While a pleasing flavor is a prerequisite to consumer acceptability of all foods, the texture of fermented milks is as important — if not more important — in determining consumer preference of fermented milk products.
Some fermented milk products must be pourable, while others must be spoonable or even spreadable. In some cases, the fermented milk product must withstand extensive mixing with other ingredients and still maintain its characteristic texture. Fermented milk products that are too thick, too thin, too firm, too soft, too grainy or too slimy are all reasons for consumer rejection. Additionally, obvious phase separation (whey off) in the product triggers consumer concerns and negative impressions about product quality (even though, in some cases, users can simply stir the whey back into the product).
Water management and texture
Most fermented milks are 50% to 80% water, yet we want these milks to behave often like a semi-solid! While this sounds like a rather daunting task, let us not forget that fruits and vegetables like celery sticks, apple, oranges and potatoes are approximately 85% to 90% water and most fresh meats are approximately 60% water. So how the water is held in the food system and the properties of the other food constituents have a big influence on the textural properties of any food, including fermented milk products.
In the case of the gel structure typically present in many fermented milk products like yogurt, conditions which form a fine pore structure result in more firmness and usually a creamier mouthfeel. Generally, higher casein content (typically via addition of concentrated nonfat milk solids sources like milk protein concentrate, condensed skim milk or nonfat dry milk), and controlled slower fermentation rates result in a finer casein network. This then results in the desired fine pore structure that can hold water well and provide a strong “scaffolding” system needed to maintain a firm gel structure to handle the challenges of product distribution.
Emerging technologies (high-pressure homogenization, five times that of conventional homogenization pressures) or high hydrostatic pressure processing have been shown to disassociate and re-associate caseins into smaller particles and denature whey proteins to form fine pore structural matrices which give better water holding and firmer gels.
Milk proteins can interact directly with water. Unfortunately, acidification makes it more difficult for milk proteins to hold water. So while lowering the pH through lactic fermentation is necessary to form the typical yogurt gel, excessively lower pHs (below 4.5) can result in less-water-holding capacity in this system and cause free whey. Proper culture selection and proper cooling of the fermented product after fermentation are critical. Also, denaturation of whey proteins associated with the appropriate higher heat treatments given to the milk system prior to fermentation enhances whey protein’s ability to hold water and enhance gel firmness. Adding higher levels of whey proteins (by the addition of whey protein concentrates) can further enhance water-holding capacity and viscosity when processed properly. Because of the wide array of dairy protein ingredients available today, manipulating the whey-protein-to-casein ratio in the milk prior to fermentation allows for an effective water management tool to control gel strength, viscosity and other rheological properties important to the product’s texture and mouthfeel.
Finally, polysaccharides (pectins, starches, gums, etc.) that are directly added to milk or are the result of fermentation (exopolysacharides produced by starter cultures) can absorb lots of water effectively and can help build higher product viscosities with or without gel formation (depending on which polysaccharides are selected). These ingredients used at relatively low concentrations provide complimentary tools for water management and texture manipulation.
For dairy products like sour cream, homogenization conditions can promote fat clustering and increase a product’s body and overall texture. Again, higher-pressure homogenization can make a much finer dispersion of milkfat globules. This tends to be less disruptive to the gel network system and will result in firmer gels with higher viscosity in fermented milk systems.
Dial in the texture
In most food fermentations, the key change (from a consumer acceptability viewpoint) is a change in the flavor due to metabolic products of the growth of the culture. There are significant flavor changes associated with fermented milks compared to unfermented milks. However, ironically, the metabolic products of fermentation coupled with additional process and ingredient technologies result in critical changes in the interactions of the water and other components of the dairy product mixture. The resultant textural properties of the fermented milk product are critical to consumer acceptability.
Understanding how and when to use the existing and emerging technological tools to manage the water in fermented milk systems and the associated interactions between milk components will empower processors to manipulate and optimize the texture, structure and mouthfeel for consumers. So whether consumers want to eat their fermented milks with a spoon or drink them from a glass, dairy processors can deliver the needed food texture and mouthfeel through dairy process and ingredient technology.