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

EFFECT OF FREEZING ON MILK QUALITY
W.L. Wendorff * and S.L. Rauschenberger
Department of Food Science
University of Wisconsin, Madison 53706
Madison, Wisconsin
Abstract
Raw whole ovine (sheep) milk was frozen at -15°C and -27°C and microbiological and
physico-chemical properties were evaluated periodically. Total bacteria decreased at a faster rate
in milk stored at -15°C than at -27°C. Acid degree values for milk stored at -15°C were significantly
higher than that stored at -27°C. Samples stored at -15°C exhibited protein destabilization
after 6 months of storage while those stored at -27°C were stable throughout the 12 month
storage period.
Frozen ovine milk was evaluated in several products including cheese and yogurt. Yogurt
produced from milk frozen at -27°C exhibited good sensory and functional characteristics while
that produced from milk frozen at -12°C for 3 mo or more exhibited poor gel strength.
(Key words: ovine, sheep, milk, freezing, whey)
Introduction
The U.S. dairy sheep industry is still in the early stages of development. In the mid-1980’s,
Dr. Boylan of the University of Minnesota conducted a number of studies in the hopes of helping
promote a dairy sheep industry in the U.S. (Boylan, 1984, Boylan, 1986). Working with cheese
researchers at the University of Minnesota, they evaluated the potential for producing Feta,
Manchego, and Bleu cheese from ovine milk (Boylan, 1986). Additional developmental work in
the upper Midwest was conducted by Dr. Steinkamp of LaPaysanne, Inc. (Steinkamp, 1994).
The first commercial dairy sheep flocks were established about 10 years ago and their numbers
have grown to approximately 100 at the present time.
In 1996, the University of Wisconsin-Madison accepted the mission of furthering research on
dairy sheep production and management. Since then, Dr. David Thomas of the Animal Sciences
Department and Yves Berger of the UW Agricultural Research Stations have been conducting
extensive research on the genetics and management systems impacting ovine milk production
(Thomas et al., 1999, 2000). Research on handling and processing ovine milk has also been
conducted by faculty within the University of Wisconsin Food Science Department (Casper et
al., 1998, 1999; Ha and Lindsay, 1991; Jaeggi et al., 2000; Kilic, 1999; Ponce de Leon-Gonzalez,
1999; Rauschenberger et al., 2000; Wendorff, 1998, 2001).
With seasonal production and low levels of milk production per ewe, raw milk typically is
frozen at the farm until sufficient quantities are accrued for further processing. In the first
several years of ovine milk production in Wisconsin, commercial cheese plants experienced
some problems with milk quality and stability with frozen raw ovine milk supplied by the dairy
sheep cooperative. Some of our research studies concentrated on the projected shelf-life of the
frozen raw ovine milk and factors impacting raw milk quality during frozen storage. This paper
will review factors affecting quality of frozen raw ovine milk and impacts on processed products
produced from frozen milk.