710
Unit 7 Working with Complex Food Systems
Precipitation
Precipitation separates two or more compounds from a mixture
by causing the compounds to form a precipitate. A precipitate is a
substance that separates from a solution as an insoluble solid. Food
manufacturers add substances to some food mixtures to cause
precipitates to form chemically. For instance, acid is added to milk
to cause the protein casein to form a precipitate. This precipitate is
the curds used to make some types of cheese.
In some cases, no substance needs to be added to cause a
precipitate to form. Precipitates form in some mixtures because of
temperature changes. Precipitation is one step in making decaffeinated
instant tea. The fi rst step is brewing tea in water and then evaporating
part of the water. If the concentrate is then cooled to 10°C (50°F), the
caffeine and tannins will naturally combine to form a precipitate. This
precipitate is large enough to be fi ltered out of the tea concentrate.
Evaporation
When a desired food component is a solute, it may be separated
through evaporation. This process simply removes all the water
from a solution. This separation technique is used to make instant
teas and coffees. Tea and coffee are brewed to suspend the fl avor
compounds in water. The solution is then heated or freeze-dried
to evaporate out the water content. What remains are the fl avor
compounds in a solid state.
Distillation
Sometimes the desired part of a food is so volatile that it quickly
evaporates out of a solution. Distillation is the collection of vapors
and the compounds they carry as they evaporate from a solution.
Distillation requires a closed system to carry the steam and/or vapors
away from the heated solution. As the collected vapors move away
from the heat, they cool and condense. Distillation works because
different compounds have different boiling points.
The fl avor compounds in tea are volatile. They are often collected
as they distill out of the brewed tea that is evaporated to make instant
tea. The low boiling point of the fl avor compounds enables them to
distill out of the tea before water does. These compounds can be
collected and mixed back into the tea solids left from the evaporation
process. The result will be increased fl avor in the instant tea product.
Distilled or purifi ed water has had minerals, salts, and organic
matter removed from it. This is done by heating the water. The pure
water leaves the solution as steam, and the unwanted solutes and
colloids remain behind. The steam is collected, condensed, and
bottled for sale. See 23-5.
The making of some alcoholic beverages involves distilling
alcohol out of a water-based solution. Alcohol has a higher vapor