Microbiology of ice cream

 

Microbiology of ice cream

Ices were known in ancient times and Chinese mixed snow and fruit juices to make an iced sweet and ice cellars were used to keep foods cool.  Currently, different types of frozen dessert are available such as Cream Ices, Ice Cream, Milk Ices, Custards, Ices or Water Ices, Sherbet, Sorbets, Mousse, Cassata, Splits, Frozen Yogurt, etc.

Ice cream and other whipped frozen desserts are foams made up of air cells surrounded by a partially frozen emulsion. Ice crystals and solidified fat globules are embedded in the continuous unfrozen liquid phase that contains proteins, carbohydrates, salts, and gums. Ice cream has water, fat, emulsifiers, milk solids non-fat (MSNF), sugars, stabilizers, air, and ice as its major components.

Manufacture of ice cream

The manufacture of ice cream is a complex operation with a number of steps.

1. The ingredients will be weighted or measured into the mixing vessels.

2. After mixing, the ingredients are pasteurized, either heat-treated in a batch system or pumped to HTST continuous-flow pasteurization

3. it will be then homogenized in a homogenizer and homogenization will reduce the size of the fat globules, producing a uniform and stable suspension of fat in the mix. The organoleptic characteristics of the product are improved by providing the correct body and texture, mouth feel, and appearance.

4. After pasteurization and homogenization, the mix is rapidly cooled to 2-4°C in order to preserve the bacteriological quality of the mix. The mixture is then stored for approximately 24h, and during

this period of “aging”, the milk proteins hydrate, the fats begin to crystallize, and any added hydrocolloids absorb quantities of water.

5. The mix is then passed to the ice cream freezer where its temperature is reduced rapidly, and at the same time the mix is subjected to considerable agitation.

6. The ice cream is packaged and then kept at a temperature of about -30°C until, and during, distribution.

Microbiology of Icecream

In the freezer, the temperature of the mix is rapidly reduced, air is incorporated and vigorous whipping of the ice cream is done. Both intra- and extracellular ice crystals mechanically injure frozen cells. Ice crystals that form outside the cell reduces the amount of free water and those that form inside the cells puncture cell membranes.

Once the ice cream leaves the freezer, it is then held at temperatures around -20°C or lower until it is consumed.  Pathogenic organisms have been shown to survive for years in ice-cream.  Therefore, it is essential that the bacteriological content of the ice cream from the freezer should be as low as possible and pathogenic organisms should be absent.

Many factors affect the microbiological quality of the finished product, including the relatively complex manufacturing process and the equipment that are used for the processing. Liquid milk, cream, and skim-milk concentrate should be subjected to adequate heat treatment.  Granulated sugar should be almost sterile. Butter and anhydrous milk fat should preferably be stored at a temperature less than  -20°C and Vegetable fats and Stabilizers and emulsifiers should not present any problems. Many other foodstuffs added to ice cream, such as fruits, nuts, chocolate, broken biscuit, colors, and flavors should be of satisfactory microbiological standard and only pasteurized eggs must be used if they are added after the heat treatment of the ice cream mix. Colors and flavors must also be handled with great care to avoid contamination. Air that incorporated into the ice cream must be filtered so as to ensure that no contamination occur. The packaging material should also be free from contamination.

The heat treatment applied to the ice cream is more severe than the pasteurization of the milk and thus is more efficient in killing microbes.  Any contamination that may occur during the handling of ingredients, packing materials, and the product during processing and distribution must be eliminated, or kept to the very minimum. The equipment that is most difficult to clean and disinfect includes the freezers and the homogenizers and mix holding tanks and the tanks used for the storage of raw materials.

 

References

Dairy Microbiology Handbook, Third Edition, Richard K. Robinson, Wiley Interscience publications