Frozen Desserts by liaoqinmei

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									Frozen Desserts
• ice-cream, sherbet, ices (sorbet) and yoghurt are popular frozen
• Crystals of ice constitutes the bulk of such frozen products and all
   contain sweetener.
• The additional ingredients differentiates the frozen products.
Ice-cream and sherbet
• are frozen, pasteurized mixes that contain one or more optional
   diary ingredients. they differ in %age of milk fat and nonfat milk
Ices (sorbet)
• contains no milk-derived ingredient and no egg except the white.
• A fermented milk product
• Sweetener, stabilizer and fruit or fruit juice are added to cultured
   yoghurt before it is frozen.
• Imitation ice-cream
1. Components of the Mix
Supplied supplied by milk, cream or fruit juice is the main
A frozen dessert is a solution, suspension and a foam
• It lowers the freezing point of water
• The proportion of sugar in a mix influences the ratio of crystals
   to unfrozen mix at a given temperature.
   –    too low then frozen product is too hard
   –    if too high then too soft.
• Helps keep the size of the crystals small
• To supply bulk.
•   Has no effect on freezing point of the mix
•   Help keep ice crystals small.
•   Fat is supplied by milk or cream.
•   Ice cream made from cream (18%milk fat) will have smaller crystals (fine
    texture) than frozen product made from whole milk with lower fat
•   Homogenized milk or cream is more effective in limiting the size of ice
    crystals because of greater number of fat globules
•   Frozen desserts made with evaporated milk have a finer texture than
    those made with fluid milk because fat has been homogenized.
•   In two frozen product with ice crystals of equal size, the one with higher
    fat content will be finer in texture.
•   low calorie substitutes may replace much of the milk fat in dairy desserts.
    Such as
       Polydextrose
       Simplesse
Nonfat Milk Solids

• Milk solids limit the size of ice crystals more effectively than
  emulsified fat.

• The greater concentration of milk solids in undiluted
  evaporated milk is another reason it makes fine textured ice
  creams. These solids

    increases the viscosity of the mix and

    favors the incorporation of air bubbles as the mix is agitated during
• An emulsifier is added is usually added to the icecream mix
  before it is homoginised.
• Emulsifiers used in ice cream are mono and diglycerides and
  polyoxyethylene derivatives of fatty acids of esters.
• The added emulsifier supplements the milk proteins, mainly
  casein, in the promotion of a finer dispersion of fat in the
  unfrozen mix.
• Function of an emulsifier in ice cream brings about partial
  destabilization of the emulsified fat droplets as the mix
• Thus fat stabilizers the foam and gives:
       -Structure to ice cream
       -Conferring stiffness
       -Melt resistance
• Excessive destabilization causes:
       - Clumping of the fat and a buttery mouth feel
       - Breakdown of the foam
• Once ice crystals form in a frozen dessert, the objective is to keep
  them small.
• Gelatin was the first stabilizer to be used in ice cream
• Stabilizers used commercially include a number of vegetable gums.
• Even under optimum conditions, frozen desserts that contain no
  stabilizer become coarse in texture when stored for an extended
  period of time.

Function of stabilizers
• have ability to make mix viscous.
• Enable frozen desserts to better withstand ‘heat shock’ or
  fluctuations in temperature with alternate thawing and freezing.
• Aid in the formation of small ice crystals as freezing occurs.
Removal of heat
• Conversion of water in a mix for a frozen dessert to ice
  crystals requires withdrawal of heat.
• This can be done by:

   – Surrounding a container of mix with brine made from ice and

   – Exposing the mix to cooling coils that contain a refrigerant (a gas
     such as ammonia). The refrigerant provides a lower and more
     uniform temperature than does the freezing mixture of salt, ice
     and brine.
The Freezing Mixture
• Salt, ice and brine are in a unstable equilibrium in a freezing
• In the freezing mixture, the salt added to the ice dissolves in
  the water on the surface of the melting ice and lowers its
  freezing point.
• Salt depresses the freezing point of water more effectively
  than sugar for two reasons:
   – the lowering of the freezing point which is dependent on the number
     of solute particles.
   – and also salt makes it possible to get colder brine than sugar.
• How cold brine gets depend on concentration of salt and
• How cold the brine actually becomes also depends upon the
  amount of ice that melts.
The three parts of a freezing mixture work together as follows.
1. Brine
• Is colder than ice
• is the part that actually withdraws heat from the mix through
   the walls of the metal container.
• Freezing does not begin until the brine forms and gets cold
   enough to lower the temperature of the mix to its freezing

2. Ice
• As the ice melts, it absorbs heat from the brine and so lowers its

3. Salt
• as it dissolves, it maintains the concentration of the brine.
• The resultant depression of the freezing point of the water
   makes it possible for melting ice to absorb more heat from the
• The freezing mixture is thus a dynamic combination of brine
  that is colder than ice and absorbs heat from the mix.
• As ice melts, it keeps the brine cold.
• As salt dissolves, it keeps the brine concentrated.
• Lower ratios of salt to ice increase the time needed to freeze a
• Ratios of salt to ice that are too high, waste salt and ice and
  drop the temperature of the mix so fast that it hardens before
  air is incorporated.
Agitation and Ice Crystal Formation
• For most frozen desserts, agitation of the mix as it
  freezes is essential for the formation of a smooth
  textured product.
• Rapid agitation of the mix should be avoided until the
  temperature of the mix is lowered to the freezing point,
  otherwise excessive de-emulsification of the fat and
  coalescence of fat droplets may occur.
• As the mix freezes, it increases in volume.
• The increase in volume is termed as overrun
• Overrun is desirable to keep the frozen product from
  being too compact.
Freezing in a Household Ice Cream Freezer
Essential part of a freezer include:
   – A metal can to hold the mix to be frozen because metal is a good
   – A dasher that fits into the can
   – A bucket, preferably wooden because it’s a poor conductor of heat –
     holds both metal can and freezing mixture.
• Bacterial contamination is a hazard in making ice cream.
• Preliminary chilling of the mix shortens the freezing time.
• With the mix to be frozen in the metal container and the
  freezing mixture of ice, salt, and brine around it, agitation of
  the mix should begin.
• A mixture for ice cream will become too difficult to
  manipulate in a freezer before enough of the water has been
  converted to ice for the frozen product to have a desirable
  consistency for serving.
• For this reason, the partially frozen mix is held in contact with
  a coolant and allowed to continue to freeze and harden
  without agitation.
• The hardness of frozen product depends on :
   – how low the temperature goes and
   – The proportion of sugar in the mix
• Allowing a frozen dessert to stand for a short time after it is
  frozen will mellow and improve the flavor as well as harden it.
• Commercial ice cream is discharged from the freezer at a
  temperature near -6ºC, with approximately half the water
  converted to ice crystals.
• Additional freezing and hardening at -35ºC follows, with high
  proportion of water now in the form of ice.
• As a result of hardening, the unfrozen portion of the ice
  cream becomes a concentrated sugar glass with glass
  transition temperature near -23ºC.
• Below this temperature water molecules are essentially
  immobile and ice crystals are stable.
• If the temperature of ice cream rises above the glass
  transition temperature, mobility will increase and ice crystal
  will grow larger.
• Temperature ≥ -23ºC contributes to speed and extent of

• Temperature ≤ -23ºC is needed to keep ice crystals from

• Most ices and ice cream have desirable consistency at a
  temperature of -12ºC to -10ºC.

• Frozen products are more flavorful at the higher temperature.
The Mix
• In the absence of agitation to aid in the formation of small
   crystals in a frozen dessert, a higher proportion of interfering
   substances is needed in the mix.
• To keep crystals small, before mix is frozen, mixes for still
   frozen products depend on
   – high proportion of fat droplets
   – Incorporation of innumerable small bubbles of air
• Whipped gelatin, whipped cream, whipped evaporated milk
  and beaten egg white are used to introduce air bubbles.
• Faster freezing also aids in the formation of small crystals
  which is a disadvantage if it contains high proportion of sugar.
Freezing the Mix
• The mix should be chilled before it is put to freeze.
• Still frozen desserts may be frozen by surrounding the mix
  with a freezing mixture of salt and ice.
• An alternative is the freezing compartment of a refrigerator.
• Good contact between the bottom of the freezing tray that
  contains the mix and the cooling surface of the refrigerator
  aids in rapid withdrawal of heat from the mix.
• A film of water between the two ensures good contact and
  eliminates air
• Rapid drop in temperature favors formation of many ice
  crystal nuclei, which help keep the crystals small.
In addition to flavor, three main characteristics by which the
   quality of a frozen dessert maybe assessed are:
1. Texture
• Refers to feel of frozen dessert on the tongue
• It may be coarse or fine depending on the size of the crystals.
• Small crystals (<35µm) → ice cream is very smooth
• Crystals between 35 - 55µm → smooth textured product
• Large crystals (>55µm) → are coarse textured
• Crystal from mixes with high proportion of fat seem finer than
  they actually are because fat lubricates crystals.
• Refers to the hardness or softness of a frozen dessert.
• Frozen products should be firm enough to hold their shape.
• This characteristics can be influenced by:
   – Temperature
   – Viscosity of the syrup that remains unfrozen.
3. Body
• Influenced primarily by the characteristics of the liquid in
  which the crystals are suspended.

1. Why is the texture of ice cream finer than
   that of ice?
2. What is the role of sugar and fat in the
   production of ice cream
3. What is the role of the following in the
   production of ice cream:
  a. Stabilizers
  b. emulsifiers

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