Ice Cream Quality

37 Post-pasteurization contamination

The consumer demands a certain quality from  the manufacturer, but more importantly they demand a product which is unquestionably safe to consume.

The ice cream industry needs to be especially concerned about product safety due to the susceptibility to contamination from post pasteurization handling. A considerable amount of processing and ingredient addition is done after pasteurization and the only control of bacterial contamination post pasteurization is through sanitation and hygiene.

Raw side

It should be assumed that all incoming raw milk products, and raw eggs, are contaminated with pathogens. This assumption, although hopefully incorrect, will ensure that safety procedures are adequate. Ideally, the raw receiving area should be completely isolated from the rest of the plant operation and all movement of plant personnel and equipment from the raw area to the finished product area should be minimized or eliminated. This includes relief workers, maintenance men, plant tours, etc.


Specifications for and routine monitoring of all incoming ingredients should be developed. Dairy product ingredients must meet established standards and be fresh and of high quality. Certification by the supplier of suspect ingredients such as egg products is one way of maintaining quality. Colors can be a source of contamination. Powdered colors used in ice cream manufacture should be prepared in the laboratory using sterilized water and should be kept reasonably fresh. Liquid flavors and fruit preparations should be monitored for coliform contamination as necessary. Although bacteria cannot grow in low water activity foods, many can survive if contaminated at the point of preparation.  All suppliers should be reliable and should have the confidence of the manufacturer but not his blind faith. Inspections of supplier facilities by the manufacturer are often warranted and welcome.


Pasteurization is the only biological control point in the system. All pathogens are destroyed by adequate pasteurization. It is imperative that the pasteurizer be maintained and operated properly. Pasteurization beyond the minimum standard is recommended to allow for a safety margin.


Product exposure and ingredient addition at this step make the freezing and filling operations critical in terms of product hygiene. Although most organisms cannot grow at temperatures less than 0oC, many can survive at these temperatures and, in the case of Listeria, it takes only a very few viable organisms to cause illness. The barrel freezers must be properly sanitized after assembly and immediately before operation. The hand assembly of the many intricate parts makes this operation a likely source of contamination.

Vacuum air intake

The air source to the barrel freezer should be assessed for quality. Ice cream is roughly 50% air by volume. If air is drawn from the plant floor through a needle valve, then the surrounding environment must be kept clean and sanitary.

Compressed air intake

If the air source to the freezer is through the compressed air line, then the quality of the compressed air lines must be assessed. Although the heat of compression is very high, the air lines could cause a recontamination of the air. These lines should be equipped with adequate dryers and filters. Drain ports at the lowest points in the compressed air system should be in place and monitored regularly. Bacterial filters are available for placement in the compressed air line as it enters the freezer and bacterial filters with very small pressure drops are also available for barrel freezers which draw air from the floor by vacuum.

Ingredient feeders

All pails, boxes, or other containers from which ingredients are dumped into the ingredient feeder should be cleaned and sanitized. These containers often come straight from the warehouse and may be covered with dust when placed next to the ingredient feeder. Ingredient exposure at the ingredient feeder should be kept to a minimum with the feeder being covered at all times. It must be remembered that no treatment is given to any ice cream ingredients beyond pasteurization. Sanitation and hygiene become an absolute necessity.

Product rework/ Waste handling

The handling of product rerun developed at the freezer needs to be assessed at each plant. Rerun is an unavoidable part of the ice cream operation and is economically important to salvage. However, it is recommended that no rerun be added back to the flavor tank at any time. The addition of rerun to the flavor tank greatly increases the chance for contamination. All rerun which can be reclaimed through filtration and blending should be repasteurized and blended with fresh mix. Any rerun that cannot be reclaimed must be clearly segregated from the reclaimable material with no chance for confusion. The handling of waste material should not provide the opportunity for product contamination from outside sources such as pails or barrels headed for pig feed.


In addition to the freezing operation, the filling operation also offers great chance for product contamination. Container storage and make-up facilities must be adequate. This is often viewed in the plant as a very mundane job, often relegated to the junior man. However, contamination from handling or from cardboard dust can occur at this step. Are empty containers exposed to contamination prior to filling? Are they covered? Employees handling product at this stage such as in manual capping operations must exercise great care to prevent problems. They should never lose sight of the fact that every package will be consumed by somebody.

Environmental variables: plant environment

In addition to equipment sanitation and hygiene, the plant environment is also critical to product safety in that many organisms can be transmitted through air borne contamination. Several aspects of the plant environment will be considered here.

HVAC ducts

The heating, ventilating, and air conditioning (HVAC) system needs attention as an agent for transmission of air borne contamination. The system ducts and piping should be free of excessive dirt and dust and filters maintained in clean condition. The production and filling area should be maintained under positive pressure with air flow moving from finished to raw product areas. The location of outside air intakes should be such that outside contamination of the air system is minimal.

Floor drain

Floor drains are another area of potential environmental contamination. They should be maintained in good condition with suitable debris baskets, screens, and traps, and flushed with sanitizer regularly. Drain location should be away from major areas of product exposure such as filling machines to avoid air borne contamination. Floors should be sloped to allow for good drainage and prevention of water pooling on floors. When possible, spills should be immediately rinsed to the drain. Air borne contamination can also be carried through the formation of moisture aerosols from high pressure hoses or centrifugal pump spray. Operation of hoses or other aerosol formation during times of product exposure should be avoided.


Condensation from equipment or frozen pipes forms readily in ice cream plants, especially in hot weather.  Stainless steel pipes can become completely covered with frost. Exposed product should be protected from the possibility of condensation drip from such a pipe. In areas where product is exposed, look up and see what could possibly drip or drop into the product.


Any items which enter the production areas such as pallets, fork trucks, milk cases, maintenance equipment, etc., are suspect and should be thoroughly cleaned and in sanitary condition.  If pallets are brought onto the production floor, have they come in to the plant from an outside source? Where have they been?

Foot baths

Traffic flow in production areas should be assessed. Employee movement from raw to finished areas should be minimal. Foot baths help to eliminate spreading of bacteria from one area of the plant to another. They must be cleaned and filled daily with sanitizer of adequate strength. If nothing else, they do help to instill in the employees a sense of hygiene every time the employee has to step on it. Farm tank truck drivers or other unauthorized persons should not be allowed access to production areas.


Viewing areas behind glass are a great way to allow visitor access to the plant without allowing product access.

Sanitation and Cleaning

Beyond pasteurization, the only control of product safety or bacterial quality is through sanitation and hygiene, both equipment and environment. Most of the ice cream plant is CIP cleanable and recommendations of the chemical suppliers should be followed. CIP lines should be self-draining or self-evacuating and removal of cross connections between raw and finished lines is often necessary.

Manual cleaning

However, many items such as homogenizer screens, ingredient feeders, variegating pumps, filler heads and freezer barrels need to be taken down to be adequately cleaned out of place. This equipment must be thoroughly stripped and scrubbed. Worn gaskets need replacement but lines must be periodically taken down to find such problems.


Absorbent items such as rags or sponges should be avoided as they often become microbial zoo’s. Disposable towels for wiping spills during filling can be used in their place. If sponges are to be used, they should be continually dipped in a sanitizer solution of adequate strength and replaced frequently. Brushes used for cleaning should be segregated and labeled for interior versus exterior cleaning of equipment and for raw versus finished product usage. Wooden handles should be avoided.


All plant functions including pasteurization, sanitation, and product handling are done by the hourly employees. Their awareness of and training in sanitation and hygiene principles is critical to product safety and quality. The development of standard operating procedures for every job in the plant will assure that each employee is aware of his/her responsibilities. Adequate training of new employees and retraining or continuing education of experienced employees is difficult but necessary. A continual effort to upgrade employee knowledge of plant procedures and concerns must be made.

Employee cleanliness and hygiene must be stressed. Hair and beard nets, clean plant clothes, and suitable footwear should be provided and helps to instill a general feeling of sanitation which will carry through to plant surroundings as well. Street clothes should not be permitted in the plant, nor should plant clothes be allowed to leave the plant with the employee. Proper clothes laundering should be provided by the company. Locker rooms, changing areas, and break/ lunch rooms should be situated so as to minimize traffic through production areas. Visitor protection must also be provided and no one should be permitted access to the production rooms without proper attire.


Product Retrieval/ Recall

Much has been said about product recalls. The importance of product coding will be emphasized here. The effects of a product recall can be minimized through ingredient tracking and product coding. All incoming ingredients should be coded and recorded by number, date, lot size, etc. This is a necessary part of inventory control and stock rotation as well. The ingredients used for each batch lot of ice cream made should then be recorded. This is the “what goes where” aspect of the program. Finished product should be coded with records available as to exactly what went into each batch. Finally, some monitoring of delivery of coded product must get done. This is the “who gets what” aspect. With this kind of ingredient tracking program, if a supplier were to call and advise that he had shipped a bad batch of ingredient, some knowledge as to where that ingredient was would be available. Conversely, returned product with some defect would be identifiable so that others of the same batch could be retrieved.



Pathogenic bacteria will not survive pasteurization. The only entry mechanism is through recontamination. With adequate sanitation and hygiene, recontamination can be avoided.




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Ice Cream Technology e-Book Copyright © by H. Douglas Goff is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License, except where otherwise noted.

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