By Jon Hopkinson, Ph.D.
In the very beginning when dairy animals were first domesticated, milk has spoiled (using the word “fermented” is a much nicer term) in one way or another. Once in a while the fermentation yielded a pleasant product. If it was fermented in a cooler climate the result was a mostly mesophilic-like fermentation yielding a product much like buttermilk. If it was fermented in a hot climate, the fermentation was mostly thermophilic and yielded a product much like yogurt. The cream that separated in the vessel became sour cream or crème fraiche. If the container was porous the fermented milk could became concentrated and yielded a product something like Greek Yogurt or Quark. If the fermented milk was diluted out (by rain or on purpose to stretch out what might be a limited food source) what resulted was a pleasant fermented drink. Of course most of this spoilage was not pleasant to taste or even safe, but over time the fermentations that resulted became more pleasant and nourishing and long lasting products everyone liked to consume were selected to be made regularly. It is from this library of improved fermentations that microorganisms were preferentially selected and grown over the years which have now become the library source or backbone for the culture suppliers that service the cultured product industry today.
Convenience has always been and will always be a driving factor in our food choices. A cultured dairy product that can be consumed straight from a bottle or cup is far more convenient than something that must be eaten with an implement like a knife or spoon. It also seems more natural to consume a dairy product by drinking than it is to eat solid or semisolid milk. Because of this, there are many delicious drinkable fermented milk products being made and consumed around the world.
Of course there are challenges to producing a convenient cultured product. Milk in general is not stable under the conditions that result when it is fermented (to a low pH). This is not a problem when the fermented product is stirred just before serving, or if it is made fresh every day. Coagulated milk with whey floating on top is very unappealing to the average consumer because it indicates that the milk is spoiled in some way. The dairy industry has come up with several strategies to overcome this challenge. Since the apparent homogeneity of the product is what the consumer is looking for, the obvious solution to this problem is to reduce the curd size (or what might be perceived as a grainy texture) in the fermented product. This can be accomplished by running the curd through some kind of shear process (mixing, stirring, blending, pumping, homogenization or other process where the product is violently agitated). Of course this is usually not a stable solution for a fermented dairy product, but if the viscosity is sufficiently high or a gelling hydrocolloid or blend of hydrocolloids is used, the product can be stable enough to reach the end of shelf life. Producers most often label the package “shake well” before consuming and put the product inside an opaque package to help hide any separation or wheying off in the retail package.
To produce a stable product with no syneresis (whey on top or bottom) it is important to understand the forces that are in play when a dairy product is acidified. Above pH=5.5 the average charge on the casein protein is positive and below pH=4 the average charge is negative. Casein between these two pH values is in transition from a positive to a negative net charge. In this region the casein micelles are able to more easily interact with one another because the electrostatic repulsion falls to near zero. Further in this pH region, the calcium phosphate that helps to physically stabilize the casein micelles dissolves removing this stabilizing effect. In this region the average diameter of the casein micelles shrinks. It is believed that the kappa casein which above pH=5 stearically stabilizes the casein micelle either retreats to the interior of the micelle or lies over on the surface. This causes the casein micelles to loose stability, the most important stabilizing factor. In this region (pH=5.5 tp pH= 3.5) the majority of the stabilizing factors for the casein micelle are reduced to near 0. This allows the casein to aggregate, form a curd and flocculate.
To produce an acidified dairy beverage (or to ferment) we will need to restore at least some of these stabilizing mechanisms. One very good method is to restore all or part of the steric stability due to K- Casein. Some kind of molecule that can be attached to the casein, in some way, and at the same time extend into the water surrounding the casein would provide some steric stability. This molecule should not be so large as to bridge casein particles together and cause them to flocculate. It should not have too many sites where the casein can connect or so few that it won’t connect well to the casein. As an added bonus the molecule should provide a yield point (very soft gel or high still viscosity) to the milk solution to help prevent sedimentation. There are two such hydrocolloid molecules that can provide this kind of an effect in milk and acidified milk. Those are a certain form of carboxymethyl cellulose (CMC) and high ester pectin. Both of these are charged positively at pH more than 3 and so are attracted to negatively charged casein particles. Also they can be manufactured (CMC) or selected (pectin) to have the correct number and placement of charges along their molecule to give the proper steric stability and to avoid bridging between protein particles. DuPont provides special CMC and Pectin for this application; both of these are labeled “AMD” for acidified milk drink in their application. These hydrocolloids are formulated, selected and standardized specifically to stabilize protein in acid dairy beverages.
Unfortunately, using these molecules to help stabilize protein is not as simple as adding a thickening or gelling agent to the acidified milk. Simply adding these to coagulated milk will result in the protein particles forming very large aggregations of protein. Unchecked the hydrocolloid will bridge the particles together primarily through charge interaction. The speed that protein particles fall to the bottom of the container is directly proportional to the particle’s diameter. What is needed is some additional process that separates the protein + stabilizer particles into as small a particle size as possible. Fortunately there exists among any standard dairy product processing equipment just such equipment, called the homogenizer! (Other shear producing equipment like shear pumps, Silverson type in-line mixers etc. also can work, but not quite as well, see the chart below, Figure 1&2).
The general process is to add the pectin (or CMC) in the form of a slurry to the acidified milk and then run this mixture through a homogenizer set to about 2500 psig. For yogurt, after fermentation is complete, the curd is broken and the pectin slurry containing pectin and sugar is added. The product is mixed and then homogenized; this is followed by cooling, flavoring and packaging. The result is a product slightly more viscous than whole milk with a clean acid dairy flavor that is stable even if the product is heated. Below is a formula for a yogurt beverage: