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Unit 7 Working with Complex Food Systems
is done as food travels on conveyor
belts. The food passes over a series
of screens. The holes or slits in
each screen are smaller than those
in the previous screen. Each screen
retains items of a different size,
from largest to smallest.
Grading is separation based
on quality. Meat is inspected and
graded based on maturity, muscle
fi rmness, color, and marbling. See
23-1. The meat from older animals
usually receives lower grades
because it tends to be tougher
than meat from younger animals.
Meat that has a very dark color
indicates the animal was under
stress just before slaughter. The
chemicals that produce this color
result in a lower-quality meat.
Marbling, which is fat distributed
throughout the muscle tissue, adds
fl avor and tenderness to meat.
Meat that has a lot of marbling
often receives a higher grade.
Testing for marbling has typically required cutting into a section
of the animal carcass. Human evaluation of marbling caused
inconsistencies in the grading of meat. The beef industry now uses
ultrasound scanning to evaluate marbling without cutting into the meat.
This method uses high-frequency sound waves to produce an image
showing the internal structure of the carcass. This technique may
be faster and more precise than cutting into the carcass. Ultrasound
is also less likely to spread contaminants. This is because a knife can
transfer bacteria from the outside of a carcass into a cut made for
inspection. A knife can also carry bacteria from one carcass to another.
Some foods are graded and sorted by color. For instance, color
indicates ripeness or maturity of many fruits and vegetables. High-
speed electronic light sensors can detect the color change that results
from mold damage. Food pieces are scanned individually. When
the sensors detect a moldy piece, machine-controlled jets of air
automatically blow it off the conveyor.
Separation by Density
Another physical property used to separate some foods is density.
See 23-2. Creaming, rendering, and sedimentation are three types of
mechanical separation based on density. Creaming is a separation
process in which low-density liquids collect above higher-density
Stephen Ausmus/USDA
23-1 USDA food technologists review beef carcass grading images.