Yogurt, a traditional dairy product, is rich in valuable nutrients and hosts a diverse range of microorganisms known for their probiotic effects. This study aimed to assess the nutritional quality, mineral content, and bacterial diversity of thirty-eight yogurt samples sourced from seven distinct commercial brands and local brands. The advent of next-generation sequencing technology has enabled the in-depth characterization of microbial communities in dairy products, such as yogurt, marking a dynamic area of research [36]. Through high-throughput sequencing, the study identified 650 Operational Taxonomic Units (OTUs), which were subsequently analyzed using the Greengenes database.
The biochemical tests found a discrepancy between high pH value (4.99–6.45) in the samples of the current study and pH value (4.0-4.4) of the earlier publication on yogurt [37]. According to the literature Boukria et al., lactose fermentation into lactic acid by Lactobacillus (LAB) may cause a pH drop in sour yogurt [38]. The variations in pH across samples of two distinct taste groups and from ten brands indicate that acidification due to bacterial fermentation is probably a factor. Furthermore, insufficient incubation period and temperature, as well as regional and source variations, may all contribute to the increased range of pH found in this study across all yogurt samples.The biochemical tests found a discrepancy between high pH value (4.99–6.45) in the samples of the current study and pH value (4.0-4.4) of the earlier publication on yogurt [37]. According to the literature Boukria et al., lactose fermentation into lactic acid by Lactobacillus (LAB) may cause a pH drop in sour yogurt [38]. The variations in pH across samples of two distinct taste groups and from ten brands indicate that acidification due to bacterial fermentation is probably a factor. Furthermore, insufficient incubation period and temperature, as well as regional and source variations, may all contribute to the increased range of pH found in this study across all yogurt samples.
The average fat level of the samples used in this study varied from 0.2 to 5.7% (w/w), with notable variations in yogurt taste. According to Lucey et al., the fat level of yogurt ranges from 0 to 10% but is typically between 0.5 to 3.5% fat [39]. It has been reported that the quantity of fat in yogurt plays a crucial role in influencing its flavor, texture, appearance, and taste of the yogurts [40]. The findings of this research demonstrated that the quantity of fat of all thirty-eight samples met these criteria which negatively influenced the overall bacterial abundance. The average fat level of the samples used in this study varied from 0.2 to 5.7% (w/w), with notable variations in yogurt taste. According to Lucey et al., the fat level of yogurt ranges from 0 to 10% but is typically between 0.5 to 3.5% fat [39]. It has been reported that the quantity of fat in yogurt plays a crucial role in influencing its flavor, texture, appearance, and taste of the yogurts [40]. The findings of this research demonstrated that the quantity of fat of all thirty-eight samples met these criteria which negatively influenced the overall bacterial abundance.
It is recommended that yogurt maintains a moisture level below 84%, as higher moisture content can impact both texture and taste [41]. Notably, the sour yogurt samples in this study generally demonstrated higher moisture content than their sweet counterparts. Conversely, sweet yogurts exhibited higher average TS and SNF content compared to sour yogurts. The anticipated TS content ranges are 15.0–22.8% [42] and 18.4–21.41% for fruit yogurt [43]. While the sour samples aligned closely with these values, the sweet samples consistently displayed elevated TS levels, hinting at potential adulteration.It is recommended that yogurt maintains a moisture level below 84%, as higher moisture content can impact both texture and taste [41]. Notably, the sour yogurt samples in this study generally demonstrated higher moisture content than their sweet counterparts. Conversely, sweet yogurts exhibited higher average TS and SNF content compared to sour yogurts. The anticipated TS content ranges are 15.0–22.8% [42] and 18.4–21.41% for fruit yogurt [43]. While the sour samples aligned closely with these values, the sweet samples consistently displayed elevated TS levels, hinting at potential adulteration.
Interestingly, sour yogurt samples in this study showed higher concentrations of certain minerals such as Zn, Na, Ca, and Mg, while sweet yogurt samples had double the amount of Fe compared to sour ones. The concentration of Fe and Cu in the tested yogurt samples exceeded standard values set by the WHO in 1996. Furthermore, sweet yogurt exhibited twice the amount of Fe compared to sour yogurt in our study. Our results for Na, Ca, Mg, and K closely align with values reported in a study by Amellal-Chibane et al. [6] on various market yogurts. Higher Cu levels were observed in commercial yogurt samples, indicating potential migration from dairy animal feed, milk type, and handling contamination during transportation.
Human studies have indicated that calcium (Ca) can hinder iron (Fe) absorption, regardless of whether it is administered as Ca salts or in dairy products [44], emphasizing the intricate relationship between these minerals. Human studies have indicated that calcium (Ca) can hinder iron (Fe) absorption, regardless of whether it is administered as Ca salts or in dairy products [44], emphasizing the intricate relationship between these minerals. In our study, a negative correlation was observed between Ca and Fe, while a positive correlation was found between Ca and ash content. Apart from Ca and Fe, all minerals studied exhibited positive correlations.
Various food components, including sugars, fats, proteins, minerals, vitamins, flavorings, amino acids, and antioxidants, along with processing factors such as heat treatments, homogenization, and fermentation temperature, as well as microbiological factors like strain type and inoculum amount, collectively influence the stability of probiotics in yogurt [45]. The physicochemical characteristics of yogurt are greatly influenced by the microbiome, which impacts the product's quality and safety [46]. Various food components, including sugars, fats, proteins, minerals, vitamins, flavorings, amino acids, and antioxidants, along with processing factors such as heat treatments, homogenization, and fermentation temperature, as well as microbiological factors like strain type and inoculum amount, collectively influence the stability of probiotics in yogurt [45]. The physicochemical characteristics of yogurt are greatly influenced by the microbiome, which impacts the product's quality and safety [46].
In the present study, Sour yogurt samples had a more diverse and abundant microbiological signature and higher nutritional content than samples of sweet yogurt. Sweet yogurt samples displayed elevated levels of the genera Streptococcus, Staphylococcus, Bacillus, Cetobacterium, Hafnia, and Pseudomonas. In contrast, sour yogurt samples exhibited a higher abundance of Enterobacter, Lactococcus, Aeromonas, Acinetobacter, and Citrobacter. Local yogurt brands had a higher relative abundance of bacterial genera like Lactobacillus, Streptococcus, Enterobacter, and Lactococcus compared to commercial brands. The microbial ecology was predominantly influenced by the starter culture genera, potentially minimizing the growth of spoilage and pathogenic bacteria. This study also revealed that, a negative correlation exists between the pH values (4.99–6.45) and the relative abundance of Lactobacillus in thirty-eight yogurt samples. Literature has confirmed that the ideal pH and temperature ranges for Lactobacillus growth are 4.5–6.5 and 30–40°C, respectively [47]. Literature has confirmed that the ideal pH and temperature ranges for Lactobacillus growth are 4.5–6.5 and 30–40°C, respectively [47]. Certain spoilage-causing and pathogenic bacterial genera, including Staphylococcus, Pseudomonas, Acinetobacter, Aeromonas, Shigella, and Enterobacter, were detected in trace quantities in sour yogurt despite its acidic environment. This finding suggests the possibility of contamination occurring after production [48]. The less prevalent species of spoilage bacteria found in yogurt samples may come from utensils, raw milk, environment, and the manufacturing process. Staphylococcus aureus, a significant pathogen indicative of unhygienic handling, processing, and packaging, was previously found in milk or its products (Incidence of Staphylococcus aureus and its enterotoxins in yoghurt). According to de Oliveira GB et al. 2015, psychotropic microorganisms, particularly Pseudomonas species, contribute to the deterioration of dairy products and reduce their shelf life [49]. Pseudomonas, along with Acinetobacter, is implicated in infections such as urinary tract and respiratory infections in humans [50]. While Acinetobacter is a part of the skin's microbial community, it can potentially lead to opportunistic infections. This finding suggests the possibility of contamination occurring after production Despite the acidic environment of sour yogurt, this study identified trace amounts of certain pathogenic bacterial genera, suggesting possible post-production contamination [48]. The less prevalent species of spoilage bacteria found in yogurt samples may come from utensils, raw milk, environment, and the manufacturing process. Staphylococcus aureus, a significant pathogen indicative of unhygienic handling, processing, and packaging, was previously found in milk or its products (Incidence of Staphylococcus aureus and its enterotoxins in yoghurt). According to de Oliveira GB et al. 2015, psychotropic microorganisms, particularly Pseudomonas species, contribute to the deterioration of dairy products and reduce their shelf life [49]. Pseudomonas, along with Acinetobacter, is implicated in infections such as urinary tract and respiratory infections in humans [50]. While Acinetobacter is a part of the skin's microbial community, it can potentially lead to opportunistic infections. Several bacterial genera, including Aeromonas, Shigella, and Enterobacter, are known to decrease the shelf life of dairy products [51]. Moreover, Shigella, a causative agent of foodborne bacterial infections, can be transmitted through contaminated water, food, or direct contact with an infected individual [52]. Several bacterial genera, including Aeromonas, Shigella, and Enterobacter, are known to decrease the shelf life of dairy products [51]. Moreover, Shigella, a causative agent of foodborne bacterial infections, can be transmitted through contaminated water, food, or direct contact with an infected individual [52].
Limitations of the present study include the lack of biochemical characterization of LAB species, the limited resolution of sequence data for identifying bacterial strains at a finer level, and the absence of exploration into the functional genomics of identified strains, among other factors. Despite these limitations, the study's findings hold significance for ensuring the safe production and preservation of yogurt quality. For future research on this traditional dairy product, emphasis should be placed on comprehensive mapping of all microbial consortia, exploring their functional implications in yogurt manufacturing, assessing the impact of yogurt consumption on gut health, and investigating adulteration using a larger sample size.