In the Lab: Popular Yogurt Blends

By: Lily Orr | Technical Specialist, Dairy Connection Inc.

The Objective

When I started working at Dairy Connection in June, it was hard for me to wrap my head around the differences in our yogurt culture blends. I could read the technical sheets and descriptions of predicted outcomes, but I knew I would not be able to grasp them without touching and tasting the end product. I am sure many of you feel similarly when surfing our product listings. Knowing the taste, aroma, and other sensory attributes are important to understanding the function and application of cultures, so I put on my white coat and into the lab I went.  

With the help of Dairy Connection Founder and Owner Dave Potter, we created fermentation trials of four of our most popular yogurt culture blends; 2651, ABY-2C, ABY-611, and ABY-653. The compositions and general characteristics of these blends are noted on the table below. You may note that, on paper, these blends contain the same (or nearly the same) bacteria; an important difference, however, is that the ratios of individual bacteria vary between the blends.

The Process

We used one gallon of store-bought, pasteurized and homogenized milk containing 3.38% fat, 3.38% protein, and 5.07% lactose. We heat-treated the milk to 180-185°F for 30 mins, cooled to 110°F, then dispensed 600mL each into four stainless-steel pots and inoculated with culture at the rates noted in the chart above. The stainless-steel pots were incubated for 6-8 hours at 108°F in a circulating water bath.

Using a continuous pH monitoring system, CINAC®, we were able to track the acidity increase (pH decrease) over time and plot on a graph for visual aid. Due to initial tech troubles, the start time of the chart below is roughly an hour after we inoculated and began incubating the samples. We do not have data for that first hour, but the milk pH was 6.7 before the initial heating step.

Once incubation ended, the samples were transferred to the refrigerator to cool overnight, then stirred and test-tasted the following morning.

The Results

The isoelectric point for casein (milk protein), noted by the dark gray line at pH 4.6, is when casein loses solubility and clumps or precipitates out of the liquid. This is what you see happening in lactic-set dairy products such as chevre, which typically rely primarily on acid production for coagulation (as opposed to rennet/coagulant enzymes).

The sample of 2651 reached pH 4.6 first at around 4.5 hours and hit the lowest pH of 4.33.

ABY-2C initially started dropping more quickly than ABY-653, but then ABY-653 ramped up acid production and reached pH 4.6 sooner.

ABY-611 had a long lag then a very gradual drop and finally hit pH 4.6 after 7.5 hours.

The Discussion

Timing: If you are looking for a yogurt blend that is fast-acting -- whether it’s the need to turnover tanks/vats or keep work shifts to a specific length of time -- then 2651 is your best choice. Both ABY-2C and ABY-653 will give you middle of the road timing, while ABY-611 will have the longest ferment time. This is based on fermentation at the optimum 108°F; lower temperatures (100-105°F) would be slower overall and higher temperatures (110-112°F) would be quicker overall, but either direction would destabilize strain proportions and not give optimal sensory results.

Flavor and aroma: You know the adage for barbecuing meat “low and slow” for best flavor? Well, that’s what came to my mind when assessing the flavor of the yogurt samples and comparing that to their pH curves. The sample of 2651 acidified the quickest but was the mildest and roundest on the palate. The yogurt sample with the most tart and most fermented flavor was ABY-611, which is the one that took the longest to reach our pH target. The ABY-2C and ABY-653 landed in the middle again – no surprise there – but ABY-2C was the milder and milkier of the two while ABY-653 was more tart with green apple flavor.

Body and texture: We do not have a viscometer so yogurt body was analyzed by mouthfeel and spoonability. All samples had medium to thick textures and stuck to their spoons well, with ABY-2C being the least heavy. ABY-653 seemed heaviest on the palate. The samples of 2651 and ABY-611 had slight ropiness to it, created by strains of organisms that produce exopolysaccharides (EPS). Strains that produce this byproduct are found in many yogurt blends. See bonus notes below about TA 40, a specific EPS producing body strain.

Post-production acidification (PPA): The samples of 2651 and ABY-653 exhibited very little to no post-acidification after sitting for one week in the fridge, getting down to pH 4.3 and pH 4.4, respectively. ABY-2C exhibited slight post-acidification from pH 4.56 at end of incubation to pH 4.4 after sitting for one week in refrigeration. The blend with the most post-acidification was ABY-611, dropping from pH 4.59 at the end of incubation to pH 4.4 after one week in refrigeration. It is important to note that 2651 and ABY-653 were already at lower pHs by the end of their incubation periods, so that could have impacted why there was little post-acidification seen from these two blends. But these observations of post-acidification for all four blends follows predicted characteristics from the product data sheets (see chart above).

At 10 days in the fridge, there was observation of free whey starting to accumulate on all samples. Then at 3-4 weeks, there was further slight post-acidification happening. The sample of 2651 dropped into the pH 4.2s while the other three samples stayed in the pH 4.3s. All developed more perceptible sourness, with ABY-611 seeming the most affected flavor-wise, becoming “sharp”.

Bonus Notes: TA 40

Along with the four yogurt blends, we also acidified a milk sample with TA 40 at 0.03g/L inoculation rate.

TA 40 is a single strain of Streptococcus thermophilus that can be used in yogurt and other cultured dairy products to enhance texture and body. Please note that both Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus are both required to satisfy the standard of identity of yogurt in the United States.

We did not track the acidification curve of this culture as it is not used alone as a starter culture in yogurt (though it does have lactic acid production), but rather as an adjunct to enhance texture and body through exopolysaccharide (EPS) production. EPS is the sugar-based polymer secreted by the lactic acid bacteria that binds water and interacts with milk proteins to enhance gel formation and mouthfeel.

The sample turned out very ropy and thick with a sheen to it and a sweet flavor. We could not stop eating spoonfuls of it! Add this to any yogurt where you want to increase body and “stretch” with each spoonful.

Conclusion

This lab experiment focused on only four culture blends for yogurt production, but this is a sliver of what is available. Our technical team is here to help you enhance your current (or create new) yogurt and other kinds of fermented milk products. We'll work with you to identify the acidifier culture blend, bioprotective cultures, and/or adjunct cultures that best fit your application and meet your product goals.