Water activity in dry foods containing live probiotic bacteria should be carefully considered: A case study with Lactobacillus rhamnosus GG in flaxseed.

This study evaluated the effect of water activity on the long-term storage stability of the probiotic Lactobacillus rhamnosus GG (LGG) in a dry food matrix. Viability of LGG was further studied in a crushed flaxseed matrix,  a new possible product matrix to deliver probiotics – as well as in reference matrices as maltodextrin. Three different water activities (aw of 0.11, 0.22 and 0.43) were used, and preparations were stored at room temperature for up to 14 months. The viability of LGG was less dependent on the matrix used, but strongly dependent on the water activity. Viability in flaxseed was lost rapidly with aw 0.43: with aw 0.22 the reduction was 2.4 log10 units and with aw 0.11 the reduction of viability was only 0.29 log10 units during the entire storage time. Taken together, regulating water activity to a low value may offer possibilities for extending the shelf life of dry probiotic products. (Authors abstract)

Viability of probiotic bacteria is required to exert health benefits in humans and keeping probiotics alive is therefore an important task for food manufacturers. Incorporating probiotics into new food matrices, and keeping them alive during their shelf life, is a significant challenge, viability can be lost during the manufacturing process, transportation or during storage of the probiotic product. Various environmental factors such as temperature, oxygen, humidity (also in the case of high water activity products), pH and presence of other cultures (starter or non-starter) can also lower the viability of probiotics.  Probiotic viability during the storage time of dry products can be enhanced by controlling the water activity, temperature and oxygen content in the product. Water activity (aw) of the final product is a key factor in maintaining probiotic viability in dry products. In this study, the effect of water activity on the storage stability of specific probiotic included in crushed flaxseed was assessed. The aim was to identify the right water activity level for storage at room temperature. 

For probiotics, it is generally recommended that the aw should be below 0.25 during long term storage.  The results showed that at the highest water activity tested (aw 0.43), the probiotic product was very unstable. Within 4 months storage time the viability was reduced 5.3 log10 in maltodextrin and 3.7 log10 in flaxseed; after 7 months storage time no viable cells were found in maltodextrin, and reduction of 3.8 log10 units was found in flaxseed. With aw 0.22, the reduction of viable cells was very slight within the first 4 months, 0.16, 0.10 and 0.81 log10 units in the matrices of sand, maltodextrin and flaxseed, respectively. However, after 14months storage, the reduction was over 2.2 log10 in all tested matrices. With aw 0.11, the probiotic was stable for the whole storage time; reduction of viability was very low in all tested matrices: 0.78, 0.58 and 0.29 log10 units in sand, maltodextrin and flaxseed, respectively.  With this water activity, there was no loss in the number of viable probiotic bacteria either in the crushed flaxseed, or within the reference matrices. Moreover, the extreme stability of probiotic bacteria in low water activity and at room temperature indicates that crushed flaxseed, and probably other vegetable or cereal matrices, potentially offers a reliable way to deliver probiotics. (Editors comments)