CHEESE TECHNOLOGY

Module 4. Pretreatments of milk for cheesemaking

Lesson 9
MEMBRANE PROCESSING OF MILK FOR CHEESEMAKING

9.1 Introduction

Membrane separation processes are commonly applied to separate a liquid under a pressure gradient through a semi-permeable membrane into two liquid streams of different composition, the permeate (which flows through the membrane) and the retentate (which concentrates those substances which do not pass through the membrane in a reduced volume of fluid). These processes are applied in dairy processing for an ever-increasing range of applications, e.g. concentration, demineralization, protein separation, or removal of bacteria.

9.2 Ultrafiltration

Ultrafiltration (UF) enables concentration of casein content and recovery of whey proteins for cheese manufacture. UF of milk at pH 6.6–6.8 concentrates mineral salts bound to casein micelles in the same proportion as proteins and increases buffering capacity, which affects acidification, pH, rennet coagulation and rheological characteristics of curd. Acidification before or during UF and/or salt addition to retentate leads to solubilisation of colloidal calcium in the permeate.

As a result of UF of milk, globular fat, caseins, whey proteins and micellar salts, are selectively concentrated in the UF retentate, whereas lactose, serum salts and peptides, are found largely in the UF permeate at their original concentration. This has two major implications for cheesemaking properties of milk:

(1) The inter-micellar mean free distance is reduced considerably, thereby forcing the micelles to interact more frequently with each other as a result of collisions induced by Brownian motion.

(2) The buffering capacity of the milk is increased considerably due to the increased concentrations of proteins and micellar minerals in the UF retentate, both of which are key contributors to the buffering capacity of milk, particularly in the pH region 5.5–7.0.

As a result of these changes, the cheesemaking properties of UF retentates differ from those of unconcentrated milk in several aspects. RCT decreases and firmness increases when cheese milk is partially supplemented with UF retentate. Cheese made from UF retentate is often characterized by a long time required to reach the desired pH and an acidic taste which is related to the higher buffering capacity of a UF retentate. Furthermore, flavour development in hard and semi-hard cheese made from UF retentate is generally slow, which has been related to a reduced rate of proteolysis of caseins during ripening of such cheese, probably resulting from retention of inhibitors of chymosin and plasmin in the UF retentate.

9.3 Microfiltration

Microfiltration (MF) may be used for partial microbial decontamination of cheese-milk and standardization of the casein content of milk, rather than the total protein content that is standardized by UF. Cheese prepared from microfiltered milk may lack flavour development due to absence of flavour producing organisms during ripening as they are filtered out in permeate during MF. MF uses larger pore sizes and lower pressures than UF. Whey proteins are smaller molecules (3 to 5 μm) compared to casein micelles (15 to 600 nm) and can be separated by use of 0.1 to 0.2 μm pore size membranes. This separation produce casein-enriched retentate and permeate containing significant amounts of native state α-lactalbumin and β-lactoglobulin. Casein-enriched milk prepared by MF has been reported to have improved rennet coagulation properties and reduced loss of fat and fines in the whey. Increasing MF concentration factor result in increased moisture, protein and calcium contents, total solids recovery, actual and composition-adjusted cheese yields, proteolysis and flavour and decreased hardness.

9.4 Standardization of Cheese-Milk by Protein Addition

Standardization of milk protein/casein levels may be used to reduce some negative defects associated with a seasonal milk supply such as variable protein/casein contents which result in poor curd-forming properties and in variations in yield and in composition and consistency of resultant cheeses. Increased yield results from reduced losses of fat and casein particles in whey and better retention of whey proteins in the aqueous phase of cheese. Furthermore, standardisation of milk protein to higher than normal levels enables increased plant throughput without installation of extra cheese vats. Protein standardization may be achieved by: use of low-concentrated retentate (LCR) produced by UF or Reverse Osmosis (RO) of cheesemilk; enrichment of casein by MF; or addition of phosphocasein powder (PC) or milk protein concentrate (MPC), typically followed by cheese manufacture using conventional equipment. Standardization of cheese is normally done to a casein/fat ratio of 0.70:1.0.