Bacteria and mycetes in affinage

Milk is sterile in the udder but is contaminated by bacteria as soon as it leaves it. Except in the case of mastitis, the bacteria at this point are harmless and few in number. Further infection of the milk by microorganisms can take place during milking, handling, storage, and other pre-processing activities.

 

Bacteria and mycetes (yeasts and moulds) intervene in the creation and affinage of cheese. Though mycetes are seldom found in milk, they too like bacteria have nutritional and physiological requirements that are specific to them.

 

When they reach the milk or cheese, the microorganisms must therefore adapt on the one hand to the characteristics of this environment affecting their development, the milk or cheese themselves (intrinsic factors) but also to the factors to which milk or cheese are subjected (extrinsic factors).

 

WHAT ARE THE INTRINSIC FACTORS?

pH:

The vast majority of bacteria and fungi have the ability to grow at a pH close to neutral ( 7 ). Yeasts, like most microorganisms, have an acidotrophic ( comfortable in acidic conditions ) character which allows them to grow at acidic pH ( Under 7 ).

 

Water activity (aw):

It corresponds to the quantity of free water available for the development of microorganisms necessary for the proper functioning of

chemical and enzymatic processes. In milk, part of the water is linked to the different constituents. The aw is rather a limiting factor for the development of microorganisms in cheese. At the start of ripening, it is mainly the salt ( NaCl ) content which regulates the aw of the cheese.

 

The redox potential:

(Oxidation-reduction) redox potential is a fundamental physicochemical property of milk and cheese that partly determines its microenvironment both during manufacture and ripening. It is determined by the presence in the milk of reducing agents (which become charged with oxygen and lose electrons) and oxidants (which become charged with electrons and lose oxygen). The redox potential resulting from this balance can influence the development of microbial flora according to its oxygen needs. Microorganisms that need oxygen to grow act as reducers and lower the redox potential. The development of certain microbial populations can lead to changes in the redox potential of milk and cause conditions to become hostile for others. Oxidation-reduction reactions have an impact on microflora (technological, alteration, etc.) and on aromatic compounds. Oxidation-reduction reactions have an impact on microflora (technological or useful microorganisms, alteration, etc.) and on aromatic compounds.

 

There are four classes of micro-organisms which can influence or be influenced by the redox potential:

Strict aerobes that can only develop in the presence of oxygen (Pseudomonas, Micrococci, moulds).

Facultative aero-anaerobes that can develop in the presence or in lack of oxygen (coliforms, staphylococci).

Strict anaerobes that can develop in the absence of oxygen (Clostridium).

Microaerophiles that require only a low level of oxygen to develop (Lactobacillus, Streptococcus).

 

Nutrient composition:

Milk is composed of a wide variety of vitamins, minerals, sugars, proteins and fats available for the development of microorganisms which must, however, have the adequate enzymatic systems to be able to metabolize these nutrients.

Fluctuations in the chemical composition of milk can favour or slow down the development of microorganisms according to their nutritional requirements and of their metabolic potential.

 

Antimicrobial systems:

There are natural inhibitory systems in milk which can act on the development of microorganisms. Among them, we distinguish systems linked to the physicochemical composition of milk (lactoperoxidase-thiocyanate-hydrogen peroxide system, lactoferrin, free fatty acids), those linked to the immune state of the animal (production of antibodies, cells) and those linked to the microbial production of bacteriocins, substances produced by certain microorganisms which will specifically or not inhibit other microorganisms. Milk can also contain viral particles, called bacteriophages. Those are obligate parasites of bacterial cells, they cannot survive alone in the medium outside. They can be present in the air, on equipment and almost systematically in whey. They can cause the destruction of bacteria responsible for acidification during cheese processing when they are virulent. All of these parameters are themselves dependent on upstream factors linked to the animal such as breed, lactation cycle, genetics but also particular food production conditions.

 

WHAT ARE THE EXTRINSIC FACTORS?

These are the factors linked to the ambient environment.

 

Temperature:

Not all microorganisms grow at the same temperature. We distinguish three groups according to the optimum growth temperature: -Mesophiles ( preference 20 to 45c ), psychrophiles ( preference -20 to 10c ) and thermophiles ( preference 60 to 80c ).

Certain lactic acid bacteria such as lactococci belong to the mesophile group while others like Streptococcus thermophilus are thermophiles. Psychrotrophic and thermotrophic germs are mesophilic, but can also grow at low and high temperatures respectively.

 

Atmospheric gases:

They do not significantly influence the quality of raw milk, except in case of strong agitation where the oxygen in the air can promote the development of aerobic microbial flora .

 

Relative Humidity:

This is the quantity of humidity in the air ( the ripening room ), it will have an influence on the growth of yeasts and moulds taking part in ripening.  

 

 

LET US LOOK NOW AT BACTERIA AND MYCETES.

They may be classified in the three following groups:

 

A) USEFUL MICROORGANISMS

In the field of the dairy industry, a large number of useful microorganisms are involved.

Among the microorganisms found in milk, bacteria are the predominant ones.

 

Lactic acid bacteria:

These are producing lactic acid by fermentation (lactic fermentation). We distinguish mainly: lactococci, leuconostocs, pediococci, thermophilic streptococci, mesophilic and thermophilic lactobacilli and enterococci.

Their essential role is to acidify milk and curds, to participate in the formation of taste (proteolysis, production of aromas), to the texture and opening of dairy products (cheese, butter, yogurt, fermented milk). These bacteria are now widely used in the form of selected cultures.

 

Propionic bacteria:

These ferment lactates to give acetic and propionic acid, as well as CO2 (propionic fermentation). They participate in the formation of taste and opening of cooked pressed cheeses (Emmental, Comté, Gruyère).

 

Micrococci, non-pathogenic staphylococci (Staphylococcus equorum, S.xylosus, S. lentus), coryneform bacteria (Brevibacterium, Arthrobacter, etc.):

These are constituents of the surface flora of matured cheeses. Lipolytic enzymes and especially the proteolytic enzymes that they emit, act on the fat and casein of the cheese, allowing both the maturation of the curd and the formation of aromatic products. They play an essential role in the formation of the taste of cheeses, in particular cheeses with washed rind, bloomy rind or brushed rind

( Munster, Camembert, Pont l’Evêque ). Micrococci early implantation contributes in allowing a “barrier effect” against unwanted microorganism growth ( the Jameson effect ).

 

As well as bacteria, mycetes are useful, they are made of yeasts and moulds.

Yeasts:

Given their great capacity to adapt to numerous substrates, yeasts are very widely distributed in the environment and are normally found in milk. They are found more significantly on the surface of cheeses (soft cheeses in particular) than on the inside. They participate in the deacidification of curds by using lactose and lactic acid, causing the pH to rise towards neutrality. Through their neutralizing enzymatic action, they contribute to the degradation of the surface of the cheeses with an effect on the texture of the dough, which makes it possible to obtain curds that are tighter and less brittle. Their action promotes the establishment of other microorganisms (Geotrichum, Penicillium or bacteria). They also play an important role in protection against contaminants, particularly against Mucor. Their slightly proteolytic action has an impact on taste, remedying any bitterness in the curd and producing a fresher and more aromatic flavour. Here are a few types found in cheese making Kluyveromyces, Geotrichum candidum, Debaryomyces, Candida, Yarrowia.  

 

Moulds:

P. camemberti ( present on the surface of soft cheeses with a bloomy rind such as Camembert or goat cheeses, and the vast majority of soft white bloom cheeses produced globally ), P. roqueforti ( the internal mould of cheeses like Bleu d’Auvergne or Roquefort ), Mucor ( the dominant mould in the surface of the Tomme de Savoie and is also present on the surface of the Saint Nectaire ).

 

Through their biochemical abilities, moulds play a determining role in the formation of sensory characteristics of cheeses.

 

B) MICRO-ORGANISMS RESPONSIBLE FOR ALTERATION

Due to their composition and production conditions, milk and dairy products can be contaminated by microorganisms which, by multiplying in the environment, cause transformations harmful to the quality of products by degradation of their constituents (proteins, lipids, lactose) and (or) release within them of undesirable compounds. These degradations may be due to bacteria, yeasts and moulds and result in taste defects, odour, appearance and texture.

 

Coliform bacteria ( E. coli, Enterobacter aerogenes, Klebsiella pneumoniae, and Citrobacter freundii ) can be responsible for premature swelling in cheeses, leading, particularly in soft cheese, to spectacular accidents (cheese with a spongy appearance).

During their development in milk and dairy products, psychrotrophic bacteria (mainly Pseudomonas genus, but also Bacillus) can produce enzymes which can cause off flavours in cheeses (rancid taste, bitterness ). Butyric bacteria (Clostridium tyrobutyricum) can grow in cheese (hard cooked and uncooked pressed ) and give defects in taste and opening (“late swelling”) by butyric fermentation (production of butyric acid and hydrogen).

 

The yeast Geotrichum candidum can cause defect in texture and taste in soft cheeses if it is allowed to become too developed with the formation of a toad skin when the rind fails to stick to paste and start sagging.

The mould Mucor is responsible for the accident known as “cat’s hair” mainly in soft cheese, characterized by aesthetic defect and by the appearance of bad tastes.

 

It should be noted that the grouping of microorganisms into useful flora or spoilage flora is to nuance according to the technologies considered. For example, Mucor is useful in Tomme de Savoie, but harmful in Camembert.

 

C) POTENTIALLY PATHOGENIC MICROORGANISMS

Contamination of milk and dairy products can also be the work of microorganisms harmful ( pathogens ) to consumer health. Staphylococcus aureus produces enterotoxins whose ingestion causes vomiting, often accompanied by diarrhoea. Salmonella can cause the same symptoms, as well as Escherichia coli. These three pathogens may be transferred and often are transferred to milk during milking and to cheese during the production or food handling of the finished product highlighting once again the need to thoroughly wash your hands before handling cheese at home. Listeria monocytogenes can cause listeriosis which preferentially affects pregnant women (abortion), newborns and immunocompromised adults (sepsis, meningitis). Listeria relishes humid environments and can develop at very low temperatures. In addition to these four pathogenic bacteria classically searched for in quality control, milk is likely to contain other potentially pathogenic microorganisms such as Campylobacter jejuni, Yersinia enterocolitica, Bacillus cereus or Aspergillus (production of mycotoxins).

 

WHAT IS AFFINAGE?

The microorganisms taking part in this process have, as established above, a sole aim which is survival, multiplication or growth while consuming and ultimately exhausting a valuable source of nutrients. While doing so they modify the texture of cheeses creating

complex flavours which we happen to enjoy.

 

The affineur job is to understand these microorganisms behaviour, provide them with the conditions needed so they may carry on consuming nutrients and produce the flavours we enjoy so much. 

Bearing in mind, all of us do not enjoy our cheese at the stage of ripening, this is the affineur's job too so he may satisfy different markets for the very same product.