Effect of different thermal treatments and storage on starch digestibility and physicochemical characteristics of unripe banana flour

Unripe banana flour is an important source of indigestible carbohydrates, mainly resistant starch (RS). However, during cooking of the fruit, the indigestible carbohydrates are transformed to digestible. The aim of this study was to test different treatments: cooking (5, 15 and 25min), cooking plus high-moisture treatment (HMT) and cooking plus HMT plus storage (HMT+S), to increase the slowly digestible starch (SDS) and RS fractions. Polarization light microscopy, leached amylose, X-ray diffraction pattern, thermal and pasting characteristics, and starch digestibility analysis of the flours were evaluated. The longest cooking time decreased the RS content in the non-gelatinized samples; HMT and HMT+S increased the SDS level. These samples could be used in products without heat treatment during their preparation. The gelatinized samples presented a significant fraction of RS in comparison with the non-gelatinized, demonstrated that the samples can be used in products where cooking is necessary, conserving a high amount of indigestible carbohydrates. PRACTICAL APPLICATIONS: Unripe bananas are an alternative source of indigestible carbohydrates, mainly resistant starch (RS) and dietary fiber (DF). Unripe banana is also considered the richest natural source of RS; however, this characteristic is lost when the fruit is cooked. The digestion rate of starch present in unripe banana flour can be modified to produce a functional or nutraceutical ingredient. The use of physical treatments on unripe banana flour such as cooking, high-moisture treatment, storage and combinations of these, have shown to increase the slowly digestible starch and RS fractions. The physical treatments on unripe banana flour produce a powder that, because of the high amount of indigestible carbohydrates, can be considered as nutraceutical ingredient and can be used in products such as pasta and bakery products in which cooking is necessary, while maintaining a high amount of indigestible carbohydrates.