Cheese making.

Some cheeses are made from raw milk, which have more flavour, whereas others go through the pasteurization process. Pasteurization occurs at 63ºC for 30 minutes or 72ºC for 16 seconds.

Cheeses generally do not undergo homogenization processes as it reduces the size of the fat globules. The natural membrane on the fat globule is replaced by casein, which results in an increased reaction between the two. Homogenization promotes lipolysis, which results to weaker curds and the development of flavour and whitening in cheeses. Low fat cheeses, and cottage cheeses, however, do undergo this process.

During the milk treatment several additives are added to encourage the coagulation process and flavour of the cheese. The following are some additives added to the milk:

• Calcium Chloride is added to replace the calcium that was redistributed during pasteurization. Calcium is bounded to the casein during coagulation, so the correct quantity of calcium must be added to attain the required balance.
• Nitrates are added to milk to inhibit the growth of Clostridium tyrobutyricum.
• Colours such as annatto, beta-carotene and paprika are used to colour the cheese orange like Cheddar. Decolourants such as titanium dioxide can also be used to whiten the cheese.
• Ripening agents are added to decreases the ripening time and to enhance the flavour of the cheese.

Acidification

Production of cheese is a method of fermenting milk where acid production is carried out by starter cultures. After being heated the milk is cooled and a starter culture containing lactic acid bacteria is added to the milk so that the lactose can be converted to lactic acid. These bacteria lower the pH of the milk inhibiting the growth of indigous bacteria. The lactic acid bacteria ripen the milk thereby contributing to the cheese flavours and aromas. 

There are two types of lactic acid bacteria that cheese producers uses: mesophilic and theremophilic culture. Mesophilic lactic acid bacteria have an optimum growth temperature between 20ºC and 30ºC, and theremophilic lactic acid bacteria have an optimum growth temperature between 30 and 45.  Mesophilic cultures are usually used for hard and fresh cheese types such as Cheddar and Gouda while theremophilic cultures tend to be used for Italian cheeses, Swiss and yoghurt. Table 1 shows the different combinations of starter cultures employed on some common cheeses.

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The lactic acid bacteria in starter cultures employ a proteolytic system, which convert the casein into peptides and amino acids. Amino acids are essential for cheeses as they contribute to the flavour. Other products produced by lactic acid bacteria such as aldehydes, alcohols, ketones, amines, esters and sulphur also contribute to the flavour and aroma. 

Coagulation

In cheese making the major processing method is gelation or coagulation, which is caused by the coagulating enzyme. Coagulation changes the protein conformation such that it is no longer water-soluble. Factors such as the presence of calcium, phosphate, pH, ionic strength and temperatures around the casein molecule effect coagulation. In cheese production coagulation is caused by the rennet. The coagulation process requires an acidic environment in the milk rennet in order to transform the liquid into a soft gel.

Rennet

Rennet contains the major proteolytic enzyme chymosin (rennin) that has the property of clotting or curdling milk. Rennet is found in the fourth stomach (abomasums) of a calf. Calves have the ability to curdle their mother’s milk so that the nutrients can be absorbed slowly enough through the small intestines. 

In order for coagulation to occur efficiently, appropriate temperatures must be employed. For most cheeses, the optimum temperature is 30-32C. Acid and rennet have a strong coagulant effect on the proteins. It is therefore essential that during pre-treatment, the milk be made acidic. 

The enzyme chymosin cuts the kappa casein and inactivates it converting it to para-kappa-casein. Since the kappa casein is hydrolysed, the alpha and beta casein micelles become unstable and are susceptible to precipitate with calcium to form curds. The lower pH increases the enzymatic activity and neutralizes the charge repulsion between the micelles and allowing the coagulation to occur more rapidly. Figure 1 illustrates the coagulation process for casein.

Figure 1: Coagulation

The production of rennet in calves is limited and other protease enzymes other then chymosin have been utilized for the production of cheese. Pepsin from pig cows, and chicken is another option for cheese makers. Cows contain bovine pepsin as well as chymosin. Bovine pepsin also clots milk but is less specific then chymosin and has an optimum pH of 2.0. 

Synthetically produced chymosin is also produced by recombinant DNA technology using strains of Eshericia coli or Klaveromyces lactis or Aspergillus niger However, there are still some controversies over the genetic modification approach as it raises question of long-term risks.

Syneresis

The process of separating the curd and the whey is called syneresis. The proteins in the cheeses gel network become highly associated by becoming hydrophobic and not interacting with water.  Several methods are practiced to accelerate syneresis, such methods include, cutting the curd, rise of temperature, and reducing pH levels.  

Cutting the curd is a simple process, whereby the whey and the curd are separated by creating an exposed surface such that the whey can escape. Correct cutting methods are important to producing quality yields. Fat and protein loss can occur when curds are shattered resulting in fine curds, which are difficult to recover in the whey. Also cheese makers are challenged in controlling consistent moisture content in each vat to produce the same body and texture of cheese.

When a blade is lifted through the curd showing a clean break the curd is prepared for cutting. If the curd is too soft the casein interaction is too fragile resulting in fat loss in the whey. Larger curds the higher the moisture content, which results in a softer cheese, whereas smaller curds create harder cheeses.

There are two methods in cutting curd, manual and automated.

1. Manual cutting is carried out by cutting with harps. The harps are pulled through the curd so that it is cut evenly.
2. Automated cutting involves mechanical sharp knives that determines the curd size.

 

Following cutting, the curd is agitated gently to prevent the formation of fine curds. The exterior of the freshly cut curd is fragile and time is required so that the edges heal which will prevent the loss of fat and proteins to the whey.

A rise in temperature and decrease in pH causes syneresis. The pH must be 4.6 the isoelectric point of casein so that the protein charges are balanced and the interactions continue to be highly associated with each other causing discharge of moisture, lactose, acid, soluble minerals, salts, and whey proteins. Heating allows the curd to contract and squeeze out the whey. As a result the curd becomes hard and is ready to be salted and shaped.

Pressing

The curd is placed in cheese molds and the whey is expelled as the curds draw together. Solidification occurs within 4 to 24 hours depending on the cheese type. The more whey is expelled, the firmer the cheese.

Salting

Once the whey is drained from the curd, the cheese can then be washed. Washing allows the lactose content to be adjusted by leaching of water, and the cheese can reach a final pH of 5.0 – 5.2. Salt is added to most types of cheeses. The purpose of salting is to provide flavour to the cheese, inhibit microbial growth within the curd, prevent the acid development within the curd, and to promote further syneresis so that the curd will shrink.

Salting usually takes place during the end of the production process, however some cheeses such as cheddar are salted in the curd. Salting in brine bath is a common technique where the cheese is treated with a 15-20% salt solution five to several weeks. The pH of the brine is in accordance with the pH of the cheese - usually at a pH of 5.2-5.6.

Prior to ripening, the cheese is treated with resins or waxes, which apply protection from drying and molds. The cheese is then transferred to a ripening cellar.

Ripening

Cheese ripening is a process of breaking down the proteins, lipids and carbohydrates to release flavour compounds and modify the cheese texture. Ripening of cheeses can vary from 1 day to 5 years at temperatures of 4 to 20ºC depending on the quality of cheese being made.

The process in which protein breaks down is called proteolysis. The protein breakdown changes the texture of cheese causing it to be less rubbery or elastic. The further the breakdown of protein, the stronger the taste of the cheese. Protein breakdown involves three sub-process:

• Proteases breaks the protein into peptides. Some of the peptides result into flavour compounds.
• Peptidases breaks the peptide bonds into amino acids
• Microorganisms in the cheese further break down the amino acids into aldehydes, alcohols, carboxylic acids and sulphur compounds which can contribute to the flavour and colour of cheese.

There are two sources of proteases in milk: Plasmin and starter enzymes, which are used in the milk treatment. Plasmins are naturally present in milk and the activity of the enzyme is dependent on whether the milk underwent heat treatment. Plasmin breaks down caseins in the cheese, specifically beta caseins, and produce the bitter off-flavours in cheeses.

Lipolysis is the process in which fatty acids are broken down. Milk has a high concentration of short fatty acid chains. With the aid of microorganisms, the fatty acids are further modified to produce flavour compounds.  Lipoprotein lipases are the primary enzymes involved in the breakdown of lipids, however, they are inactivated at low heat treatment.

L Lactose is being continuously broken down during cheese production. During the start of the procedure starter cultures are added to break the lactose into lactic acid. Microorganims such as yeast and moulds are able to thrive on the cheese during the ripening since the conditions are favourable. The cheese is at a pH of 5.2- 5.6 and the temperature ranges from 4-20 C. Lactose is utilized to produce favourable compounds as well as carbon dioxide, which forms the holes and pipes in some cheeses.

As lactose is broken down into lactic acid, the pH of the cheese decreases to approximately 4.6. At low pH, the calcium phosphate begins to dissolve. The presence of calcium is important as it keeps the cheese compact, however, as calcium phosphate is dissolved, the cheese becomes softer.

Molds

There are several enzymes that are employed which lead to the development of flavour coupounds. Please refer to Table 3 for the list of compounds and the flavours. Molds are also used in order to create the desired cheese. Molds are resistant to adverse conditions allowing them to survive in certain environments so they can develop a new mycelium. The most commonly used mold species are Penicillum candidum, Geotrichum candidum for surface ripened cheeses such as Bries and Camembert, and Penicillium roqueforti for Blue cheese.

Molds have three basic functions in cheesemaking:

• Protection – inoculating the surface of the cheese with a selected strain prevents any other microorganisms from growing.
• Flavour – the enzymes from molds have the capability of synthesizing aromatic compounds such as methylketons which is a major Blue cheese flavour.
• Appearance – parameters such as view colours or rind colours can be created by using molds

Packaging

Cheeses are packaged to maintain the quality and protect it from factors such as, light, air, loss of aroma, flavour, and moisture. Cheeses with rind are usually transported without packaging as it already provides protection from bacteria and mechanical or chemical factors. Other cheeses tend to be packaged in plastic or foil.

Conclusion

There are over 1000 types of cheeses, and the production of each of them is slightly different. The basic materials of milk, rennet, microorganisms and salt are the same for all, yet slight variations in the choice of microorganisim create distinct flavours and aromas. The processes of cheese production can vary, but the biological principles tend to be the same.

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