Fermented dairy products such as yogurt are widely consumed worldwide due to their rich pool of bioactive proteins, hydrolyzed carbohydrates, vitamins, and minerals, and are associated with positive effects on human health. Yogurt and related products are prepared by milk fermentation using starter cultures, mainly lactic acid bacteria. Yogurt is prepared with the protocooperation of Streptococcus thermophilus and Lactobacillus bulgaricus. Initially, Streptococcus thermophilus, which is resistant to oxygen, begins to acidify and lowers the pH to 5.2, while at pH 4.4, the growth of acidity is dominated by Lactobacillus bulgaricus. At this pH, fermentation is stopped by rapid cooling to 4 °C. The optimal acidic conditions of commercial yogurt should be in the range of 7-9 mg/g of lactic acid and pH 4.4-4 to avoid the excessively acidic taste of the product. Acid-resistant Lb bacteria continue to produce lactic acid at a slow pace during refrigerated storage, transportation, and marketing leading to the well-known phenomenon of post-acidification.
Factors affecting post-acidification
The quality parameters of any fermented dairy product are affected by several factors such as starter culture, milk composition, temperature and pH, homogenization and stirring, pre- and probiotics, and packaging materials.
1- Type of starter culture
Starter cultures are broadly classified based on morphology (rod or cocci), fermentation pathway (homo-, facultative hetero- or heterofermentative), incubation temperature (mesophilic or thermophilic), and composition (mixed or defined). Most of the dairy starters used during milk fermentation belong to the genera of Lactococci, Streptococci, Leuconostoc and Lactobacilli. To offset post-acidification, yogurt production units use starters with a high cocci/bacilli ratio, which leads to a decrease in the production of the main flavor compound of yogurt, i.e. acetaldehyde. The genetic changes of the specific strain in a starter culture lead to variation in enzyme activity and reduce post-acidification.
2-Milk composition
The kinetics of acidification and acid development after the process are different depending on the milk of different ruminants, milk composition, total solids level, and interaction between milk components. LB strain showed faster growth, acidification, and higher peptidase activity in goat milk. Similarly, goat’s milk showed faster acidification during yogurt production and a constant pH in the range of 4.1 during 29 days of storage, while the pH was 3.9 in yogurt made from a 50:50 ratio of cow and goat’s milk. Higher post-acidification potential of ST in milk as compared to plant substrate (equal weight of 10% hydrolyzed oat powder and soy milk). Skim milk powder (SMP), milk protein powder (MPC), and casein hydrolysate (CH) showed significant differences in fermentation time compared to pure milk, and casein hydrolysate improved fermentation rate and probiotic stability. Yogurt enriched with casein hydrolysate having varying degrees of acidification (8.5%, 14.6%, and 26.7%) showed a higher pH (4.18-4.37) after 30 days of storage, indicating its post-acidification control potential. However, higher CH concentration leads to bitterness and low viscosity in yogurts.
Modification in milk components, especially protein and lactose, causes acidification kinetics in the stages before and after fermentation. Skimmed milk and whole milk treated with transglutaminase enzyme (40°C/ 2h) before fermentation lowered the post-acidification in yogurt during 25 day storage period due to reduction of gel pore size, higher water holding capacity (γ-Glu)-Lys bonds, regular distribution of proteins and reduction in low molecular weight peptides required for microbial growth. Reducing the lactose content of skimmed milk to less than 2% is an alternative strategy to prevent post-acidification but it produces a soft coagulum. Usually, a good coagulum is obtained with lactose above 2%, and protein above 8% leads to an increase in viscosity.
3-Temperature and pH
The rapid cooling of yogurt in refrigerated conditions (<10°C) is the most important factor in controlling its final pH (4.0-4.4) by limiting the metabolic activity of the starter culture. Storage <1 °C is used to control the yogurt’s acidity. However, similar approaches are unable to prevent post-acidification due to the residual activity of microorganisms. The yogurt prepared with fresh sheep’s milk showed more sensitivity to post-acidification than yogurt prepared with frozen/thawed milk due to higher buffering capacity.
The termination pH of fermentation may also affect starter culture activity, levels of organic acid, gel formation kinetics, and probiotic survivability. Probiotic-supplemented yogurt has increased proteolysis which relies on termination pH of fermentation and strain. Enhanced proteolysis showed greater survival of Lactobacillus bulgaricus and higher organic acid production during 28 days, which is due to the availability of free amino acids and peptides produced during proteolysis.
4- Homogenization and stirring
During yogurt production, milk is generally homogenized at a pressure of 10-18 MPa at 55-65 °C to prevent fat separation and improve stability, whiteness, viscosity, and water-holding capacity. Homogenization reduces the globular size of fat, which creates a better incorporation of fat into the protein network, thus enhancing the interaction of fat with casein and denatured whey protein during acidification, and subsequently improving gel properties.
Based on the texture and production method, yogurt is divided into set (fermented in containers) and stirred (or strained or Greek). Stirred yogurt is viscous, creamy, and has a smooth texture compared to the continuous gel structure of set yogurt. The conversion of set yogurt into their stirred version showed better water holding capacity, but objectionable coarse and grainy texture due to the ability of stirred gels to recover their structure during post-acidification at refrigerated storage.
5-Pre- and probiotics
Probiotics are living microorganisms that increase the host’s health if consumed in sufficient amounts. amounts. Fermented dairy products serve as an excellent platform for incorporating probiotics as these products promote probiotics growth during the fermentation phase in addition to providing excellent nutrient density. The co-culturing of probiotics with lactic starters affect the kinetics of acidification and post-acidification. Yogurt prepared with ST and probiotic Bifidobacterium lactis had less post-acidification due to the limited ability of Bifidobacterium to produce acid at refrigerated temperature. Synbiotic soy yogurt prepared with a combination of probiotics (Lactobacillus acidophilus, Lactobacillus plantarum, and Lactobacillus rhamnosus) showed the greatest pH reduction for L. plantarum and L. rhamnosus combination compared to other single or binary combinations during storage at 4°C for 28 days.
Prebiotics are non-digestible food compounds that are specifically used by the group of beneficial microbes in the gut microbiome and thus promote the health of the host. Galacto-oligosaccharides (GOS) and fructo-oligosaccharides are two common prebiotics that are often used as food by probiotics. Several studies suggested the role of prebiotics in enhancing the fermentative behavior and post-acidification rate. Lactulose fortified skim milk (4%) showed an increased acidification rate in fermentation with LDB, Lactobacillus acidophilus, and Lactobacillus rhamnosus. Lactobacillus acidophilus showed the highest post-acidification (4%) rate after the seventh and thirty-fifth days due to its superior metabolizing power of fructose moiety. Similarly, the effect of co-cultures containing probiotics and prebiotics (maltodextrin, polydextrose, and oligofructose) on the fermentation kinetics of fermented products based on skim milk was investigated. Oligofructose and polydextrose supplemented the growth of probiotics with the highest positive effect on B. lactis, and all three prebiotics stimulated the post-acidification. Based on the studies mentioned above, it is a definite conclusion that prebiotics can undoubtedly amplify the post-acidification in fermented dairy products due to the selective use of certain strains of probiotics.
6-Packaging materials
Different combinations such as aluminum foil/plastic, paper/plastic laminate, thermoformed HIPS (high impact polystyrene), glass containers, HDPE (high-density polyethylene) bottles, and LDPE (low-density polyethylene) pouches are the most common options available for both set and stirred yogurt type of fermented dairy products. Higher post-acidification was reported in ethylene vinyl alcohol (EVOH) and glass containers but exhibited better flavor scores due to improved retention of certain aroma compounds. These high-barrier packaging materials resulted in negative redox potential due to oxygen consumption by microbes, thereby creating stressful conditions for aerobic microflora. Polypropylene, polystyrene, and glass containers were found to affect post-acidification in 0 and 4% fat yogurt, with a more significant effect in 4% fat yogurt in polypropylene containers and the least effect in 0% fat yogurt in glass containers. The post-acidification analysis of probiotic yogurt in plastic containers with different oxygen transmission rates (OTR) and fortified with glucose oxidase enzyme showed a higher post-acidification rate in containers with lower OTR. These containers had less dissolved oxygen and higher probiotic microflora count.
It was reported about modified atmosphere packaging (MAP) of yogurt by injecting nitrogen in the headspace of yogurt containers, reducing the residual oxygen in the headspace of the containers to 0.1-0.2% and increasing its shelf life to 8 months at 4.4 °C. Therefore, packaging material should be meticulously selected after considering the starter type and its metabolism, food composition, and storage conditions for controlling post-acidification.
Conclusion
Yogurt and related products have a wide consumption pattern all over the world. The quality of such products decreases during cold chain storage and transportation due to residual metabolic activity of viable starters leading to post-acidification, which in turn reduces the product’s shelf life, decreases consumer acceptance, and is even detrimental to the stability of probiotics. This phenomenon is prevalent in tropical and resource-poor countries, hence, there is a growing interest in inhibiting post-acidification without affecting product quality. Factors such as the type of starter culture, milk composition, processing parameters, pre- and probiotics, and packaging materials are influential factors in the post-acidification of yogurt and fermented products.
Reference:
Deshwal, G.K., Tiwari, S., Kumar, A., Raman, R.K. and Kadyan, S., 2021. Review on factors affecting and control of post-acidification in yoghurt and related products. Trends in food science & technology, 109, pp.499-512. DOI: 10.1016/j.tifs.2021.01.057
