The production of a high free-fat whole milk powder for the chocolate industry : the spray chilling technology : a thesis presented in partial fulfilment of the requirements for the degree of Master of Technology in Food Technology at Massey University
Whole milk powder containing down to 80 percent free fat was manufactured by spray chilling suspensions of skim milk in milk fat, using a modified laboratory spray drier. Also, the pure unmodified milk fat, and the soft (SBF23), medium (SBF27) and hard (SBF42) fractions of the same were transformed into powder by spray chilling the molten samples. The effect of chilling with air and nitrogen was investigated. The powders were satisfactorily stable at 5°C, but were relatively unstable at ambient temperatures. The powders' particle size distributions ranged from 4.44 to 215.56 μm. The powder characteristics were influenced by the size of the nozzle, the atomising gas pressure, the chilling temperature, the feed flow rate, and to a lesser extent, the feed temperature. The shelf life of the unmodified milk fat powders stored at 20°C, 5°C and -10°C was assessed. The powders chilled with air had excessively oxidised after one month of storage at all the temperatures, whereas powders processed with nitrogen were still usable after the same period of storage. Lower peroxide values were recorded for the powders stored at - 10°C and 5°C, while significantly higher values were obtained for the samples stored at 20°C. The powdered fats dry-blended successfully with skim milk and calcium caseinate powders at the ambient temperature. In comparison, the hard fraction mixed better than the other softer fractions. Up to 50 percent of the hard fraction, and just 30 percent of the softer fractions, could be blended with the skim milk powder. An upper level of 70 percent hard fraction, and of 50 percent for the softer fractions, were mixed with calcium caseinate. The repose angles of the skim milk and milk fat blends increased with the increasing fat content, and the blends containing up to 20 percent fat were free flowing. For the calcium caseinate and skim milk blends, the repose angle decreased with the increasing fat content, and all the blends were not free flowing. The bulk densities of the skim milk blends decreased with the increasing amount of fat, while those of the calcium caseinate blends increased with the increasing fat content.