ANSWER
Moisture migration is a cumulative cause for quality deterioration in multicomponent food, such as the undesirable textural changes. Moisture migration can occur in different parts of food, such as from crumb to crust, from intermediate-moisture fruit pieces to cereal flakes and from a pie filling to the crispy crust. Moisture redistribution will inevitably arise whenever there is difference in water activity within multicomponent foods, until the food system reaches equilibrium (Hao et al., 2017).
For instance, biscuit and strawberry jam fillings are differing in their water activity (0.4 and 0.84 respectively for biscuit and jam). When these two components are in contact, water will migrate from jam to biscuit which leads to softening of the outer biscuit. As a result, thicken the jam and induces sugar crystallisation. Consequently, the latter may lead to quality loss since sugars crystallisation may cause syneresis from the filling. To avoid softening and further quality loss, the water activity gradient should be limit (Skibsted et al., 2010).
This could be achieving by lower the water activity of the moist phase (jam fillings) such as sugars, salts, high fructose corn syrup or polyols. These solutes strongly interact with water and thus supressing the water activity of the fruit filling (Skibsted et al., 2010).
Pectins are the most preferred gelling agents for acidic fruit gels because of being acid stable (Saha & Bhattacharya, 2010). A small amount of pectin helps to impart a firmer texture to the filling. This results in maintaining low water activity in jam filling so that crispiness of biscuit could be preserved. Pectin requires acidic medium otherwise it will not react and the filling will remain watery as compared to the desirable sticky and firm texture of the usual fruit jam. Similarly, acids such as maleic or citric acids may be added in suitable amount to enhance acidic taste and assist in the setting of the gel (US Patent No. US6660314B2, 2000).
Besides, barrier technology can be employed to prevent moisture migration between the base biscuit and fillings/coatings. For biscuits, the barrier coating could be the mixture of a high-melting fat and icing sugar that are able to retard moisture migration from the higher moisture content component to the biscuit in order to retain the crispiness of the biscuit during storage and prior to consumption. By incorporated with the moisture barrier, the crunchiness of the biscuit is significantly improved over its shelf life (US Patent No. US20100119661A1, 2010).
On top of that, the moisture-impermeable film packaging could help to reduce potential for moisture movement from atmosphere and hence the biscuits remain crisp-eating (Cauvain & Young, 2008).
In short, addition of humectants, mixture of pectin and acid with appropriate proportion, barrier coating and packed in moisture-impermeable film can be employed to maintain the biscuit crispiness, by the meaning of reducing moisture migration.
References
Cauvain, S., & Young, L. (2008). Bakery Food Manufacture and Quality (p. 150). New York, NY: John Wiley & Sons.
Hao, F., Lu, L., & Wang, J. (2017). Finite Element Analysis of Moisture Migration of Multicomponent Foods During Storage. Journal Of Food Process Engineering, 40(1), e12319. https://doi.org/10.1111/jfpe.12319
Lecat, A. M. P. & Myers, L. J. L. (2010). Coated biscuit product. US Patent No. US20100119661A1.
Martinville, J. S. D. & Chirife, J. (2000). Low water activity flavoured filling for baked flour based products. US Patent No. US6660314B2.
Saha, D., & Bhattacharya, S. (2010). Hydrocolloids as thickening and gelling agents in food: a critical review. Journal Of Food Science And Technology, 47(6), 587-597. https://doi.org/10.1007/s13197-010-0162-6
Skibsted, L., Risbo, J., & Andersen, M. (2010). Chemical deterioration and physical instability of food and beverages. Boca Raton: CRC Press.
