The ice cream mix is a complex colloidal system. It is both an emulsion and a foam. The milk fat exists in tiny globules that have been formed by the homogenizer. The fat globules are in coarse dispersion. Some constituents occur in true solution (sugar, lactose and salts) others are colloidally suspended (casein micelles, stabilizers,insoluble sweetener solids and some calcium and magnesium phosphates).
Mix stability refers to the resistance to separation of milk proteins (in colloidal suspension) and milk fat (in emulsion). Instability results in separation of fat globules due to creaming, protein particles as coagulated or precipitated material or a clear serum of whey from mix or melted ice cream.
Generally, ice cream mix is homogenized to reduce the relatively large fat globules to fine particles (mean size 0.5 – 1.0 µ and a maximum size of about 2 µ) with a high degree of dispersion. These fat globules are kept from creaming due to their small size after homogenization, the increased density (due to the addition of protein and stabilizers), and the high viscosity in the mix due to the addition of proteins and stabilizers. The optimum stability is that which allows the mix to pass through the processing stages (especially the plate-type pasteurizer in which plates may be pushed apart by high viscosity mixes) while permitting whipping and freezing process to destabilize an adequate amount of fat. The displacement of proteins by emulsifiers helps create this optimum in stability by weakening the emulsion
Protein stability results from the state of the proteins and the appropriate balance in the solution of pH and salts. Excessive heat in pasteurization, e.g. may denature the whey proteins, leading to their adsorption to casein micelle and eventual precipitation. Likewise, any change in solvent conditions may lead to enhanced protein precipitation.
Whey separation from mix generally arises from phase separation between milk proteins and polysaccharide stabilizers. Stabilizers tend to move apart from each other (even though they are hydrophilic) leading to formation of clear serum layer in the mix after standing or to leakage in serum from ice cream during melting. To prevent this from happening carrageenan is normally added as a secondary stabilizer.
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Density of Mixes
The density or specific gravity(density relative to water) of ice cream mix varies with its composition. Measurements of specific gravity can be made with a hydrometer and of density by weighing a known volume of mix at a known temperature on a gravimetric balance. Density can also be calculated based on composition. Investigations indicate that the density of mix may vary from 1.0554 to 1.1232 g/ml, with an average for a 10% fat mix of approximately 1.1 g/ml.
Acidity of Mixes
The normal titratable acidity of mixes varies with the percentage of MSNF and may be calculated by multiplying the percentage of MSNF by the factor 0.017. Thus, a mix containing 11% MSNF would have a normal titratable acidity of 0.187% lactic acid. The normal pH of ice cream mix is about 6.3. The acidity and pH are related to the composition of the mix – an increase in MSNF raises acidity and lowers pH.
The freezing point of ice cream is dependent on the concentration of soluble constituents and varies with composition. The freezing temperature can be calculated with considerable accuracy and can also be determined in the laboratory with a cryoscope or avapour pressure osmometer.
An average mix containing 12% milk fat, 11% MSNF, 15% sugar, 0.3% stabilizer and 61.7% water has a freezing point of approximately –2.5°C. The freezing point of mixes with higher sugar and MSNF contents may range downward to –3°C while for mixes with high fat, low MSNF or low sugar content it may range upward to –1.4°C. Generally, the differences in type and amount of sweetener solids and lactose concentration used in the mixes are primarily responsible for the differences in freezing points of mixes.
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Freezing point depression
When a solute is added to water the physical properties of freezing point and boiling point change. Water normally freezes at 0oC and boils at 100oC. As more solute is added, the freezing point drops (“freezing point depression”). The freezing point depression ΔTf is a colligative property of the solution, and for dilute solutions is found to be proportional to the molal concentration cm of the solution:
ΔTf = Kfcm
Where Kf is called the freezing-point-depression constant.
The viscosity of the liquid is important. If the liquid is too viscous, it is difficult to beat and therefore to incorporate the air; if it is not viscous enough, the film between the air bubbles rapidly drains, and the bubbles coalesce.
The mix viscosity can be measured in three ways
- By the time required to flow under a fixed pressure through a pipette or specially constructed tube.
2. By measuring the force required to move one or two parallel plates or coaxial cylinders between which a layer of liquid sample is placed or
3. By measuring the fall of a ball through a column of mix
Conclusion – The impact of physico-chemical properties of mix is carried forward in handling of mix, processing including freezing and quality of finished product. With precaution in selection and balancing ingredient viz. Fat, Protein, sugar along with emulsifier, stabilizer and various flavouring components may provide the better quality of the final product. The proper proportion of different ingredients, improved processing can provide better quality and lower cost of Ice-cream with appreciable and acceptable quality.
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Article Written By:
Name – RASHMI SINGH
Profession – MASTER OF EDUCATION
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