Salting-VI. Attainment of equilibrium

Salt absorption is a relatively rapid event taking perhaps 15-30 min for curd chips of Cheddar cheese, ~7.5 h for Camembert and perhaps 15 d for Parmigiano-Reggiano. After NaCl is absorbed into cheese curd, there then must be established a salt equilibrium across the cheese mass.

In dry-salted cheeses, true equilibrium is rarely is ever fully achieved and substantial intra- and inter-block variation in NaCl distribution are common for Cheddar. NaCl equilibrium is established quickly within each curd chip but there is poor transfer of NaCl across the chip boundary. In brine-salted cheeses, however, there is initially a very large NaCl gradient from the outer portion of the cheese towards its centre; attainment of equilibrium is very slow indeed, taking up to 10 months for a large wheel of Parmigiano-Reggiano.

Further reading:

Guinee, T.P. and Fox, P.F. (2004). Salt in cheese: physical, chemical and biological aspects, in Cheese: Chemistry, Physics and Microbiology, Vol. 1 General Aspects, 3rd edition, P.F. Fox, P.L.H. McSweeney, T.M. Cogan and T.P. Guinee (eds), Elsevier, Amsterdam, pp. 207-259.

(Picture credit)

Salting-V. Brine salting

In brine salted cheeses, the difference in osmotic pressure between the (often saturated) brine and the cheese aqueous phase is the driving force for NaCl migration. Diffusion of brine into cheese is an impeded diffusion process (Na+ and Cl- ions must migrate around fat globules and the hydrated casein matrix of cheese).

Factors that affect NaCl uptake in brine-salted cheeses include:

• Concentration gradient. Uptake increases as brine concentration increases from 5-25% (w/w) NaCl.


• Salting time. Uptake increases but at a diminishing rate with salting time.


• Brine temperature. Uptake increases as brine temperature increases from 5 to 20 C. There is a minimum temperature for uptake at ~32C as fat exudes at the cheese surface (above this temperature fat is more liquid and is lost more easily).


• Surface area:volume ratio of the cheeses. Salt uptake increases with increasing surface area:volume ratio.


• Shape of cheese. Assuming an equal surface area:volume ratio, rectangular cheeses brine more quickly than cylindrical cheeses, which in turn brine more quickly that spherical cheeses. (Faster brining is caused in this case by the presence of corners in the cheese which allow brine diffusion from two or more directions.)

• Moisture content of the curd. Rate of NaCl uptake increases as curd moisture increases.


• Fat content of the curd. Increasing fat tends to impede diffusion.


• Curd pH. Uptake decreases as pH increases from 4.7 to 5.7. This effect is caused by increased charge on the caseins as one moves away from their isoelectric point causing greater hydration of the protein and thus tending to impede diffusion.

Further reading:

Guinee, T.P. and Fox, P.F. (2004). Salt in cheese: physical, chemical and biological aspects, in Cheese: Chemistry, Physics and Microbiology, Vol. 1 General Aspects, 3rd edition, P.F. Fox, P.L.H. McSweeney, T.M. Cogan and T.P. Guinee (eds), Elsevier, Amsterdam, pp. 207-259.

Guinee, T.P. (2007). What factors affect salt uptake in cheese curd? In Cheese Problems Solved, P.L.H. McSweeney (ed), Woodhead, Cambridge, pp. 87-89.

Photo shows an industrial brine bath used to brine Mozzarella-type cheeses moulded into fist-sized pieces.

Salting-IV. Dry salting

Relatively few cheese varieties are dry salted but a high percentage of the world's cheese production is dry salted! This discrepancy can be explained by the fact that Cheddar, the variety produced in the largest quantity, is a dry-salted cheese. In dry salting, the curd is milled (cut) into small pieces at the end of acidification and dry NaCl is sprinkled on the surface of the curd chips.

NaCl dissolves in whey on the surface of curd chips and diffuses inwards (in effect, each individual curd chips is "brined" in its own salty whey) and each curd chip acts as a "mini-cheese". The milled curd chips have a very large surface area: volume ratio which leads to very rapid uptake of salt in Cheddar and related varieties. As high salt-in-moisture inhibits the growth of starter organisms, it is important that the curd has achieved close to its final pH before dry salting.

Factors affecting salt uptake in dry-salted cheese include:

• Quantity of salt added to milled curd: Salt uptake increases as salting level increases)
  
• Mellowing time: Salt uptake increases as mixing time increases from 20 s to 6 min.
  
• Holding time between salt addition/mixing and pressing: Uptake increases as holding time increases.
  
• Curd temperature: Uptake decreases as curd temperature increases from 24 to 41C (this effect is largely due to increased liquefaction of milkfat at the surface of the curd chips which inhibits NaCl uptake).
  
• Surface area: volume ratio of curd chip: NaCl uptake increases as surface area is increased (i.e., by reducing chip size).
  
• Curd moisture: NaCl uptake decreases as curd moisture increases. As moisture increases, there is more syneresis of press whey which removes more NaCl from the chip.
  
• Curd acidity: Salt uptake decreases as acidity decreases (i.e., lower uptake at higher pH).
  

Further reading:

Guinee, T.P. and Fox, P.F. (2004). Salt in cheese: physical, chemical and biological aspects, in Cheese: Chemistry, Physics and Microbiology, Vol. 1 General Aspects, 3rd edition, P.F. Fox, P.L.H. McSweeney, T.M. Cogan and T.P. Guinee (eds), Elsevier, Amsterdam, pp. 207-259.

Guinee, T.P. (2007). What factors affect salt uptake in cheese curd? In Cheese Problems Solved, P.L.H. McSweeney (ed), Woodhead, Cambridge, pp. 87-89.