705
Chapter 23 Separation Techniques: Mechanical and Chemical Methods
liquids. Creaming works best with
a cold mixture or a liquid-in-liquid
mixture. The cream, or milkfat, in
unhomogenized milk will rise to
the top if allowed to sit long enough.
Fat is nonpolar and less dense
than water. Therefore, milkfat will
separate from the water portion
of milk and fl oat to the top. This
layer of cream can then be skimmed
from the top of the milk.
Rendering is a separation
process in which a component is
extracted out of a food product by
melting. Rendering works on the
same principle as creaming. Fat in
meat is an example of a substance
that might be rendered. Meat can
be simmered in liquid until the
fat melts and separates from the
meat. The cooking liquid can then
be chilled, and the fat will rise to
the surface and solidify. At this
point, the fat can easily be
removed from the broth.
Sedimentation uses gravity to pull the denser particles or liquid
droplets in a mixture to the bottom. This separation process is used
to remove pieces of organic material from many beverages, including
beers and wines. The grains and hops used to make beer are allowed
to settle after brewing. The liquid, called wort, is then drawn off
the top without disturbing the sediment. Yeast used to make wine
must be removed before the wine is bottled to keep off fl avors from
developing. The yeast is separated from the wine in a step called
racking. During this step, the wine sits until the yeast cells and other
suspended material settle to the bottom. Then, as with beer, the wine
is drawn off without disturbing the sediment.
Separation by the Addition of Force
Allowing fats to rise and sediments to sink can be time-consuming.
Many food manufacturers add force to speed such separation
processes. Engineers use their knowledge of physics to design
force-based equipment to meet the separation needs of each food.
Centrifugal Force
Some foods are separated by the force that acts on objects when
they move around a central point. This force is called centrifugal force.
It tends to propel objects outward from a center of rotation. This force
© Lijuan Guo/Shutterstock
23-2 Cranberry bogs are fl ooded so the low-density cranberries can fl oat
to the top for collection. Understanding the physical properties of foods
can help engineers develop effi cient production systems such as this one.