh, mild and pleasant and the aroma should have a characteristic yogurt flavor [Kudełka 2005Kudełka, W. (2005). Characterization of fermented milk beverages in the EU and Poland [Charakterystyka mlecznych napojów fermentowanych w Unii Europejskiej oraz w Polsce]. Zesz. Nauk. Aademii Ekonomicznej Wrocław, 678, 149–160 [in Polish]. Google Scholar]. According to the sensory analysis we carried out, the tested beverages were characterized by an appropriate color, appearance and consistency, and the analyzed features obtained very high scores, from 4.4 to 5.0 points during 14 days of storage at 2°C and 8°C (Table 4). Moreover, on the 21st day of storage after the expiry date, the tested yogurt received slightly lower scores for these sensory parameters (notes from 3.66 to 5.0 points).

Table 4. Effect of temperature of storage on sensory features
Tabela 4. Wpływ temperatury przechowywania na zmiany właściwości sensorycznych

Days – Dni

Appearance and color (point)
Wygląd i barwa pkt.

Consistency (point)
Konsystencja pkt.

Aroma and taste (point)
Smak i zapach pkt.

Desirability (point)
Pożądalność pkt.

Temp. 2°C

Temp. 8°C

Temp. 2°C

Temp. 8°C

Temp. 2°C

Temp. 8°C

Temp. 2°C

Temp. 8°C

s

s

s

s

s

s

s

s

0

5.00

0.00

5.00

0.00

5.00b

0.00

5.00

0.00

5.00b

0.00

5.00a

0.00

5.00a

0.00

5.00a

0.00

7

4.60

0.00

4.60

0.00

4.70b

0.00

4.40

0.00

4.96b

0.05

4.40b

0.07

4.84b

0.03

4.43a

0.04

14

5.00

0.00

5.00

0.00

5.00b

0.00

4.67

0.00

3.53c

0.18

1.94c

0.13

4.12c

0.11

3.08b

0.08

21

4.26

1.65

5.00

0.00

3.66a

1.48

4.33

0.00

3.87c

0.30

2.47d

0.60

3.87c

0.80

3.31b

0.36

a, b, c – Means marked by different lower-case letters differ significantly (P ≤ 0.05).
a, b, c – Średnie oznaczone różnymi małymi literami różnią się od siebie istotnie (P ≤ 0,05).

The results of organoleptic assessment did not show a statistically significant effect of storage time on the appearance and color of yoghurts at 2°C and 8°C (Table 4). The only significant differences (P ≤ 0.05) were found in the assessment of consistency of yoghurts stored at 2°C. Yoghurts rated on the 21st day of storage (3.66 points) obtained lower scores at this temperature compared to the yoghurts tested on the 7th and 14th day of storage (from 5.0 to 4.7 points).

Immediately after purchase and on the 7th day of storage, the yoghurts received the highest notes for taste and aroma (from 5.0 to 4.40 points), but together with the extension of the storage time a significant (P ≤ 0.05) deterioration of their taste and aroma at 2°C and 8°C was observed. The lowest notes for these parameters were recorded in yogurts stored at 8°C. On the 14th day of storage the score was 1.94 points, and on the 21st day of storage, 2.47 points. The statistical analysis of the general desirability of yogurt, which consisted of the results taken from all the assessed factors, showed that the yogurt is the most desirable immediately after purchase and in the first week of storage (Table 4). Products immediately after purchase and after the first week of storage, regardless of storage temperature, obtained significantly (P ≤ 0.05) the highest notes (from 5.0 to 4.43 points) compared to yoghurts tested on 14th and 21th day of storage after the shelf life (4.12 to 3.08 points) (Table 4).

From the beginning to the end of the study, yoghurts stored at 2°C received higher notes for the general desirability, compared to yoghurts stored at 8°C (Table 4).

Cais-Sokolińska and Pikul [2001]Cais-Sokolińska, D., Pikul, J. (2001). The effect of refrigeration storage temperatures on the quality and durability of natural yoghurt [Wpływ chłodniczych temperatur przechowywania na jakość i trwałość jogurtu naturalnego]. Chłodnictwo, 8–9, 84–88 [in Polish]. Google Scholar noticed the deterioration of sensory features of yogurt along with the extension of storage time. These authors have also shown a significant effect of temperature on the quality of yogurt. Just as in the above analysis, they proved that yoghurts stored at lower temperatures kept better desirability for longer than yoghurts stored at higher temperatures. Also Wichrowska and Wojdyła [2014]Wichrowska, D., Wojdyła, T. (2014). Sensory and physicochemical assessment of the selected natural and ecological yoghurts [Ocena sensoryczna i fizykochemiczna wybranych jogurtw naturalnych i ekologicznych]. Inż. Ap. Chem., 53(6), 421–423 [in Polish]. Google Scholar found that color, taste, aroma and texture of yoghurts deteriorated with the increasing storage time. However, according to research conducted by Chougrani et al. [2009]Chougrani, F., Cheriguene, A., Bensoltane, A. (2009). Sensorial and Physico-Chemical Characteristics of Yoghurt Manufactured with Ewe's and Skim Milk. World J. Dairy Food Sci., 4(2), 136–140. Google Scholar storage time had a positive effect on the taste of yogurt made from skim milk, where at the end of the study the taste of yogurt was rated better than at the beginning.

According to our studies, both storage time and storage temperature were the factors that differentiated the sensory quality of yogurt. The tested yoghurts stored at lower temperature were more stable in terms of the tested parameters than those kept at higher temperature. Keeping the appropriate sensory characteristics during the storage of yogurt is very important for the consumer. Taste, aroma, texture or color are the reflection of their quality and play a decisive role in the final choice of products by the buyer.

Food manufacturers must pay attention to the sensory quality of produced food, to offer a competitive product with repeatable quality characteristics, that meets the growing requirements of consumers [Rój and Przybyłowski 2012Rój, A., Przybyłowski, P. (2012). Evaluation of the color of natural yoghurts [Ocena barwy jogurtów naturalnych]. Bromat. chem. toksykol. XLV,(3), 813–816 [in Polish]. Google Scholar].

Effect of storage time on chemical composition

The results of protein and fat content tests in the examined samples on the day of purchase were close to those declared by the manufacturer (Table 5). However, the sugar content did not coincide with the data presented on the product label. Fluctuations in sugar content in samples tested on the day of purchase and during the cool storage can be explained by the fermentation activity of microorganisms added to dairy drinks during the production process, which at 4°C are still able to digest lactose [Cais-Sokolińska et al. 2009Cais-Sokolińska, D., Danków, R., Pikul, J. (2009). The dynamics of yogurt acidity changes with the addition of cereal products during refrigerated storage [Dynamika zmian kwasowości jogurtu z dodatkiem produktów zbożowych podczas chłodniczego przechowywania]. Nauka Przyr. Technol., 3(4), 1–5 [in Polish]. Google Scholar, Jankowska and Reps 2013Jankowska, A., Reps, A. (2013). Factors affecting the shelf-life of yoghurt during storage [Czynniki decydujące o trwałości jogurtu podczas przechowywania]. Prz. Mlecz. 11, 2–5 [in Polish]. Google Scholar].

CONCLUSIONS

  1. A significant effect of storage time on the titratable and active acidity of natural yoghurts was found.
  2. There was no statistically significant effect of storage time on the color of yoghurts; only in yoghurts stored at 8°C a significantly (P ≤ 0.05) lower proportion of green color (a) with the longer storage time was noted.
  3. As the storage time increases, the number of yogurt microorganisms decreases. The normative amount of lactic acid bacteria was recorded on the day of purchase and after one week of storage at 8°C.
  4. The hygienic quality of the determined yogurt samples was very good. In the tested samples no infections with molds, yeasts or bacteria from the Enterobacteriaceae family were found.
  5. As the storage time and the storage temperature increases, the sensory characteristics of yogurt deteriorates.
  6. Yoghurts had the best sensory quality immediately after purchase and after the first week of storage. These yoghurts were characterized by the best taste, aroma and general desirability.

ACKNOWLEDGEMENT

The study was financially supported by the UPB Subsidy, task A – Animal Husbandry and Fisheries, grant no. 503–01–083–11/4.

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This Article

Received: 28 Nov 2019

Accepted: 11 Dec 2019

Published online: 9 Jan 2020

Accesses: 276

How to cite

Jakubowska, M., Karamucki, T., (2019). The effect of storage time and temperature on the quality of natural yoghurt. Acta Sci. Pol. Zootechnica, 18(4), 29–38. DOI: 10.21005/asp.2019.18.4.04.