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

DISCUSSION
Stability of Frozen Raw Milk
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. Gross composition of the milk is shown in Table 1. The raw milk was packaged
in sterile 170 ml polyethylene containers and 13.5 kg polyethylene pails with a 2-mil
polyethylene liner. One set of containers and pails was frozen and stored in a home freezer at -
15°C. The other set of containers and pails was frozen and stored in a commercial freezer at -
27°C. A sample container and pail were removed from each freezer after 3, 6, 9 and 12 months
of storage and thawed in a cooler at 4°C. When thawed, samples were analyzed for total bacteria,
coliform bacteria, acid degree value (ADV) and intact protein. Intact protein was defined as
the total protein content of milk minus the protein present in sediment at the bottom of container
or pail.
Both total bacteria and coliforms decreased at a faster rate in milk stored at -15°C than milk
stored at -27°C (Table 2). Milk that was frozen at -15°C developed larger ice crystals than milk
that was flash-frozen at -27°C. Those larger ice crystals formed at higher freezing temperatures
tend to be more destructive to bacteria than smaller ice crystals formed in flash-freezing at lower
temperatures (Jay, 2000). It has also been reported that gram-negative rods tend to be more
susceptible to freezing damage than cocci (Georgala and Hurst, 1963). ADVs for milk stored at -
15°C were significantly higher than for those samples stored at -27°C (Table 3). In spite of the
increases in ADV with storage, samples did not exhibit a rancid flavor within the 12 months of
storage. Several researchers (Antifantakis et al., 1980; Needs, 1992) have reported an increase in
free fatty acids with frozen storage of ovine milk. Needs (1992) reported a significant loss of
residual lipase activity in ovine milk after 6 months of storage at -12°C and -20°C. However,
Voutsinas et al. (1995) reported no significant differences in lipolysis or lipid oxidation when
concentrated ovine milk was stored at -20°C for up to 6 months.
After 6 mo of frozen storage at -15°C, thawed milk samples exhibited protein destabilization
with flocculated protein settling at the base of containers. After 9 mo of storage, over 20% of the
protein was lost in the sediment (Table 3). Samples stored at -27°C exhibited good protein
stability throughout the 12 mo of storage. Storage stability results were comparable for the 170
ml containers and the 13.5 kg pails throughout the study. Previous researchers have reported
good protein stability in frozen ovine milk if stored below -20°C (Antifantakis et al., 1980;
Bastian, 1994, Young, 1985). Young (1987) did observe separation and recombination problems
in ovine milk stored at -12°C for 12 months. Koschak et al. (1981) reported that frozen bovine
milk and milk concentrates stored at -20°C or lower remain stable for long periods of time but
stability decreases greatly as the temperature is raised above -20°C.
Protein stability of frozen bovine milk has been extensively researched over several decades.
The destabilization of proteins in bovine milk during frozen storage was primarily due to the
casein fraction (Desai et al., 1961). Several factors impacting casein destabilization included;
time and temperature of storage ( Tracy et al., 1950; Koschak et al., 1981); milk concentration
(Bell and Mucha, 1952); lactose crystallization (Desai et al., 1961); and prefreezing heat treatment,
(Braatz, 1961). El-Negoumy and Boyd (1965) concluded that free calcium in milk was the
primary cause of protein instability in frozen bovine milk. Several pretreatments were recommended
to improve protein stability in frozen milk concentrates: lactose hydrolysis (Stimpson,