Freeze-drying processes are frequently used to preserve microorganisms. Indeed, the low operating temperatures of freeze-drying processes lead to minimal damage in microorganisms. Moreover, it is known in the art that low storage temperatures can be an essential element in maintaining the viability and stability of such freeze-dried cells. It is also known in the art, that cells which are freeze-dried in the presence of protective agents are better suited to maintain their viability and stability than cells which are freeze-dried without the addition of said protective agents. For example, U.S. Pat. No. 6,919,172 (Wisconsin Alumni Research Foundation) relates to the use of a protective mixture (i.e. sucrose and potassium phosphates) mixed with biological material in order to form a preservation medium, prior to the biological medium undergoing at least one preservation process (eg. freezing, freeze-drying, vacuum-drying or spray-drying). After being subjected to a preservation process, the biological material is capable of remaining stable when stored in dry conditions at room temperature. However, the probiotic microorganisms of U.S. Pat. No. 6,919,172 which have been preserved to provide increased stability during shipping and storage in sealed containers cannot usually withstand the simultaneous high temperature and/or high humidity conditions which are typically encountered in long-term storage and secondary processes used in feed products, food products (e.g. cereals, nutritional bars, recombined cheese, infant formulas), consumer healthcare products (including dietary supplements, nutraceuticals, over-the-counter products) as well as agri-products. Indeed, the viable microoganisms are often inactivated by heat and/or moisture and/or other processing conditions or storage conditions, thereby decreasing their viability in the final product over time, during storage and/or during consumption, in such a way that the desired effect of the final product containing the microorganisms is eventually lost.
In order to overcome this problem, numerous coating processes for microorganisms have been developed to protect them against undesirable elements such as high temperature and high humidity levels. These coating processes are often based on coating materials such as fats or low permeability polymers such as shellac and hydroxypropylmethyl cellulose (HPMC). For example, U.S. Pat. No. 7,157,258 (Lallemand) deals with coating of dehydrated microorganisms by using hydrophobic barriers such as fat and waxes with melting points greater than 60° C., while EP 1753440 B1 (Merck), U.S. Pat. No. 6,365,148 (Il Yang Pharm Co. Ltd.) and EP 0778778 B1 (University New South Wales) use polymeric barriers such as HPMC or starch. These moisture barriers are intended to provide protection of the dehydrated microorganisms by preventing or delaying penetration of water into the coating, thereby avoiding diffusion of moisture to the underlaying dehydrated microorganism. If the coating allows moisture to pass, then the dehydrated microorganism may absorb moisture, thereby altering its viability.
However, the Applicant has prepared dehydrated microorganisms coated with commonly used compounds such as fats, shellac or HPMC and noticed that the viability of such microorganisms containing products was not sufficiently maintained. Indeed, as can be seen in the results section of the present patent application (especially Example 4, Table 5 and Example 7, Table 8), the materials used in the prior art to coat the microorganisms do not enhance the thermostability of the microorganisms nor provide enhanced viability over a longer period of time in medium or high humidity environments when compared to non-coated dehydrated microorganisms.
Consequently, despite the teachings above, there still exists a need in the art for coated dehydrated microorganisms, whereby the microorganisms are able to retain most of their activity during and after manufacturing and are able to resist the damaging conditions encountered for example in secondary processes used for the preparation of feed/food products, consumer healthcare products or agri-products or during long term storage of these products, wherein said processes/long term storage often involve a combination of high temperature and/or high moisture, thereby responding to a widely sought-after and currently unmet demand for dehydrated microorganisms with improved stability.