Magnesium enrichment of skim milk : a thesis presented in partial fulfilment of the requirements for the degree of Master of Food Technology at Massey University, Auckland, New Zealand

Abstract

Effective magnesium enrichment of dairy products depends on the amount of magnesium salt added and the nature of its distribution between the serum and micellar phases. Thus, this study firstly aimed to profile the magnesium distribution in skim milk as a function of pH (pH 5.50 to 7.20), preheating temperatures (20 to 80 ± 1 °C) and concentration of added magnesium chloride (0 to 40 mmol L-1). The second aim was to investigate the rheological properties of magnesium-induced skim milk gels as a function of different concentrations of magnesium chloride (0 to 20 mmol L-1) added to heated skim milk. The total concentration of magnesium in skim milk and serum was determined using a complexometric titration (EDTA titration) and the EDTA results were validated by atomic absorption spectroscopy (AAS). The measurement of ionic magnesium (Mg2+) in milk is of importance for understanding the bioavailability of magnesium-enriched dairy products. Hence, the concentration of ionic magnesium (Mg2+) in the serum phase was measured using a novel magnesium fluorescence dye (Magnesium 510 probe). In all the samples, a reduction in the pH increased the total soluble magnesium and ionic magnesium (Mg2+) concentrations in the serum phase, regardless of whether magnesium chloride (15 mmol L-1) was added or not to skim milk at 20 ± 1 °C. At pH 5.50, more than 92% magnesium was found in the serum phase for both added (15 mmol L-1) and no added magnesium chloride samples. The concentration of magnesium in the serum phase remained unchanged as the preheating temperature was increased from 20 to 80 ± 1 °C. The addition of magnesium chloride to skim milk reduced the milk pH and increased the ionic magnesium (Mg2+) and total soluble magnesium concentration in the serum phase at 20 ± 1 °C. The pH values are important for new magnesium-enriched dairy products as the distribution of added magnesium chloride in the serum phase was different dependent on pH adjustment between the pH 6.50 and natural pH 6.70. Rheological measurements using cone and plate geometry at constant strain showed that the addition of 5 mmol L-1 magnesium chloride induced the gelation of skim milk after 22 min of heating at 80 °C in the rheometer. The time and temperature for reaching the gelation in skim milk depended on preheating and concentration of added magnesium chloride. A higher concentration of added magnesium chloride achieved gelation at a lower temperature in the rheometer. With the magnesium-induced gels, G’ values obtained were found to increase with increasing concentrations of added magnesium chloride (0 to 20 mmol L-1), increasing holding times (10 to 60 min) and increasing heating temperatures (70 to 80 °C). Compared with samples with 5 mmol L-1 magnesium chloride, those with 15 mmol L-1 magnesium chloride showed a more rapid increase in G’. The G’ value obtained with 15 mmol L-1 magnesium chloride was 15.01 Pa at 80 °C during 10 min of holding and cooling to 20 °C rapidly increased the final G’ value to 81.44 Pa. In contrast, the addition of 5 mmol L-1 magnesium chloride resulted in the formation of a weak gel with a final G’ value of 6.87 Pa after cooling to 20 °C. The oscillation stress of milk gels also increased with increasing magnesium chloride concentration, heating temperatures and holding times in the rheometer. Preheating significantly (P < 0.05) affected the strength of magnesium-induced skim milk gels. The addition of 20 mmol L-1 magnesium chloride to preheated skim milk followed by heating at 85 °C then cooling to 20 °C formed strong skim milk gels. In parallel, samples with no added magnesium chloride did not undergo gelation. In conclusion, the distribution of magnesium was influenced by pH and magnesium concentration added and the strength of magnesium-induced gels was influenced by magnesium concentration added, preheating and the heating time and temperature in the rheometer. The technology of making magnesium-induced skim milk gels can be exploited commercially for the formation of non-fermented dairy products supplemented with magnesium.