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The Bacterial Content of Ice Cream^*

A Report of Experiments in Bacterial Control in Six Commercial Plants

Kansas Agricultural Experiment Station, Manhattan, Kansas

ABSTRACT

One of the objects of this investigation was to determine the possibility of producing a product of unquestioned sanitary quality under the most trying plant conditions and to verify the results obtained in previous work. Table 4 emphasizes the results very well. The average count for all the unsupervised finished ice cream was 390,225 bacteria per gram, while the average for the supervised finished ice cream was only 39,127 bacteria per gram. If the results obtained in plant V are not averaged with the rest, the difference would be even more pronounced. The highest count of the finished product under supervision was 91,000 bacteria per gram while the lowest was 3200 bacteria per gram.

Only 5 of 12 unsupervised mixes were pasteurized as required by the Kansas law (mixes 1, 2, 14, 17, 18; table 2).

It is apparent from a study of tables 4, 5 and 6 that not enough effort is expended on the part of the manufacturers toward securing a more desirable quality of milk products.

The results included in this work verify in every detail the summary of the results of previous work (1).

From the data presented in this paper the following conclusions seem justifiable:

1. Cream and milk are the most important source of bacteria in the raw ice cream mix. In most instances these products supplied over 99 per cent of the total bacteria in the raw mix.
   
2. Condensed milk may, at times, be an important source of bacteria in the raw mix.
   
3. Gelatin, water, and sugar were negligible factors as sources of bacteria in the ice creams studied.
   
4. Thorough washing with an alkaline washing powder and sterilization of all equipment with live steam are essential factors in the production of ice cream of low bacterial content.
   
5. Carelessly washed tubular surface coolers are a considerable source of bacterial contamination in ice cream.
   
6. Conveying pipes, pumps, and homogenizers cannot be properly cleaned without being taken apart.
   
7. Hypochlorite solutions, when properly used, give satisfactory results in sterilizing ice cream plant equipment.
   
8. The use of pasteurizing equipment for two or more successive mixes without washing may result in high counts due to the growth of thermophilic bacteria.
   
9. The aging, without agitation, of improperly cooled mix (65°F.) in cans in a cold room (38°F.) results in a material increase in bacterial content in twenty-four hours.
   
10. Proper pasteurization is the most important factor governing the bacterial count of ice cream. Pasteurization at 145°F for thirty minutes and homogenization at pasteurizing temperature results in counts of less than 100,000 bacteria per gram in the finished ice cream, provided equipment contamination is reduced to a minimum. These results were obtained even with raw mixes containing as high as 34 million bacteria per gram.
    
11. If the bacterial flora of the mix is constituted of types easily destroyed by heat, high efficiency of pasteurization may be obtained in less time than is regularly prescribed.
    
12. Extreme pasteurizing temperatures (165°F.) are unnecessary in producing ice cream of low bacterial content.
    
13. Homogenization, even though contamination is negligible, usually results in an increased count, as determined by the plate method, probably due to breaking up of bacterial clumps.
    
14. Ice cream mix should be cooled as soon as possible after homogenizing, to prevent bacterial growth.
    
15. It is possible and practical to produce ice cream containing less than 100,000 bacteria per gram under all plant conditions, provided that efficient pasteurization is practiced, that the temperature is controlled during aging and that equipment is properly washed and sterilized. High bacterial counts indicate carelessness at some point in the manufacturing process which in turn indicates an undesirable, if not an unsafe, product. There can be no valid excuse for ice cream containing more than 100,000 bacteria per gram as determined by the plate method.
    

^* Contribution No. 51 from Department of Dairy Husbandry and No. 73 from from Department of Bacteriology.