Milk transforms into an array of dairy foods when its pH is lowered. This is readily accomplished via the addition of starter cultures or acidulants, both of which make milk proteins coagulate and thus contribute to the finished product’s texture and mouthfeel. Starter cultures and acidulants also enhance flavor through various chemical pathways. Depending on the choice of culture or acidulant, the end result can be buttermilk, cottage cheese, cream cheese, natural cheese, sour cream, spreads or yogurt.
Indeed, the essential step in the manufacture of all the aforementioned dairy foods is controlled production of lactic acid from lactose by lactic acid bacteria (LAB), which is also known as a fermentation. It is possible to skip the fermentation and directly add acid, which more often than not is lactic acid. However, depending on the product, other acids can be used to enhance flavor.
Starter culture genomicsAccording to a review published in the Journal of Dairy Science (2007, 90:4005-4021), today’s starter cultures are developed mainly by design rather than by traditional screening methods or trial and error. Advancements in genetics and molecular biology provide scientists the opportunity to study these organisms and engineer them to improve their functionality and efficiency.
Genomes of LAB contain both plasmid and chromosomal DNA. The characterization of plasmids in LAB has been an ongoing area of study for more than 30 years and advancements have been very useful to dairy processors.
For example, the starter cultures Lactococcus lactis and Streptococcus thermophilus are both readily attacked by phages. Bacteriophages are viruses that attack and destroy bacterial cells. Their presence in a dairy fermentation environment inhibits the activity of starter cultures. In other words, starter cultures are destroyed and no fermentation takes place. Being able to choose phage-resistant strains for cheesemaking can have major economical impact on operations.
Years of research has shown that in lactococci, bacteriophage resistance is one of several industrially important traits that may be encoded by plasmid DNA. Scientists have found that many lactococcal phage-resistance plasmids can be transferred into other strains of L. lactis by conjugation. Because conjugation is considered a natural form of gene transfer, dairy LAB that are genetically improved by this method are not considered genetically modified organisms according to regulatory and activist groups. In fact, conjugation-derived, phage-insensitive dairy starter cultures have been in commercial use for many years.
Characterization of LAB chromosomes has also been going on for a long time; however, the most exciting developments in LAB genomics are now being fueled by nucleotide sequence information for complete genomes. Currently, the genome sequence is known or is being determined for more than 20 LAB. Some of this research is being conducted by culture suppliers for their own proprietary use, while other research is publicly funded and is available to the general scientific community. (A complete list can be found in the Journal of Dairy Science reference.)
Why obtain the genome sequence for dairy-related LAB? Here’s an example of how such data is helpful to cheesemakers.
Bacteriophage is a trait that varies from strain to strain. Thus, when multiple genome sequences within a species are known, it allows for the study of strain-specific traits. A comparison of the complete genome sequences of two strains of Lactobacillus delbrueckii ssp. bulgaricus identified regions involved in bacteriophage resistance. Suppliers can screen the species for such strains and deliver to cheesemakers only those strains that resist phage.
Here’s an example involving proteolysis, which is a critical part of cheese making. Scientists at the University of Wisconsin-Madison have spent more than a decade examining the proteolytic system of a specific strain of the cheese starter Lactobacillus helveticus. The outcome of these efforts was the characterization of 12 genes in L. helveticus CNRZ32 that encode proteolytic enzymes. Despite these concerted efforts, initial annotation of the CNRZ32 genome sequence revealed a large number of additional genes in CNRZ32 whose products are predicted to contribute to the proteolytic enzyme system of this bacterium, according to the Journal of Dairy Science article reviewers.
From their perspective, such data underscore both the power of genome sequence information for applied bacteriology, and the challenges one must face in interpreting and applying that information. Although sequencing efforts expanded the genetic database for the CNRZ32 proteolytic enzyme system by about five-fold, efforts to confirm and characterize all the new gene assignments require more time and resources. Nonetheless, functional analysis of the newly discovered endopeptidase genes has already identified enzymes with important roles in the hydrolysis of bitter peptides in cheese.
“Access to genomic information has provided researchers with an unprecedented opportunity to refine old, and develop new, hypotheses concerning how LAB effect the conversion of milk into a variety of fermented dairy products,” according to the Journal of Dairy Science. “However, testing these hypotheses is likely to take several years. Then, of course, perhaps the greatest challenge remains-taking this new knowledge and converting it into new or improved products for the consumer.”
Going the acidulant routeAs mentioned earlier, some dairy processors choose to use acidulants to lower the pH of milk. Acidulants help dairy processors achieve a desired pH quickly, in fact, instantly, if the acidulant is not added properly.
It’s helpful to dilute the acidulant and deliver it to the milk via a metering or dosing system. Slow addition with constant agitation is a must. Keep in mind, no matter how slow, direct acidification is still much faster than lowering pH through fermentation.
In addition to lactic acid, other acidulants such as acetic, citric, hydrochloric and phosphoric acids have application in select dairy foods. For example, acetic acid, also known under the more common name of vinegar, is used in the manufacture of pasta filata-style mozzarella (e.g., string cheese, balls, etc.). Acetic acid contributes a desirable sour flavor to mozzarella cheese, which is inherently a very mild-tasting cheese. Sometimes a blend of acetic and lactic acid is used. In this instance, the lactic acid mellows the sour notes, allowing milk’s creamy notes to come through.
Depending on the application, acidulants alone, or in combination with starter cultures, might be the preferred method for lowering milk’s pH. Regardless of the process, advancements continue to be made in this area and it’s imperative that dairy manufacturers work closely with their suppliers to formulate the right ingredient system.