The impact of seasonal variations of New Zealand raw milk on the heat stability of skim milk

Abstract

Seasonal changes in milk composition is well documented in several countries. The concentration of many constituents and the physicochemical properties differ throughout the year to different extents. In New Zealand (NZ), the milk composition changes during a milking season are linked to the weather cycle. To the best of our knowledge, how seasonal variations of raw milk constituents affect heat stability of skim milk in NZ has not been established. The objectives of this research are to investigate how the season changes affect fresh raw milk components in NZ dairy farms in Canterbury and its impact on the heat stability of skim milk. Milk used in this research was collected fortnightly from the Lincoln University research farms in Canterbury, NZ. During the research period fresh whole milk (FWM) and fresh skim milk (FSM) were used. During the study period, FWM and FSM were measured for the general composition (GC), pH, free calcum ion concentration (Ca++), particle size distribution (PSD), sedimentation rate(DS%), ethanol stability (ES%), total phospholipids (TPL), buffering capacity (BC), and composition of fatty acids (FA), minerals, and proteins. Fresh skim milk was obtained by centrifuging FWM at a rate of 3000 x g for 30 min and it was used to investigate the milk heat stability by eliminating the impact of milk fat globules and their associated materials. Di-sodium hydrogen phosphate (DSHP), tri-sodium citrate (TSC), sodium dihydrogen phosphate (SDHP) and di-sodium hydrogen phosphate (DSHP) were added as stabilizing salts to reduce heat-induced sedimentation. One set of FSM was homogenized at 11,000 rpm for 10 min and heated to 850C another set of FSM was heatd to the same temperature and then homogenized. This study showed seasonal variation in milk protein and fat concentrations, pH and BC, TPL, total whey protein and a-casein, Ca++, FA, minerals, surface area mean [D (3, 2)] and volume weighted mean [D (4, 3)], ES%, and DS%. No seasonal variation was evident in total milk solids (TS; %). In addition, the average mineral (calcium , potassium , magnesium, sodium, phosphours, zinc, and sulphur) concentrations varied significantly (p<0.05) between seasons (p<0.05). Stabilizing salts DSHP (salt 1; S1), TSC (salt 2; S2) in the study), and a mixture of 2 parts of SDHP and 1part of DSHP (salt 3; S3) when added to milk, improved the heat stability. The addition of stabilizing salts to milk increased pH, decreased Ca++ concentration resulting in a lower sedimentation rate of milk components thus may help in extending the shelf-life.