The interaction of ionic calcium and milk proteins during heat treatment

Ramasubramanian, Lakshmi (2013). The interaction of ionic calcium and milk proteins during heat treatment PhD Thesis, School of Agriculture and Food Sciences, The University of Queensland.

       
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Author Ramasubramanian, Lakshmi
Thesis Title The interaction of ionic calcium and milk proteins during heat treatment
School, Centre or Institute School of Agriculture and Food Sciences
Institution The University of Queensland
Publication date 2013
Thesis type PhD Thesis
Supervisor Hilton Deeth
Eustina Oh
Bruce D'Arcy
Total pages 180
Total colour pages 8
Total black and white pages 172
Language eng
Subjects 0908 Food Sciences
Formatted abstract

Calcium supplementation of milk is nutritionally significant but challenging because it can affect the stability of milk during processing. The study aimed at understanding the interactions of calcium with milk proteins that occur during the heat treatment of calcium-added milk. Changes that occurred in milk at 70°C during large (20 to 200 mM) and small (10 to 20 mM) additions of calcium, in the form of calcium chloride, were explored. Coagulation was observed at large calcium additions while gelation occurred with the small additions. The products resulting from the calcium- and heat- induced coagulation or gelation of milk were characterised. To distinguish the effect of the calcium ion from a divalent cation effect, a study of the coagulative effect of magnesium ion on milk proteins at 70°C was also conducted.

Addition of 20 to 200 mM calcium to milk caused coagulation on heating to 70°C. Calcium addition increased the ionic calcium concentration and decreased the pH of milk, thereby facilitating the coagulation of milk proteins. Milk also coagulated at pH values close to the normal pH of milk, when ≥ 50 mM calcium was added, thereby signifying the direct role of calcium in coagulation. Measurement of protein in whey showed that milk pre-heat treatment, temperature of coagulation and the pH of milk coagulation significantly affected milk protein coagulation. Pre-heating milk at 90°C for 10 min or UHT treatment at 140°C for 6 s increased the sensitivity of milk proteins to coagulation. The former treatment was more effective than the latter in coagulating proteins. A maximum of 98% of the protein in milk that was pre-heated at 90°C for 10 min, was coagulated by 50 mM added calcium when held at 70°C for 5 min. Gel electrophoresis was used to determine the proteins that were coagulated by calcium. At 50 mM calcium addition, most proteins in milk were coagulated, except for small amounts of κ-casein and α-lactalbumin.

The texture, microstructure and composition of a calcium-induced milk coagulum, obtained by coagulating milk with 50 mM calcium at 70°C were evaluated. The microstructure differed from that of an acid-induced milk coagulum since it was comprised of casein micelles that were interlinked by appendages, or fused with each other depending on the nature of the pre-heat treatment employed. In addition, the casein micellar network entrapped fat globules. The calcium-induced milk coagulum made from pre-heated milks showed numerous distinct hairy appendages surrounding the casein micelles which interconnected adjacent micelles. A direct relationship was established between the microstructure and the texture of the calcium-induced milk coagulum. The pre-heat treatment of milk and the pH of milk during coagulation significantly affected the hardness, adhesiveness and composition of the calcium-induced milk coagulum. UHT pre-heat treatment imparted a soft and adhesive texture to the coagulum while pre-heat treatment at 90°C for 10 min produced a firm coagulum. A pH of 6.6 during coagulation produced a calcium-induced milk coagulum with high calcium retention.

Rheological measurements showed that the addition of 10 to 20 mM calcium caused thickening or gelation of milk on heating at 70°C. Thickening was observed with 10 mM addition while gelation was evident with ≥ 12.5 mM additions, as indicated by an increase in the storage modulus (G´) of the calcium-added milk. The final G´ and breaking stress of milk gels made from ≥ 12.5 mM added calcium increased with calcium addition. Pre-heat treatment significantly affected the strength of calcium-induced milk gels. The strongest gel was obtained by the addition of 20 mM calcium to milk pre-heated at 90°C for 10 min and held at 70°C for 60 min followed by cooling to 20°C. Pre-heat treatment of milk at 90°C for 10 min also increased the viscosity, strength and hardness of container-made calcium-induced milk gels. A significant effect of the total solids concentration (TS), from 10 to 15% on the gelation of reconstituted milk containing 12.5 mM to 20 mM added calcium was established.

The coagulation of milk proteins by magnesium contrasted sharply with coagulation by calcium. The protein yields from magnesium fell sharply after 20 mM addition whereas those from calcium addition from 20 to 200 mM only slightly decreased. A threshold for maximum protein yield was not observed with magnesium coagulation. About 92% of protein was coagulated by 20 mM magnesium. Gel electrophoresis showed that magnesium was not as efficient at coagulating the caseins in milk as calcium. Wheys obtained from milk coagulated with ≥ 50 mM magnesium retained significant amounts of α-, β- and κ-caseins.

Therefore, it is concluded that the nature of the calcium- and heat-induced gelation or coagulation of milk is influenced, and can be manipulated, by a combination of factors including ionic calcium concentration, temperature, milk pH and milk solids level. The technology of manufacturing calcium-induced milk coagulums and gels has potential for the development of novel dairy products.

Keyword Calcium
Magnesium
Protein
Texture
Microstructure
Micelle
Coagulum
Gel
Rheology

 
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Created: Sun, 10 Feb 2013, 18:29:24 EST by Ms Lakshmi Ramasubramanian on behalf of Scholarly Communication and Digitisation Service