Dairy Sheep Symposium - the Department of Animal Sciences ...

1954), prefreezing heat treatment to resolubilize lactose (Braatz, 1961), and addition of sodium
hexametaphosphate (Riddle, 1965). To determine if any of these pretreatments might aid in
stabilizing the protein in ovine milk during frozen storage, mid-lactation ovine milk was processed
by one of the following treatments and frozen at -12°C:
1) addition of 4 g of sodium hexametaphosphate per L of raw milk.
2) heat the milk to 68.5°C for 25 min prior to freezing.
3) a combination of pretreatments (1) and (2).
4) enzymatic hydrolysis of at least 30% of the lactose in milk.
5) control, no pretreatment.
Samples were frozen in 170 ml polyethylene containers. A comparative control sample was
also frozen and stored at -27°C for 12 mo. A sample container of each pretreatment and control
was removed from the -12°C freezer after 3, 6, 9 and 12 months of storage and thawed in a
cooler at 4°C. When thawed, samples were analyzed for acid degree value (ADV) and intact
protein. Complete results are being reported in Abstract 346 at this Annual Meeting
(Rauschenberger et al., 2000). In summary, lactose hydrolysis and treatments with the addition
of hexametaphosphate did help stabilize the proteins in milk frozen and stored at -12°C for up to
12 mo. The results from these pretreatments indicate that higher calcium in ovine milk most
likely contributed to the instability of the milk frozen at -12°C. These results were similar to
those observed in bovine milk concentrates (Muir, 1984). Since the addition of
hexametaphosphate has been reported to contribute a salty flavor to milk and cause potential
destabilization of the fat (Muir, 1984), this may not be a viable pretreatment to use in freezing
ovine milk in a home freezer at -12°C. At -27°C, stabilizing effects of high viscosity and low
kinetic energy limit lactose crystallization and protein aggregation (Johnson, 1970).
Quality of Yogurt from Frozen Milk
Since the majority of ovine milk is seasonally produced, yogurt manufacturers are dependent
on treatments, such as freezing, to extend the milk supply throughout the year. Several researchers
(Antifantakis et al., 1980; Voutsinas, et al., 1996) have reported that good quality yogurt
could be produced from frozen ovine milk. However, several yogurt manufacturers in the U.S.
have reported quality problems when producing yogurt from frozen ovine milk. The objective of
this study was to determine the effects of frozen storage of ovine milk on the quality of yogurt.
Mid-lactation ovine milk was obtained from the University of Wisconsin Experimental
Station at Spooner, WI. The milk was immediately cooled to 4°C and transported to the laboratory
in Madison. The raw milk was packaged in 2.1 L polyethylene containers. One set of
containers were frozen and stored in a home freezer at-12°C. Another sample was frozen and
stored in a commercial freezer at -27°C for comparative purposes. A sample container was
removed from the -12°C freezer after 1, 3, 6, 9 and 12 months of storage and thawed in a cooler
at 4°C. When thawed, milk was heat-treated at 82°C for 30 min and cooled to 44°C. Milk was
inoculated with 0.02% commercial yogurt culture of Streptococcus thermophilus and Lactobacillus
bulgaricus (YC-470, Chr. Hansen, Inc., Milwaukee, WI) and 0.02% culture of Lactobacillus
acidophilus (LA-K, Chr. Hansen, Inc., Milwaukee, WI). The fermentation was continued until
the pH of the yogurt reached 4.6. The yogurt was immediately cooled to 4°C with ice water and
placed in a 4°C cooler. The pHs of the yogurts, at 24 hr, ranged from 4.3 to 4.4. Samples were