One solution is to formulate using another modified food starch that results in a smoother finished texture.

Bob Roberts
Editor's Note: Difficult to define, but, "you know it when you see it," "graininess" in yogurt is a perennial topic of conversation during the annual Penn State Cultured Dairy Products Short Course. Graininess in yogurt is a complex problem with many potential causes including compositional factors, cultural factors and general manufacturing parameters.

Q: What compositional factors can cause graininess in yogurt?

A: One of the primary compositional factors leading to graininess in yogurt is improper selection or use of modified food starch. Modified food starch, when combined with other ingredients such as gelatin, creates a beautiful yogurt stabilizer. However, not all modified food starches lead to smooth-textured yogurt. The size of the starch granule and the post-process integrity of the granule influence the "look" of the finished yogurt base. Some modified starch systems result in a "rough-textured" product. Although these particles do not feel rough on the palate, you can observe particles that look like dust in the finished yogurt. Unfortunately, these particles are so small they cannot be smoothed using conventional smoothing devices. One solution is to formulate using another modified food starch that results in a smoother finished texture.

Q: What cultural factors can result in graininess in yogurt?

A: Cultural factors that can lead to graininess in yogurt include incubation temperature and inadequate mixing of the starter into the yogurt mix.

The optimum incubation temperature for yogurt cultures ranges from about 102°F to 109°F. However, setting yogurt at temperatures near the optimum may lead to graininess because the rate of acid development is so rapid that protein flocculation occurs. By lowering incubation temperature, which reduces the rate of acid formation, a smooth-textured yogurt can be produced using the same starter culture system. Inadequate mixing of the culture into the white mass may result in "hot spots" where the localized rate of acid development is quite high. Graininess may be the end result of this situation. It is also possible to have hot spots as a result of temperature differentials within the vat. The solution to problems of hot spots is to allow a longer time for mixing of the starter culture into the mix prior to quiescent incubation and to ensure uniform temperature distribution within the vat.

Q: What general manufacturing parameters can cause graininess in yogurt?

A: Two manufacturing issues can lead to graininess in yogurt: Insufficient smoothing and residual sanitizer in the vat.

When preparing stirred-style yogurt, the cultured yogurt is pumped through a smoothing device to ensure a smooth texture. Common smoothing devices include back pressure valves, in-line wire mesh screens or perforated plates, or a "kiss" valve. If you are experiencing graininess and using a back pressure valve as your smoothing device, simply close the valve to increase the line pressure to smooth the yogurt. When using an in-line screen, graininess may be eliminated by decreasing the mesh or pore size. When using a kiss valve, graininess is usually a result of some other factor.

Small amounts of leftover acid sanitizer are often found on the bottom of large cone-bottom processors. When the yogurt base is pumped into the fermentation vat, it will immediately coagulate (or directly acidify), resulting in very stable flocs of milk protein. This curd will not re-dissolve in the base during incubation and may not be broken down by the smoothing device, resulting in a grainy product. The curd that is formed at the beginning of vat fill will carry over into the flavoring tanks and make white particles visible throughout the yogurt after it has been flavored with yogurt fruit (especially the darker flavors). To avoid this problem, assure residual sanitizer from process lines is removed prior to the fermentation vat with yogurt base.

Clearly, the grainy defect in yogurt can result from a variety of causes. However, none of these causes is insurmountable and, by paying attention to a few details, production of consistently smooth yogurt can be assured.

Special thanks to Douglas Vargo, senior technical service representative for cultured products, Danisco USA Inc., New Century, Kan., for his insight and helpful discussions on this topic.