Occasionally, two defects that can appear in Swiss style yogurt are white specks in the product and a runny or loose body and texture. What causes this phenomenon, and how do you correct it?
The white speck defect appeared in a 3000 gallon batch of yogurt set in a coned bottom setting or incubation tank. What happened at this dairy was that small white specks looking much like white sand particles were appearing in their darker colored flavors of Swiss style yogurt; namely the raspberry and blueberry flavors. It was commonplace for their production sequence to go from lighter colored yogurt flavors to darker colored yogurt flavors, so as not to have to flush out the filler bowls each time when changing from one flavor to another. One flavor was simply run down right after the previous flavor, the change of color or fruit pieces was observed in the filler bowl, and the new flavor of cups were fed to the machine after the filler operator was sure the old flavor was gone. The sequence went something like this: plain, pineapple, peach, strawberry, raspberry, and blueberry. The white specks were not noticed in the lighter colored flavors. However, they appeared more noticeable in the darker raspberry and blueberry flavors. Why? Where did the white specks come from, and how did they get in there? Upon investigation it appeared that after washing or CIPing the large 3000 gallon tank, some residual acid sanitizer had been left in the tank, and was not emptied out of the vessel before the yogurt base was run into the tank. It was from this source or from the possibility that acid sanitizer from the fill lines was not being drained from the lines out of the bottom of the tank outlet valve. So in this case, some acid sanitizer was being flushed into the bottom of the incubation vat during the first fill of the day.
Acid (from the low pH sanitizer solution) causes dairy proteins to precipitate. In fact, directly acidified curd was being made upon contact with the first 20 gallons or so of yogurt mix entering the setting vat from the fill pipe when it hit the 5-10 gallons of residual acid sanitizer left in the 3000 gallon setting vat. Now, 10 gallons of sanitizer doesn’t seem like much in a 3000 gallon vat, but once that solution has turned skim milk into coagulated or acidified curd (much like directly acidified cottage cheese curd), that hard curd can create problems within the white mass yogurt base after the yogurt mix was pasteurized, cooled and set with the yogurt culture. This coagulated curd could not be smoothed out with the white mass smoothing device that smoothed all yogurt coagulum from the setting tank to the yogurt flavor batching vat, thus causing small white curd specks to appear in the yogurt. This was especially visible in the darker Swiss style flavors such as raspberry or blueberry after the darker colored fruit base was mixed into the plain white yogurt base. This defect can also occur if your fruit is mixed in-line with an in-line static mixer. So please, watch out for any residual sanitizer left in the setting vats, or sanitizer that might be flushed on the way to the setting tank through the fill lines from the HTST system, especially at start-up, or when running the first product of the day to that setting tank!
If your plant makes a vat set yogurt base, and then uses that base to make your yogurt flavors, either in a batch process or mixed with the fruit in an in-line static mixer, it is important not to cool the yogurt base down too far after it is broken if that yogurt contains a stabilizer that uses gelatin as part of the stabilization system. Granted, cooling yogurt base down to a certain temperature is good for control of finished yogurt pH, and to make a yogurt that is not too sour for the average American consumer. However, that means strict temperature control must be maintained. When using gelatin, hydrogen bonds start to form that build the gel and create the “set texture” or typical gelatin-like wiggle. Destroying or shearing those bonds that have started to form will make a runny or loose textured yogurt. That means not pumping, stirring, or filling yogurt that has been cooled to much less than 60F, since with gelatin the hydrogen bonds start to form and set at temperatures of 60F and below.
At the dairy that I worked at, we would make and culture the yogurt base in a pasteurized setting tank at an incubation temperature of 106F. It would incubate to a pH of 4.7 or less. At this time slow stirring was applied to the yogurt base tank and chill water was run through the jacket to bring the temperature of the yogurt base down to 65F with constant or intermittent stirring. We cooled our yogurt base to 65F recognizing that it would be combined with cold fruit at a 15% level before stirring in that fruit and pumping the blended yogurt to the yogurt fillers. This temperature control more or less quickly stopped the yogurt culture from growing and producing lactic acid that would lower the pH further. By the time the cooling process was done and the yogurt was mixed with the fruit flavor and additional sugar and pumped to the filler, the pH was in the 4.3 range. Research has shown that Americans don’t particularly prefer a yogurt that has a pH of 4.10 or less. It is too tart for them. After the fruit was mixed in the temperature of the blended yogurt was 58-60F at the filler. Doing this saved the gelatin functionality preventing a loose runny texture in the finished yogurt. That preserved the set yogurt texture after that yogurt was palletized and cooled to <45F in the cooler before shipping it to the grocery store warehouse and eventually the end user.
While plants don’t make that heavy “gelatin set” type of yogurt much anymore, it is also possible to totally destroy a gelatin set in yogurt after the finished yogurt has cooled. This occurs by slamming the package or the whole case of yogurt onto the pallet too harshly after the yogurt is totally cooled to 40F and ready for shipment. I have seen some “rowdy palletizing employees” palletize gelatin set yogurt too harshly or drop it down 2-3 feet on top of other cases, which will also destroy that gelatin type of set. This can also happen when picking cases off of a pallet for individual order picking, or when placing the product on shelves in the retail supermarket. In this case the yogurt will appear curdy or lumpy in the package, and not have a completely smooth gel texture like it should. Instruct line pickers or palletizers to handle yogurt gently with no more than a 6” drop onto the pallet or into the dairy case when loading out full cases of yogurt when hand selecting product.
So if your yogurt doesn’t have the finished product body and texture you developed it to have, and you are using gelatin as part of the yogurt stabilization system (modified food starch and gelatin are common), you may just need to look at how the yogurt is being handled before and after packaging. Don’t get it too cold in the vat after incubation. And if the yogurt is fine in the cooler when packaged and looks as you intended it to look at the dairy, but does not appear the same at the store level when the consumer gets it, you might look to see if you have any “rough handlers” out there in the distribution chain; palletizers, delivery truck drivers, and grocery store stockers (and that includes the checkout cashier who actually puts the yogurt into the grocery bag).