Syneresis- V

Stirring the curds-whey mixture facilitates heat transfer during cooking, prevents the curd pieces from fusing and promotes syneresis by encouraging collisions with other curd pieces and the vat wall. It is important to stir gently after cutting to avoid curd shattering (and thus yield losses); indeed, some cheesemakers leave a 5-10 min "healing time" after cutting before starting to stir.

In Cheddar-type cheeses, the curds-whey mixture is stirred and cooked to a desired pH (e.g., 6.1-6.2) but in many varieties (e.g., Emmental), the whey is drained at a target temperature. In a few varieties (e.g., stirred-curd Cheddar or Colby cheese), it is normal to stir the curd pieces after drainage ("dry stirring") which also promotes syneresis.

Note: Figure shows the effect of stirring (solid curves) and no stirring (lower broken line) on percentage syneresis as a function of time afer cutting.

Syneresis- IV

Cooking temperature is a major factor that determines the rate of syneresis. Cooking temperature varies from ~31C (Camembert) to 52-55C (Emmental or Parmigiano-Reggiano) and temperature must match the starter. Acid production by lactococci is slowed ~35C and many strains are killed >40C (which is very close to the Cheddar cooking temperature of ~38.5C). Thermophilic starters, while surviving high temperatures do not gro >~52C so syneresis in Swiss cheese (cooked 54-55C) is mainly due to heat; starter grows as curd cools.

Cooking is normally achieved using a jacketed vat although "curd washing" (removal of perhaps 30-40% of the whey and replacement with hot water) is used in Dutch-type cheeses. In addition to increasing the temperature, curd washing reduces the lactose levels and helps to control the final pH of the cheese.

The rate of cooking is important. If it is too fast in the early stages, case hardening can result.

Syneresis- III

Processing variables that influence syneresis include the size of the curd particles. The smaller the pieces, the greater the greater the surface area for whey expulsion hence the greater the syneresis. Indeed, curd for high moisture cheeses not cut but scooped into mould.

Acidification also has a major influence on syneresis. The lower the pH, the greater is syneresis.

Other processing variables that influence syneresis will be discussed in future posts.

Syneresis- II

Factors affecting syneresis are associated with the milk or with the processing operations. Factors that affect syneresis associated with the milk include:

• % Fat. Increasing fat decreases syneresis as fat globules inhibit the movement of moisture to the surface of the curd piece. Increasing the fat content of milk increases cheese yield (Ya, actual yield) by 1.2 x mass of added fat due to increased retention of moisture.


• % Casein. Casein is the structural element in the reticulum of the curd and so increasing casein content of milk results in better syneresis.


• pH of milk. Reducing the pH of milk improves syneresis which is optimal at the isoelectric point of the caseins (pH 4.6). As pH moves towards 4.6 the net charge on the casein is reduced facilitating their interaction.


• Ca2+ generally improves syneresis.


• NaCl added to milk. At low levels, salt added to the milk improves syneresis but at higher levels it reduces syneresis. Addition of salt to milk is a practice used only in the manufacture of a very small number of varieties (e.g., Egyptian Domiati).
  

However, in general processing operations have a greater effect on syneresis than milk composition and will be discussed in future posts.

Syneresis- I

Rennet-induced milk gels are relatively stable if left undisturbed. However, if the gel is cut, broken or exposed to pressure, the para-casein matrix contracts on itself expressing its aqueous phase (as whey) in a process known as syneresis. Controlling syneresis is the key to cheesemaking as it allows the cheesemaker to control moisture which, in turn, largely determines the quality, ripening and stability of the cheese. After the gel has formed, it is subjected to various treatments (cutting, cooking, stirring, acidification, pressing...) to encourage the expulsion of whey.

In hard cheeses such as Swiss and Cheddar syneresis occurs mainly in the vat while the gel for soft (high moisture) varieties such as Camembert may be scooped directly into moulds where whey expulsion is driven mainly by the decrease in pH.

Perhaps surprisingly for such an important parameter, methodology for measuring synersis is relatively poor. Approaches used have included measuring the volume of whey produced or the volume, moisture content or conductivity of the curd. Tracer/marker methods have also been used and some authors mimic the cheesemaking protocol (e.g., addition of starter, cooking) when measuring syneresis. However, all methods have inherent drawbacks.