The Role of Bacteria in the Health Potential of Yogurt

Fig. 1. Yogurt as often seen and consumed. Courtesy of http://beezwaxpromo.com

Introduction

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The Biochemistry Behind Yogurt

Fig. 2. Overview of biochemical processes in yogurt production. Courtesy of The Food and Agriculture Organization of the United Nations.

Fig. 3. Lactose catabolism into glucose and galactose. Courtesy of Thomas M. Terry at the University of Hamburg.

Fig. 4. Glycolysis and homolactic fermentation. Courtesy of Dr. Todar's Online Textbook of Bacteriology.

Introduce the topic of your paper. What microorganisms are of interest? Habitat? Applications for medicine and/or environment?

Yogurt Production

Fig. 5. Scanning electron micrograph of Lactobacillus bulgaricus. Courtesy of The Microscopy Facility at Utah State University.

Fig. 6. Scanning electron micrograph of Streptococcus thermophilus. Courtesy of Dennis Kunkel Microscopy, Inc.

Benefits of Yogurt

Include some current research, with at least one figure showing data.

Fig. 7. Bacterial enzyme activities (µmol/min per gram of protein) in fecal samples obtained from non-yogurt consumers (Group N) and yogurt consumers (Group Y). Only β-galactosidase activity levels were significantly different. Figure courtesy of Alvaro et al., 2007.

Probiotics

Fig. 8. Beneficial effects and therapeutic applications of probiotics as proposed by Fuller (1989). Courtesy of Lourens-Hatting and Viljoen (2001).

Include some current research, with at least one figure showing data.

Fig. 9. Scanning electron micrograph of Lactobacillus casei. Courtesy of The Microscopy Facility at Utah State University.

Fig. 10. Gram stain of Lactobacillus acidophilus. Courtesy of Dr. Todar's Online Textbook of Bacteriology.

Fig. 11. Scanning electron micrograph of Bifidobacterium. Courtesy of Dr. Sandy Smith, Dept. of Food Science, University of Guelph, Canada.

Fig. 12. The effect of probiotics on suppressing Helicobacter pylori density and infection. (A) Number of H. pylori colonies depending on the concentration and type of probiotic bacteria present on the plate. La5 refers to Lactobacillus acidophilus and Bb12 refers to Bifidobacterium lactis. (B) Characteristics of biopsy sites before and after yogurt treatment containing La5 and Bb12. H. pylori density was graded from 0 to 5, while activity of gastritis and gastric inflammation were graded on a scale of 0 to 4. Each value is a sum of 2 biopsies. Courtesy of Wang et al., 2004.

Fig. 13. The amount of ACE-inhibitory activity as depicted by the bars and IC50, the sample concentration in mg/mL of probiotic cultures in soy yogurt needed to inhibit ACE activity by 50%, as depicted by the lines. Fermentation was halted at pH 4.50 and the samples were stored at 4°C for 28 days. Courtesy of Donkor et al., 2005.

Improving Yogurt

Current Problems

Include some current research, with at least one figure showing data.

Fig. 14. Hydrolytic breakdown of lactoferricin, as indicated by the % peptide remaining, at pH 7.0 by various strains of L. bulgaricus and S. thermophilus. Courtesy of Paul and Somkuti, 2010.

Fig. 15. Hydrolytic breakdown of a hypotensive peptide by various strains of L. bulgaricus and S. thermophilus at different pH values. Courtesy of Paul and Somkuti, 2009.

Improving functionality of Yogurt

Fig. 16. The stimulating effect of various prebiotics on probiotic growth and function. Courtesy of Ranadheera, Baines, & Adams, 2009.

Towards a "Superior" Yogurt

In addition to improving the health potential of yogurt, from an industrial point of view, there is interest in manufacturing a more palatable type of yogurt that appeals to the masses and can be produced efficiently. Such yogurt is generally smooth, mild, and pleasantly sour, though these characteristics may be subject to individual taste. The outcome of yogurt is affected by the quantity and rate of lactic acid addition. L. bulgaricus and S. thermophilus are known to be facultative anaerobic bacteria that can grow in oxygenated environments. It has been found that these species remove the dissolved oxygen in the yogurt mix during fermentation, and only actively begin to produce lactic acid after the dissolved oxygen (DO) concentration in the yogurt mix is lowered to 0 mg/kg (Horiuchi et al., 2009). This suggests that lactic acid production is suppressed by dissolved oxygen in the yogurt mix. By altering the DO concentration to begin with, one could essentially control when lactic acid production begins. In addition to jumpstarting, lactic acid production was also prolonged in a culture that started with 0 mg/kg DO, or reduced dissolved oxygen fermentation (ROF), compared to the control of around 6 mg/kg DO. This means that it takes less time to reach a desired acidity level if yogurt is produced by ROF (Fig. 17A). Moreover, despite initiating lactic acid production much earlier and maintaining it for a longer time, the viable cell counts of the bacterial species and characteristics of the ROF yogurt such as acidity and curd tension were no different from the control yogurt (Horiuchi et al., 2009). One advantage of using ROF is the reduction of production time, as the cultures enter the exponential growth phase sooner. It has been shown that yogurt made at a low temperature produces smooth yogurt and the physical properties of yogurt are improved as the starter culture are given more time to produce aroma substances and other accessory molecules that affect the taste, and are able to block fast acid production (Guzel-Seydim, Sezgin, & Seydim, 2005). Given this fact, Horiuchi et al. (2009) set out to make a yogurt that was smooth but could be counteracted with ROF to reduce the wait time compared to traditional low temperature production, which requires more time in exchange for a smooth product. Using low temperature reduced dissolved oxygen fermentation (LT-ROF), the researchers achieved a "superior" set yogurt with a smooth texture and a strong curd structure. Moreover, the marketability of this type of yogurt production is increased, as Horiuchi et al. (2009) have shown that yogurt produced by LT-ROF takes less time than traditional yogurt produced at 37°C (Fig. 17B), and is comparable to the faster common yogurt production that takes place at 43°C (compare control of Fig. 17A and LT-ROF of Fig. 17B).

Fig. 17. The difference in change in % acidity over time between two levels of dissolved oxygen. The culture contained both L. bulgaricus and S. thermophilus. (A) Experiment carried out at 43°C. The circle represents the reduced oxygen fermentation treatment and the square represents the control with normal levels of dissolved oxygen. (B) Experiment carried out at a lower temperature of 37°C. The circle represents the reduced oxygen fermentation treatment and the triangle represents the control with normal levels of dissolved oxygen. Courtesy of Horiuchi et al., 2009.

Regardless of the actual percentage of acidity, what is important is how the yogurt appeals to consumers. The LT-ROF yogurt was evaluated to be smoother and milder on average by 200 consumers compared to control yogurt produced at the standard temperature of 43°C (Fig. 18). However, the LT-ROF yogurt turned out to have a smooth texture comparable to the control yogurt made at a lower temperature (37°C). The major difference between these two types of yogurt was the firmness of the curd (Fig. 19). Horiuchi et al. (2009) suggested that having a firm curd was beneficial since manufactured yogurt needs to be transported in trucks, thereby requiring a firmer texture that can withstand the shaking. The researchers thus concluded that the LT-ROW method of producing yogurt was the superior method as it takes about the same time as producing yogurt at 43°C and results in a smooth yogurt that can hold its shape while in transit.

Fig. 19. Difference in firmness and appearance between the control yogurt produced at 37°C and the yogurt produced by low temperature (37°C) reduced dissolved oxygen fermentation. Courtesy of Horiuchi et al., 2009.

Fig. 18. Evaluation of the taste and sensation of yogurt produced under varying conditions. A represents low-temperature (37°C) reduced dissolved oxygen fermentation and B represents the control fermentation at 43°C. Each characteristic was evaluated on a scale of 1 to 5 by 200 yogurt consumers. Courtesy of Horiuchi et al., 2009.

Another study focused on the effect of temperature and starter culture type on the quality of yogurt. Guzel-Seydim, Sezgin, and Seydim (2005) compared how the quality of yogurt changes based on whether it is produced at high (45°C) or low temperatures (35°C) and whether exopolysaccharide-producing or non-producing strains are used. These exopolysaccharides are of interest because it is a ropy, mucoid substance that increases the viscosity of yogurt and decreases whey separation. Guzel-Seydim et al. (2005) quantified the quality of yogurt by looking at the pH, lactic acid percentage, total volatile fatty acids content, acetaldehyde content, tyrosine content, consistency, viscosity, and extent of whey separation of the yogurt samples. They found that the ropy exopolysaccharide-producing strains had a better texture overall when incubated at the lower temperature, but the non-exopolysaccharide strains actually had a better taste, as evaluated by the higher acetaldehyde content. Nonetheless, the researchers suggest that these exopolysaccharide-producing strains may be useful in replacing additives like fat that are used to improve the texture of yogurt. Thus, as is the case with most foods, one would hope to improve not just the taste and texture of yogurt, but also the nutritional value, which also ties back to the health benefits that were described earlier.

Conclusion

Yogurt has a long history and its benefits have been valued by many people, particularly those with gastrointestinal problems. The production behind yogurt is well understood, allowing for improvements and advancements in both the quality and efficient manufacturing of the product. Improving the health potential of yogurt has become a popular field, and for industrial reasons, enhancing the taste and texture, as well as storage life of yogurt is an appealing advancement for yogurt consumers. Yogurt in its basic form is a very eco-friendly product, as humans are essentially consuming the waste products of acidic fermentation. Additionally, the unique taste, texture, and potential for even better health benefits make yogurt an attractive food for people of many cultures.

References

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Edited by student of Joan Slonczewski for BIOL 238 Microbiology, 2010, Kenyon College.