Technical:
Starters and Lacto-acid cultures
From spontaneous acidification to controlled fermentation:
the role of lactic acid bacteria in cheesemaking
For centuries, lactic acid fermentation has been at the heart of cheesemaking. In the past, this process occurred largely spontaneously. Raw milk was processed immediately after milking, often still warm and without refrigeration. That milk naturally contained microorganisms: bacteria originating from the animal, the barn environment, the milking equipment, and the air. Among this microflora were lactic acid bacteria (LAB) capable of converting lactose (milk sugar) into lactic acid. As a result, the pH of the milk decreases, leading to acidification, preservation, and even an initial coagulation — the beginning of cheese.
However, natural acidification is difficult to predict. The composition of the microflora varied by season, by farm, and even by day. Sometimes fermentation proceeded ideally, resulting in a flavorful and well-preserved cheese. At other times, undesirable microorganisms dominated, causing off-flavors, poor texture, or spoilage. Cheesemaking was therefore partly craftsmanship and partly dependent on microbiological circumstances.
With the advent of improved milking hygiene, this changed fundamentally. Better barn hygiene, clean milking equipment, and especially rapid cooling of raw milk drastically reduced the total microbial count. This was a major step forward in food safety and quality, but it also marked the end of spontaneous acidification. The milk simply contained too few — and too unpredictable — lactic acid bacteria.
For this reason, cheesemakers now work with starter cultures: carefully selected lactic acid bacteria added to milk in controlled quantities. These cultures ensure a rapid and predictable conversion of lactose into lactic acid. As a result, the pH decreases at the correct moment and at the appropriate rate. But their role goes much further. Lactic acid bacteria in milk and cheese are essential for:
Proper rennet activity
Lactic acid bacteria (LAB)convert lactose into lactic acid, lowering the pH of the milk. This is essential for optimal activity of rennet (chymosin), which cleaves K-casein and thereby allows casein micelles to aggregate. At a slightly reduced pH, coagulation proceeds more efficiently, resulting in a firmer and more uniform gel.
Correct curd texture
Acidification affects the electrical charge of casein micelles and thus their interactions. As pH decreases, electrostatic repulsion between micelles is reduced and a more compact protein network forms. This determines whether the curd becomes firm, elastic, or crumbly. Acidification therefore initiates the first aggregation of casein in milk. This is the basis of lactic cheeses, where only very small amounts of rennet are used.
Moisture expulsion (whey separation)
As acidification progresses, the protein network of the curd contracts. This process, known as syneresis, causes whey to be expelled from the curd. The rate and extent of pH decline determine how much moisture is removed and therefore how dry or creamy the final cheese will be.
Flavor and aroma development
Lactic acid bacteria produce not only lactic acid but also numerous secondary metabolites such as diacetyl, acetaldehyde, and various enzymes. During ripening, the bacteria themselves — and especially their intracellular enzymes released after cell lysis — further break down proteins (proteolysis) and fats (lipolysis) into flavor and aroma compounds. They thus form the foundation of each cheese’s characteristic flavor profile.
Inhibition the growth of undesirable organisms
The production of lactic acid lowers the pH, inhibiting the growth of many spoilage and pathogenic bacteria. In addition, some lactic acid bacteria produce bacteriocins and other antimicrobial compounds. Combined with competition for nutrients (a high LAB population suppresses other microorganisms), this results in a microbiologically stable and safer product.
A starter culture therefore consists of specific strains of genera such as Lactococcus, Lactobacillus, Streptococcus thermophilus, and/or Leuconostoc, depending on the type of cheese. Where chance once played a role, microbiological knowledge is now central. Yet the principle remains unchanged from centuries ago: without lactic acid bacteria, no cheese.
Lactic acid bacteria were — and remain — the silent engine of cheesemaking: once invisibly present in raw milk, today deliberately selected for quality, safety, and flavor. Lactic acid fermentation is also central to sauerkraut, kimchi, salami, sourdough, and milk kefir.
CHOOSING STARTER CULTURES
There are various types of lactic acid bacteria, each with specific functional properties. Most commercial starter cultures consist of mixtures of different strains.
The primary distinction is between mesophilic and thermophilic cultures. Mesophilic cultures grow best at temperatures above 15°C and are important for most cheese varieties. Thermophilic lactic acid bacteria, however, truly thrive when milk is heated above 40°C. At these temperatures, most mesophilic LAB cease multiplying.
One well-known thermophilic LAB is Lactobacillus delbrueckii subsp. bulgaricus, used in yogurt production.
Inoculating milk for cheesemaking, there are several options:
1: Propagating a mother culture
An easy method for home cheesemakers. Simple dosing. Mix one sachet of cheese starter with 1 liter of pasteurized or UHT milk at 20°C. After 48 hours, the milk will have the consistency of a drinkable yogurt. This is your bulk starter (mother culture): a mesophilic all-purpose LAB culture used to inoculate milk at 1%. It can be stored for several days in the refrigerator. If not used immediately, divide into portions and freeze. When using frozen culture, double the inoculation rate (2% or 20 ml per liter of milk).
2: DVS cultures (Direct Vat Set)
Ready-to-use starter cultures that can be added directly to the final milk volume. Simply sprinkle onto the milk while heating, allow to rehydrate, stir thoroughly, and let ripen for 30 minutes. Primarily used in professional cheesemaking for larger volumes. A wider selection of mesophilic and thermophilic strains and combinations is available, often under proprietary names such as STI-15, CHN22, or Flora Danica. Major suppliers include Chr. Hansen, Danisco, and DuPont.
Dosage: follow manufacturer’s instructions.
3: Buttermilk
Like cheese, cultured butter is a fermented product made with mesophilic lactic acid bacteria. These bacteria are also present in buttermilk. Look for buttermilk containing live cultures.
Dosage: 2% of milk volume (20 ml per liter).
4: Milk kefir
Kefir is a SCOBY: a Symbiotic Community of Bacteria and Yeasts. Lactic acid bacteria constitute a major part of this community. Among the yeasts, Geotrichum is commonly present — recognizable by the wrinkled rind it forms on fresh, moist cheeses.
Dosage: 2–3% of milk volume.
5: Yogurt
Thermophilic cultures are readily available in DVS form. Home cheesemakers may substitute a few spoonfuls of plain yogurt. Not identical, but practical for beginners.
FUNCTIONAL DISTINCTIONS BETWEEN STARTERS
Starters differ in functional properties. For proper application and optimal results, it is important to understand their composition and metabolic behavior.
The broad group of lactic acid bacteria includes all bacteria capable of converting lactose into lactic acid. Lactose is hydrolyzed into glucose and galactose, then metabolized through multiple intermediates to pyruvic acid, which is reduced to lactic acid: lactose > pyruvic acid > lactic acid
LAB can be classified in several ways. Growth temperature was discussed earlier.
Another classification is based on lactose metabolism, which occurs in two ways:
• In one case, exclusively lactic acid is produced.
• In the other case, various by-products are formed, such as carbon dioxide, volatile aroma compounds, and alcohol.
Bacteria producing only lactic acid are called homofermentative. Those producing lactic acid along with other compounds are heterofermentative.
Lactose conversion continues until acidity increases to a level that inhibits bacterial growth due to the accumulated lactic acid. This typically occurs at a pH between 4.0 and 4.5.
Formation of Metabolic Products
All Lactic Acid Bacteria (LAB) produce lactic acid. Some also produce carbon dioxide, alcohol, and aroma compounds. Milk contains small amounts of citric acid. Certain bacteria can metabolize citrate into volatile compounds (aromas). In cheese, some starter bacteria — notably Leuconostoc species and Lactococcus lactis subsp. lactis biovar diacetylactis — produce gas (primarily CO2) from citrate metabolism. This is important for eye formation in cheese. Large eyes may be desirable (as in propionic fermentation) or undesirable (as in butyric fermentation defects).
Based on metabolic end products, starters are classified as follows:
O-type starter: Non-gas-producing starters. Contain only Lactococcus lactis and Streptococcus cremoris (now Lactococcus lactis subsp. cremoris).
L-type starter: Contains at least one homofermentative strain of Lactococcus lactis and a heterofermentative strain of Leuconostoc mesenteroides subsp. cremoris (formerly called B-type starter).
D-type starter: Contains a homofermentative strain of Lactococcus lactis plus the citrate-positive variant Lactococcus lactis subsp. lactis biovar diacetylactis, capable of gas production from citrate.
LD-type starter: Contains all of the above strains.