Much might be written regarding the factors affecting the growth of bacteria, the preparation of the media, the incubating temperature, the counting; this is all discussed in the various text-books on bacteriology. Here only the relation of bacteria to ice-cream will be considered.

It is commonly recognized that ice-cream contains large numbers of bacteria. The table No. VI⁠[37] on page 171 shows the average bacterial count of ice-cream and the highest and lowest counts in various cities and at different times in the same city.

This table indicates that ice-cream made in different sections of the United States has some extremely high counts and the average is comparatively high when one considers the count of milk and cream produced under clean conditions. The redeeming feature is that ice-cream can be produced with low bacterial count. This shows the need of a bacterial as well as chemical standard.

133. Sources of bacteria in ice-cream.

—There are two sources of bacteria in ice-cream, the materials employed and the utensils which come in contact with the ice-cream. The latter source is entirely under the control of the manufacturer. If materials used have a low bacterial count, there is no reason why the ice-cream should not be low in bacteria. If this is the case and the ice-cream has a high count, it would indicate that the maker was careless and the utensils dirty.

Table VI
Summary of Bacterial Investigations of Ice-cream

Ellenberger⁠[38] found the minimum and maximum number of bacteria in the materials used in the mix as given in Table VII:

Table VII
The Minimum and Maximum Bacterial Content of the Ingredients used in the Mix

The important fact brought out by the above table is that the milk products are the source of most of the bacteria in the ice-cream. This emphasizes the need of dairy products manufactured and marketed under the most cleanly conditions. The ice-cream-maker in most cases has little control over these factors. For the production and handling of milk, see Chapter II. The numbers of bacteria may be reduced materially by pasteurization.⁠[39] This destroys the viscosity so that milk or cream that has been aged and then pasteurized must be aged again to regain it. There is great danger while ageing either pasteurized or raw cream of a material increase in the numbers of bacteria. Great pains must be taken to cool and hold or age it at a low enough temperature so that the organisms will not develop. If this is not done, the result would be practically the same as if it had not been pasteurized, so far as numbers of organisms are concerned.

Hammar gives the count of different samples of gelatine as follows:

Table VIII
Bacterial Count of Samples of Gelatine

This table shows the need of testing to make sure that the supply of gelatine is low in number of bacteria. Usually in the better or more expensive grades, the bacterial content is lower, but this is not necessarily true. The method of preparation has much influence on the bacterial content of ice-cream. (See Chapter IV.)

Sugar contains very few organisms. The greatest danger is that dirt may get into the sugar due either to exposure or sifting through the cloth sacks.

The flavoring materials may have a decided influence on the bacterial content of the ice-cream. Vanilla contains very few organisms, but such flavors as fruits may have large numbers of bacteria, especially if they are not sound.

Hammar⁠[40] reaches the following conclusions regarding the bacterial content of the materials used in the manufacture of the ice-cream:

1. The cream unless pasteurized is the greatest source of the bacteria in ice-cream.

2. After pasteurization cream can be stored at 0° C. for several days with no important increase in the number of bacteria developing at 37° C. on agar.

3. The bacterial content of gelatine is very valuable and some probably add large numbers of bacteria to ice-cream in which they are used.

4. The vanilla extract plays a very small part in detaining the bacterial content of ice-cream.

5. The sugar is comparatively unimportant as regards the number of bacteria in the ice-cream made with it, if it has been properly protected from dirt.

All the utensils, unless clean, may be a source of contamination to the ice-cream. When machines are selected, the ease of cleaning should not be overlooked. All seams should be flushed with solder. This leaves no crevices in which bacteria and dirt can lodge, and makes cleaning easy. The cleaning of the freezer should receive special attention.

134. The effect of freezing and hardening on the bacterial count.

—Authorities agree that there is an increase in the number of organisms in ice-cream during the freezing process, as determined by the plate method. This may be accounted for by the agitation in the freezer breaking up the clusters of bacteria. This cluster would give only one colony on a plate, but after being broken up might result in several colonies. This is not a real increase in the number of bacteria. The same holds true with other machines, such as the clarifier.⁠[41]

Ellenberger found that there were no radical changes in the total number of bacteria in ice-cream during hardening. There seems to be a tendency toward a slight decrease in the first two or three days with a more noticeable increase and corresponding decrease again between the sixth and twentieth days, after which there is only a very gradual falling off in numbers. The lower temperatures may have a destructive effect on some types of organisms. There may be a reorganization with the survival of the fittest.

135. Types of organisms in ice-cream.

—The bacterial flora of ice-cream in summer and winter was studied by Ayers and Johnson.⁠[42] They divided the samples into summer and winter and the bacteria into groups by the milk-tube method of differentiation.

Table IX
Showing the Groups of Organisms and the Percentage in Each Group

136. The total-acid groups.

—“As seen in Table No. IX of the average bacterial flora of summer ice-cream 70.54 per cent is made up of the total group of acid-forming bacteria, and during the winter 68.87 per cent. While using the milk-tube method of differentiation the reactions of the litmus milk tubes are recorded after 2, 5 and 14 days, and the total acid-forming group is composed of those bacteria which produce acid in litmus milk during the 14 days’ incubation. Those bacteria which form acid and peptonize the milk are included in the peptonizing group. The total-acid group can be further divided into those which produce acid and coagulate the milk and those which simply form acid within 14 days. Since the reaction is recorded after 2, 5, and 14 days, the rapidity of the growth of the acid-forming bacteria can be determined, and this serves as an additional means of separating the group. In Table No. X the percentages of the acid-coagulating and the simple acid-forming groups of bacteria are shown, based on the 2, 5 and 14 day reaction in litmus milk.”

Table X
Changes in the Percentage of the Total-acid Group of Bacteria in Ice-cream when Determined by Litmus-milk Reactions after Various Lengths of Incubation

“An examination of Table No. X shows that among the summer samples 49.82 per cent of the bacteria produced acid and coagulated the milk after 14 days. After 2 days 26.31 per cent produced this reaction. This shows that a little more than half, or 52.81 per cent of the bacteria which were in the ice-cream produced the reaction within 48 hours. The remaining 47.19 per cent coagulated milk more slowly and may represent a different variety of acid-forming bacteria. Turning again to the table and considering the acid-coagulating group of the winter series, it will be seen that of the 30.84 per cent which produced the reaction only 8.20 per cent produced acid and coagulated milk in 2 days. Therefore only 26.69 per cent of the acid coagulating group of the winter samples were active enough to produce the reaction in 48 hours, while 52.81 per cent of this group in the summer samples brought about the change in 2 days.

“There is little to be said regarding the acid-forming bacteria which simply produce acid. Many of them grow slowly and do not show an acid reaction for several days in litmus milk. The milk-tube method furnishes a means of determining the difference in the rapidity with which the bacteria produce acid. As may be seen in Table No. X the percentage of the acid-forming group of bacteria was highest when determined by the 2-day reactions and lowest when based on the 5 and 14 days’ reactions. This is explained by the fact that many bacteria have simply formed acid after two days in litmus milk and later may coagulate or peptonize the milk, and are therefore thrown into another group.”

137. The inert group.

—“The inert group of bacteria in ice-cream comprises those which produce no change in litmus milk during the 14 days’ incubation at 30° C. (86° F.). By this method of grouping there are, of course, included in the inert group those cultures which fail to grow in milk and tubes of litmus milk, and which would also be included even though the lack of growth were caused by failure to inoculate the tubes properly. However, this last possibility is small. The inert group is of little interest, on the whole, since the bacteria produce no apparent change in milk, and in all probability the same is true of ice-cream.”

138. The alkali group.

—“The alkali-forming group of bacteria is made up of organisms capable of producing an alkaline reaction and no other apparent change in litmus milk during the 14 days’ incubation at 30°C (86°F). This group does not include bacteria which produce an alkaline reaction together with visible signs of peptonization. While there are in the literature references which deal with types of alkali-forming bacteria, this group has rarely, if ever, been considered when the flora of milk has been under discussion. The authors in some previous work on bacteria in milk showed that considerable numbers of this group were present in milk. In a later piece of work we have shown the numbers of this type of bacteria in milk, together with some of the cultural reactions of the alkali-forming bacteria. These bacteria, however, give very few positive reactions with the usual cultural media, and it is impossible to give much information regarding this group. A detailed bacteriological and chemical study of these organisms is under way in the research laboratories of the Dairy Division.

“It will be seen from Table XI that during the summer series of ice cream samples the average sample contained 1.86 per cent of the alkali group of bacteria, and during the winter series 5.42 per cent. In general, the alkaline reaction is not noticeable until after four or five days’ incubation in litmus milk. Occasionally, however, the reaction is in evidence in 48 hours. This group percentage for the summer season was 1.86 after 14 days and only 0.15 per cent based on the 2-day reaction. Therefore, only 8.06 per cent of the bacteria of the alkali group produce an alkaline reaction within 48 hours. Among the samples collected during the winter season only 3.13 per cent of the bacteria of this group were capable of producing the reaction within two days. Whether this indicates a different variety of organism can not be said with assurance.”

Table XI
Changes in the Percentage of the Alkali Group of Bacteria in Ice-cream when Determined by Litmus-milk Reactions after Various Lengths of Incubation

“At present we are unable to state the significance of this group of bacteria in milk and ice-cream, but it is evident that they are not present in ice-cream in large numbers, as are the bacteria of other groups.

“Alkali-forming bacteria were not found in each sample examined, but this does not prove that there were none present in the ice-cream. Since these organisms are present in small numbers compared to the rest of the bacteria, it is not surprising that none should be found on plates in which the dilution had to be high in order to take care of the large total number of organisms.”

139. The peptonizing group.

—“The peptonizing group is probably the most interesting if not the most important group of bacteria in ice-cream. This group consists of what are commonly known as the putrefactive bacteria; that is to say, they attack primarily the proteins, decomposing them into less complex organic bodies. Bacteria of this class are usually considered undesirable in articles of food, and it is to them that intestinal troubles are sometimes attributed, perhaps with or without justification. Whatever their true effect is will not be discussed in this paper, but because bacteria of this group are looked upon with suspicion it is therefore of great importance.

“Among this group there are a large number of different types of organisms. Many rapidly peptonize the casein of milk and render milk alkaline or slightly acid, while others first attack the lactose and only produce a slight peptonization after several days’ growth. From the milk-tube method of differentiation of the bacterial groups it was possible to gain some information as to the extent of these different classes of peptonizers. In Table No. XII are shown the average percentages of the peptonizing group in summer and winter samples of ice cream. Based on the 14-day reaction among the summer samples, 13.62 per cent of the bacteria belonged to the peptonizing group. According to the 2-day reaction, there were 5.93 per cent. Therefore 43.54 per cent of the peptonizing bacteria were sufficiently active to produce a peptonization within two days. Among the winter samples 34.06 per cent of the peptonizing bacteria were sufficiently active to peptonize milk within 48 hours. These active peptonizing bacteria are more important than the slower-acting varieties, since their peptonizing action is usually more complete than that of the latter-named varieties, and if any harm is produced by this group, they are most likely to be the organisms concerned.”

Table XII
Changes in the Percentage of the Peptonizing Group of Bacteria in Ice-cream when Determined by Litmus-milk Reactions after Various Lengths of Incubation

140. Colon bacilli in ice-cream.

—Since the presence of colon bacilli has been understood in water analysis to indicate fecal contamination, many investigators and boards of health apply the same tests to milk and naturally then to ice-cream with the same idea. In water analysis, lactose-bile fermentation tubes are employed for the examination for colon bacilli. By using different dilutions, the maximum number of gas-forming bacteria in a given amount of water may be determined. This preliminary test has to be followed by confirmatory ones in which cultures are isolated and their characteristics studied in order to prove the presence of colon bacilli. Ayers and Johnson used this method to some extent but endeavored to prepare a synthetic medium which would restrict the growth of the larger number of bacteria in ice-cream and at the same time would allow colon bacilli to develop and produce characteristic reactions. Ice-cream contained a much larger number of gas-forming organisms in the summer season. A large number of media were used in an attempt to devise a suitable medium for the detection of Bacillus Coli in ice-cream and the results show that there is no entirely satisfactory method known at present.

141. Difficulties in studying the bacteriology of ice-cream.

—As has already been pointed out, one of the greatest difficulties in studying the bacteriology of ice-cream is the lack of a suitable culture media. Because of the low temperature at which the ice-cream is hardened and held, some investigators believe that there is a gradual change of the types of organisms or the survival of the fittest. This brings up the question as to the temperature at which the cultures should be incubated. Until a suitable media is prepared and a uniform temperature of incubation used, the counts of the organisms in ice-cream made by different investigators will not be comparable.