Yogurt Monograph Series: Production
Daniel Wilbanks, Technical Contributor
Perhaps no other food contains as robust of a nutritional and probiotic profile as cultured milk. Due to its ancient origin, many varieties exist today but yogurt is the most popular cultured milk product in the US. The fermentation of milk using Lactobacillus bulgaricus and Streptococcus thermophilus cultures differentiate yogurt from other cultured milks, and in the US the FDA regulates the standard of identity for yogurt under 21 CFR 131.200. Notably, the FDA does not regulate many varieties, such as Greek or French styles. This monograph series will overview the production of yogurt and highlight tools to help manufacturers produce high quality yogurt. Subsequent parts will cover narrow topics, such as viscosity determination and sensory evaluation.
The transformation of milk into yogurt is biochemistry in action. Biologically, microorganisms metabolize lactose and convert it into lactic acid. Chemically, the high acidity lowers the pH < 4.6 (titratable acidity > 0.7%) which gives yogurt its distinct sour or tart flavor and causes proteins in the milk to coagulate and form a gel. Flavor compounds, such as diacetyl and acetaldehyde, are also produced during fermentation and additional cultures may be used to provide more flavors, modify texture, impart therapeutic effects, or protect against spoilage.
The gel, or white mass, that forms from the acidification of milk is a result of insoluble complexes aggregating. Caseins, which make up ~80% of the proteins in bovine milk, are naturally insoluble near pH 4.6. Other components in milk are soluble in acidic conditions, such as lactose, lactic acid and milk salts, and so are found in the water – or whey – phase. Water and soluble components do not coagulate and are loosely held within the gel, much like water in a sponge. Whey proteins are natively soluble – hence their name – but can be made insoluble by severely heating milk prior to fermentation. These insoluble denatured whey proteins will aggregate with caseins to form a stiffer white mass that more firmly traps water. This helps to reduce the amount of whey that sometimes separates from yogurt (Figure 1). If milk is homogenized before fermenting, milkfat globules acquire a coating of casein proteins and become insoluble – to a degree – and provide additional stiffness to the casein-based gel.
A process flow diagram for yogurt production is shown in Figure 2, though many more varieties exist. For instance, yogurt may be concentrated, such as with Greek or Icelandic styles. Concentration can be thought of as a water removal process, and the water (and water-soluble components) may be removed after fermentation by straining, which generates acid whey. Yogurt may also be concentrated prior to fermentation by evaporating milk or by a more modern approaches like membrane filtration, though these come with their own challenges that we will cover in a later issue. Additionally, yogurt may be heated < 150ºF after fermentation to extend the shelf life or heated at high temperatures (>162ºF) to inactivate microbes and allow storage without refrigeration.
This article originally appeared in the Winter 2023 issue of the Dairy Pipeline.