For its production, fermented milk and cheese require a specific fermentation or series of fermentations. Therefore, any abnormalities in these fermentations will harm the product’s quality and may even cause it to spoil. Even the final product may be susceptible to deterioration by microbes.

Fermented Milks 

• Most fermented milks are produced by adding a starter to pasteurised milk, which is then incubated until the desired acidity is achieved.
• Minor amounts of flavouring chemicals may also be created or added, but lactic acid is the primary result of fermentation.
• If the starter bacteria remain inactive, other bacteria can proliferate and taint the flavour and texture of the curd.
• Proteolytic bacteria, which cannot normally compete with lactic bacteria, might generate a weak curd and unpleasant tastes.
• Coliform bacteria and lactose-fermenting yeasts should not be present, but they can enter through equipment and other sources, causing off flavours and gas.
• If surface air is present, the completed product is prone to deterioration by moulds from air or equipment.

Cheese

Cheese defects may be caused by mechanical or biological factors, however only the latter will be treated here. The varieties of spoilage can be separated into those that occur during the production and ageing of cured cheese and those that occur in the final product.

Spoilage During Manufacture 

• Throughout the production of the majority of cheese varieties and during the draining process, lactic acid fermentation is stimulated.
• If the lactics are inefficient or if contamination with other microbes is very high, unexpected changes may occur that negatively impact the cheese’s quality.
• The gas-forming organisms in cheese prepared from raw milk may produce off-flavors and gas holes in the curd.
• Although lactose-fermenting yeasts are rarely present in significant numbers, they can also induce flatulence. Sporeforming gas producers, notably Clostridium species, can cause problems in raw-milk or pasteurized-milk cheese if the lactic starter bacteria are not functioning properly; less frequently, aerobacilli*, spore-forming Bacillus species such as B. polymyxa, can produce gas and other abnormalities.
• These sporeformers can also induce cheese ripening problems. Other bacteria may compete with the beginning organisms, with outcomes not becoming apparent until the curing process, when the body and flavour of the food may be altered.
• Thus, acidproteolytic bacteria may produce a bitter taste, while Leuconostoc spp. may cause holes or openings in pasteurised milk-based Cheddar cheese.
• Particularly susceptible to deterioration during storage prior to consumption is cottage cheese. If the starting bacteria create insufficient acid or produce it too slowly, there will be no useable curd or the cheese will be of poor quality due to the proliferation of unwanted organisms.
• The product may be ruined by proteolysis, gas generation, sliminess, and off-flavors. Soil or water bacteria, such as Pseudomonas, Pseudomonas fragi, and Alcaligenes metalcaligenes*, frequently produce a lack of acidity in cheese caused by the addition of cream or the failure of the starter culture.

Spoilage During Ripening 

• Cheeses undergo physical and chemical changes during ripening or curing as a result of the action of enzymes released by the autolyzed cells of bacteria that proliferated during manufacture and the action of microorganisms that increase during ripening.
• In severe circumstances, the growth of organisms other than those wanted leads in cheese that is inferior or even worthless due to variations in texture, body, general look, or flavour.
• The most significant types of cheese deterioration are, however, sufficiently cheese-specific to preclude generalisation.
• Generally, “late gas” is created by heterofermentative lactic acid bacteria or lactate-fermenting Clostridium spp., however bacilli, propionic bacteria, or heterofermentative lactics may also be responsible.
• Eyes, or gas holes, are preferred in Swiss and related cheeses, but not in other varieties.
• In Swiss and similar cheeses, cracking or splitting due to gas or the production of too many, too small, or malformed eyes are particularly undesirable.
• The generation of gas by sporeformers is followed by the production of unpleasant tastes, such as butyric acid by anaerobes.
• Bitterness may be generated by specific lactic streptococci; proteolytic bacteria, such as acidproteolytic kinds; coliforms; micrococci*; other bacteria; and (rarely) yeasts, which typically provide a sweet, fruity, or yeasty flavour.
• Putrefaction may develop locally or globally in cheese if the lactics fail to create adequate acidity or if the acid is eliminated by a lactate fermenter, such as Clostridium tyrobutyricum.
• Possibly implicated are putrefactive anaerobes, such as C. sporogenes or C. lentoputrescens*. Discolorations of ripening cheese may be caused by the action of microbes on chemicals formed during curing or on additional colouring material, such as the annatto used in Cheddar cheese, or by the growth of pigmented colonies or organisms on or within the cheese.
• The reaction of hydrogen sulphide produced by organisms with metals or metallic salts may result in a blue, green, or black colouring.
• bacterially produced sulfhydryl groups give annatto a pink to murky colour. Reddish-brown to grayish-brown hues may result from the bacterial oxidation of tyrosine in soft cheese.
• Rusty spots of Cheddar and similar cheeses are caused by colonies of Lactobacillus plantarum var. rudensis* or L. brevis var. rudensis*, whereas yellow, pink, or brown spots on the surface of Swiss eyes are caused by colonies of pigmented species of Propionibacterium.

Spoilage of the Finished Cheese 

• In general, the moisture level in aged cheeses enhances their perishability. Consequently, soft cheeses like Limburger and Brie are the most perishable, whilst hard cheeses like Cheddar and Swiss are the most stable.
• The most dreaded spoiling organisms are the moulds that prefer to grow on the surface of cheese and in the crevices of trier holes.
• Even cheeses that rely in part on a certain mould for ripening might be spoiled by other moulds.
• The rinds of the majority of natural cheeses provide some protection for the anaerobic core, but are typically not sufficiently dry to prevent mould formation.
• The acidity of the cheese is not an impediment to growth, nor is the storage temperature excessively low.
• The majority of moulds develop in colourful colonies on the surface or in cracks, penetrating the cheese only minimally; however, certain species generate genuine decay.
• Mycotoxins and antibiotics, which are produced by mould, can migrate into cheese.
• Not only are there visible discolorations, but also off-flavors are formed locally. The following are among the moulds that grow on cheese surfaces:
  1. Oospora species (Geotrichum) Oospora (Geotrichum) lactis, sometimes known as the dairy mould, grows on soft cheeses and suppresses other moulds and surface-ripening bacteria during ripening. The curd liquefies gently beneath the felt. O. rubrum and O. crustacea generate a crimson hue, whereas O. aurianticum produces orange-to-red dots. Swiss and similar cheeses are afflicted with “cheese cancer” due to O. caseovorans. The growing bumps become filled with a white, chalky substance.
  2. Cladosporium spp. These moulds’ mycelium and spores are black or smoky and impart dark colours to the cheese. The most frequent species is C. herbarum, which has colours ranging from dark green to black. Other species create discolorations that are green, brown, or black.
  3. Penicillium spp. Due to its spores, P. puberulum and other green-spored species grow in cracks, fissures, and trier holes of Cheddar and kindred cheeses to impart a green hue. They may induce mottling and discoloration in annatto. Camembert cheese is discoloured by both P. casei and P. aurantio-virens.
  4. Monilia spp. M. nigra causes black patches to penetrate the rind of hard cheeses. Species from numerous other genera, such as Scopulariopsis, Aspergillus, Mucor, and Alternaria, may discolour cheeses and impart off-flavors.

Yeasts may generate coloured colonies or patches if the surface is sufficiently moist, and film yeasts may pave the way for the yellow to red growth of Brevibacterium linens. In some surface-ripened cheeses, this characteristic is favoured, but not in others.