Acidification and Coagulation of Cheese

The production of acid at the optimum rate is critical to the manufacture of good quality cheese. Cheese can broadly be categorized as acid or rennet cheese.

Acid cheeses are made through direct addition of acid to milk. Fresh cheese varieties, such as cream cheese and queso fresco, are made this way. Addition of 1.5%–2% starter cultures is common to reduce the pH of milk. Historically, addition of starter to cheese milk 30–60 min prior to rennet addition was common practice to reduce the pH of the milk and improve rennet action.

Bacteria may be wild in milk, or they may be added as a starter. Homofer-mentive bacteria produce only lactic acid during fermentation, versus heterofermentive bacteria that, in addition to lactic acid, produce other compounds such as alcohol, aldehydes, ketones, and carbon dioxide. Homofermentive bacteria are commonly used in cheeses such as cheddar where a clean acid flavor is required. Heterofermentive bacteria produce a wide range of flavors, including fruity flavors. Typical starter culture bacteria include Lactococcus lactis subsp. lactis or cremoris, Streptococcus salivarius subsp. thermophilus, Lactobacillus delbruckii subsp. bulgaricus and Lactobacillus helveticus. Adjunct cultures may also be added to further enhance flavor and improve texture. These can result in the formation of eye holes in the cheese. Mold spores may also be added to the milk when making mold-ripened cheeses. Starter cultures are still used for some cheese varieties; however, many have discontinued this practice due to risk of bacteriophage infection.

As an alternative to ripening, milk can be direct pre-acidified either through addition of acid (lactic or hydrochloric) or acidogen (gluconic acid-&-lactone). Chemical acidification is mainly used for cheese varieties where texture is more important than flavor. Acidification times range from 5–6 hr for cheddar to 10–12 hr for Swiss cheeses. Target pH levels vary depending on the cheese variety being processed. Swiss and blue cheeses, for instance, require pH levels of 6.2–6.5.

Coagulation is an essential step for the formation of cheese. It involves coagulation of casein to form a gel entrapping fat, if fat is present. Most cheese varieties (75%) are produced using rennet coagulation. Rennet contains the enzyme chymosin, which converts K-casein into para-K-caseinate and glycomacropeptide. Calf rennet or rennet produced through microbial processing can be used. Rennet acts on the milk proteins to form the curd. After rennet is added, the curd is not disturbed for 30 min or longer.

The third method of coagulation is acid/heat coagulation. This method is less common and is used to produce cheese from whey or a blend of whey and skim milk. Ricotta is an example of an acid/heat coagulated cheese.