Use of Stabilizing Agents in Culture Media for Growing Acid Producing Bacteria

The present invention relates to bulk starter media for cheesemaking and, in particular, to a bulk starter media which contains particulate, insoluble materials as neutralizing agents. In the commercial production of cheese, large vats of milk are treated with a milk clotting agent such as rennin and inoculated with lactic acid producing bacteria such as Streptococcus lactis, S. cremoris, S. thermophilus and Lactobacillus bulgaricus. These lactic acid producing bacteria are capable of fermenting lactose or other similar carbohydrates to produce lactic acid.

The bacteria is generally propagated from a mother culture in large enough quantities of aqueous medium to produce a bulk starter which can then be used for fermenting the final batch of milk to produce cheese as the final product. In a typical procedure, a bulk starter medium is prepared by dissolving a dry powder comprising a nutrient base and a growth stimulant, together with alkali metal phosphate and/or polyphosphate phage control agents in water to a level of about 8% solids. The solution is sterilized and cooled whereupon it is inoculated with the lactic acid producing bacteria which is allowed to incubate. After incubation, the "ripened" starter is added to the cheese milk at a level of from about 1/2 to 3/4 volume percent. The cheese milk at this point will have a pH of approximately 6.6, however, as the bacteria grows, the acid it produces will gradually lower the pH.

Neutralization is very important to the growth of the lactic acid producing bacteria because the acid produced impairs the growth of the bacteria and will eventually kill them, especially at a pH of 4.8 or below. Neutralization can be accomplished by the continuous addition of a water soluble base or an aqueous basic solution such as sodium hydroxide or ammonia gas so as to provide a pH which is maintained at 5.2 or above. Specialized equipment has been developed to monitor the pH of the growth medium and to add the neutralizing agent as needed. More recently, there has been disclosed in U.S. Pat. No. 4,282,255 the use of an essentially water insoluble or temporarily water insolubilized and thus initially solid form of a neutralizing agent. This patent mentions the use of magnesium phosphate and magnesium ammonium phosphate in particular. A magnesium ammonium phosphate neutralizing system in which magnesium ions, ammonium ions and phosphate ions are provided so as to form insoluble magnesium ammonium phosphate in situ is disclosed in U.S. Pat. No. 4,402,986.

While the employment of essentially insoluble or temporarily insolubilized neutralizing agents eliminates the need for the constant addition of base, their use presents another problem because of their insolubility. This is the case because the presence of the insoluble neutralizing agent requires agitation to keep it in suspension. This agitation adds cost to the process and is traditionally considered an undesirable procedure because culture growth may be detrimentally affected due to oxygen incorporation into the ripening starter. Accordingly, it would be desirable and it is an object of the present invention to provide a method for maintaining a particulate, insoluble material as neutralizing agent suspended in the ripening starter without the need for agitation.

The present invention is an improvement in the method of propagating a mother culture of an acid producing bacteria by inoculating an aqueous bulk starter medium containing a nutrient base, a growth stimulant, a phage control agent and an essentially insoluble or temporarily insolubilized, particulate material as neutralizing agent. The improvement comprises incorporating into the bulk starter medium a foodgrade protective hydrocolloid as stabilizer in sufficient amount to prevent settling out of the neutralizing agent from the medium.

The acid producing bacteria is normally propagated from a mother culture in large enough quantities of aqueous medium to produce a bulk starter which can then be used for fermenting the final batch of cheese-milk to produce the end product. A good starter must produce lactic acid in the cheese vat at a vigorous and steady rate as slow starters produce cheese of inferior quality. Some of the causes of starter slowness include infection by bacteriophage (phage), bacteria of low viability and a low bacterial cell population. The composition of the starter medium exerts a considerable influence on the loss of cell viability due to phage attack, and on maintenance of a desirable pH for the growth of acid producing bacteria.

A typical dry bulk starter medium may contain milk, whey and dextrose as nutrient base sources, an enzyme hydrolyzed casein and/or yeast as growth stimulant and mono-ammonium phosphate, di-ammonium phosphate and sodium hexametaphosphate or a combination thereof as the phage control agent. Suitable neutralizing agents include calcium carbonate, magnesium hydroxide, magnesium phosphate and magnesium ammonium phosphate. These inorganic salts are essentially insoluble in water which makes them desirable as neutralizing agents because they provide a buffering effect over a long period of time. Dissolving the dry bulk starter in water or milk at a level of about 8% (w/w) solids provides a suitable growth medium for the acid producing bacteria, but the insoluble nature of the particulate material, i.e. the neutralizing agent, necessitates constant agitation of the bulk starter medium to keep it in suspension. It has now been discovered that by adding a foodgrade protective hydrocolloid to the bulk starter medium before ripening, in an amount sufficient to prevent separation of the mixture, the necessity of constant agitation during the ripening process is eliminated.

In many food applications, stabilizers are used as thickening, suspending and emulsifying agents. All stabilizers increase the viscosity of the liquid medium and have the ability to bind or hold relatively large amounts of water. Exemplary of hydrocolloids useful as stabilizing agents are sodium carboxymethylcellulose, locust bean gum, sodium alginate, propylene glycol alginate, carrageenan and guar gum. Other suitable stabilizers include gelatin, gum exudates, karaya, tragacanth and gum arabic. It has been observed that care must be taken with the use of gelatin as stabilizer as it may thicken excessively and require agitation after ripening of the culture. Xanthan gum is preferred for use as the stabilizing agent because it is economical and effectively maintains the particulates in suspension.

In the practice of the present invention, the stabilizer is preferably mixed with the dry blended bulk starter media at the time of its preparation. Thus, the nutrient base, growth stimulant, phage control agent and neutralizing agent, which comprise the starter media, are mixed with the dry stabilizer and packaged as a single product. The addition of this material to an aqueous medium, usually with agitation, results in dissolution of the soluble ingredients. The neutralizing agent, which does not dissolve, is prevented from settling by the presence of the stabilizing agent, so that agitation, with its consequent disadvantages, can be terminated shortly after inoculation with the acid producing bacteria. Alternatively, the culture media can be put into water solution without the stabilizer and the stabilizer then added either as a dry material or a solution thereof. In either emoodiment, the stabilizer is normally used initially in the form of a dry powder which assumes its functionality once it is rehydrated. The powder becomes uniformly distributed by the blending action required for preparation of the starter formulation.

A typical starter medium may contain milk and/or whey as the nutrient base source. Growth stimulants such as yeast extract and/or an enzyme hydrolyzed casein, are suitable growth promoters and the phage control agent can be of the citric acid type or, more commonly, an inorganic phosphate salt such as monoammonium phosphate, di-ammonium phosphate, sodium hexametaphosphate or a mixture thereof. The phage control agent of choice for the formulation described in U.S. Pat. No. 4,402,986 is an alkali metal tripolyphosphate. As mentioned above, it has been found desirable in certain instances to combine these ingredients with an essentially insoluble or temporarily insolubilized particulate material such as a neutralizing agent of the type described in U.S. Pat. No. 4,282,255. This is typically a base, a basic salt or a mixture thereof adapted to provide a controlled reaction with the acid produced by the bacteria without substantially raising the pH of the medium. This material is typically employed in a particulate form (preferably having a particle diameter of from 45 to 710 microns) and in an amount of from 0.5 to 8 weight percent of the combined dry weights of carbohydrate source, growth promoter and phage control agent.

The stabilizer prevents settling of the particulate material and thus eliminates the need for agitation which may interfere with the ripening of the media after it is inoculated with the acid producing bacteria. The stabilizer, as previously described, will be added to the dry blend bulk starter media or to its water solution in an amount sufficient to prevent settling of any insoluble, particulate material contained therein. This amount will vary depending on the amount and size of insoluble particulates involved, but will typically range from 2% to 6% of the dry bulk starter media on a weight basis.

The method of practicing the present invention is further illustrated by the following examples in which all percentages are on a weight/weight basis unless otherwise indicated.

To a portion of the above formulation was added Xanthan Gum to form Medium XG having a final composition as follows:

The medium without Xanthan Gum (designated Control) and Medium XG were rehydrated with water to 8.25% total solids, heated with agitation to 87.8.degree. C. for 40 minutes and cooled to 22.8.degree. C.

A one percent inoculation of both the Control and Medium XG was made with an active culture of WP Streptococcus cremoris. The inoculated media were incubated at 22.8.degree. C. for 16 hours. An activity test was carried out by inoculating 0.5% (v/v) of the above 16 hours growth into steamed 11% (w/w) nonfat dry milk and incubating at 89.degree. F. for 6 hours. The developed acidity was determined by subtracting the 0 hour value from the 6 hour value. Settling of particulates was observed at the times indicated in Table I.

From Table I it can be determined that culture growth and performance in Meiium XG without agitation were equal to or greater than the Control with agitation and Medium XG with agitation. The Control without agitation shows the expected reduced subculture growth and cell numbers caused by settling of the particulates. Culture growth indicates that the stabilized medium is not detrimental to growth and that incubation without agitation yields a very active culture.

Certain lactic acid producing bacteria are more sensitive to the presence of oxygen than others. Those culture strains which are the most sensitive are frequently inhibited by the agitation that is necessary to keep insoluble salts suspended. The procedure described in Example I was repeated using Culture CJB, an oxygen sensitive strain of Streptococcus cremoris from Marschall Products Divisions of Miles Laboratories, Inc. The results of this study are set out in Table II.

Table II illustrates the distinct advantage to the use of a bulk starter medium containing a stabilizer as contemplated by the present invention for the propagation of oxygen sensitive cultures. In all cases, even with stabilizer present, agitation severely inhibited growth due to oxygen incorporation; whereas, the use of a medium containing stabilizer without agitation resulted in normal culture development. Since there are a number of commercial cultures that are sensitive to the oxygen levels which occur during normal agitation in many starter tanks, the present invention will enhance the utility of media containing insoluble salts.

A dry bulk control starter medium (without Xanthan Gum) was prepared as described in Example I. The Control was rehydrated with water to 8.25% total solids and two samples of Medium XG (prepared as in Example I) to 8.25% and 15.25% total solids, respectively. One percent inoculation was made with the CJB culture which had been grown for 16 hours as described in Example I. The incubation was carried out at 22.8.degree. C. for 16 hours with the results being reported in Table III.

The data set out in Table III show that the use of Xanthan Gum in the medium resulted in complete and sustained suspension of the particulate material regardless of the solids level. At the normal medium solids level (8.25% w/w) suspension was maintained for 5 days at refrigeration temperature of 4.degree. C. At 15.25% (w/w) suspension was maintained for at least 24 hours at the temperature of 4.degree. C.

A second medium containing insoluble salts was prepared with the following composition:

To a portion of the above Control formulation was added Xanthan Gum so that the final composition was as follows:

The Control growth medium and the growth medium with Xanthan Gum (Medium XG) were hydrated with water to 7.0% total solids, heated with agitation to 87.8.degree. C. for 40 minutes and cooled to 22.8.degree. C.

One percent inoculations were made with cultures CC7 and EV4 strains of Streptococcus cremoris and M24, a strain of Streptococcus lactis all from Marschall Products Division of Miles Laboratories, Inc. grown as described in Example I. Incubation was carried out at 22.8.degree. C. for 16 hours with the Control samples being agitated and the samples with Xanthan Gum grown quiescently. An activity test was carried out by inoculating 0.5% (v/v) of the above 16 hour growth into steamed 11% (w/w) nonfat dry milk and incubating at 89.degree. F. for 5 hours. The results are set out in Table IV.

From Table IV it can be determined that culture growth and performance in Medium XG were equal to or greater than the Control with agitation.