Chemistry Reference
In-Depth Information
The structure of ice cream has been studied in detail using electron microscopy.
Trapped air bubbles are found to be separated by only few micrometer-thick layers
of the continuous phase.
Ice cream consists of ca. 40% air-frozen foam. The continuous phase in foam
consists of sugars, proteins, and emulsifiers. A typical ice cream contains
Fats
9%
Milk
10%
Sugar
15%
Emulsiiers
1%
Salts
1%
Ice cream emulsion has a very characteristic degree of stability. The air bubbles
should remain dispersed, but as soon it melts in the mouth, the emulsion should
break. This leads to the sensation of taste, which is very essential to enjoy its spe-
cialness. The sensation of taste on the surface of the tongue is known to be related
to molecular shape and physicochemical properties. As soon as these molecules are
separated from the emulsion, the taste sensation is recorded in the brain. Therefore,
the various components must stay in the same phase after the breakup of the emul-
sion. Emulsifiers that are generally used have low HLB values (for W/O), and have
been found to have considerable effect on the structure of the ice cream.
In food emulsions, one comes across a variety of products in which interfaces play
an important role. One such example is churning, where air bubbles are whipped into
milk or cream, the process being the same as that employed in the case of whipped
cream. A large amount of air is incorporated into the liquid in bubble form, and fat
globules (being surface active) collect in the bubble walls. This shows that surface-
active molecules, such as fats, are collected at the air-water interface under churning
or whipping. However, where whipped cream is kept cold and the agitation stopped
when a stable, airy foam is produced, churned cream is warmed to the point that the
globules soften and, to some degree, liquify. The ideal temperature range is said to
be 12°C to 18°C. Persistent agitation knocks the softened globules into each other
long enough to break through the protective membrane, and the liquid fat cements
the exposed droplets together. The foam structure is broken both by the free fat and
the released membrane materials, which include emulsifiers such as lecithin. These
materials disrupt thin water layers and so burst bubble walls, and once enough of
them have been freed in the process of whipping or churning cream, the foam will
never be stable again. As churning continues, the foam gradually subsides, and the
butter granules are worked together into larger and larger masses. Paddles slowly
agitate the cream, causing it to thicken and separate into butter grains and butter-
milk. Cold water at 10°C is then added, and it is agitated again. The added water
is necessary to help the cream to “break,” but the water should not exceed 25%
of the total volume of the cream. Churning continues until the butter granules are
about the size of wheat grains. In butter, the fat globules vary from 0.1 μm to 10 μm
in diameter. The fat globule membrane comprises surface-active materials such as
phospholipids and lipoproteins.
Fat globules typically aggregate in three ways:
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