Interactions of protein, dietary fiber, and co-passengers in plant-based side streams - technological and digestive perspectives

A globally sustainable food system calls for the reduction of food waste.Among different food waste categories, plant-based food industry side streams represent high volume streams with underutilized potential as food ingredients. Often, plant-based side streams contain high amounts of dietary fiber (DF) as many food processes harness the intrinsically more functional components of plants rich in starch and lipids. In return, enormous amounts of edible materials comprising nutritionally beneficial compounds such as DF and protein are underexploited and used for feed or energy production.
This thesis explored the potential of side streams from white rice production and rapeseed oil pressing as functional and nutritionally valuable food ingredients. The role of the secondary plant metabolite, phytic acid, was specifically focused on. Phytic acid (myo-inositol hexaphosphoric acid or IP6) is the main phosphorus storage form in plants and is enriched in the outer layers of cereal grains and in the cotyledon of oilseed. Due to this, the two studied side stream materials, rice bran and rapeseed press cake, contain considerable amounts of phytic acid.
Interest in phytic acid from both technological and nutritional perspectives derives from its chemical nature. Phytic acid carries a net negative charge at pH values found in food and the human digestive tract. It strongly chelates cations and other positively charged compounds such as proteins below their pI. Both mechanisms are relevant for technofunctional properties but in humans, only the adverse nutritional implications related to mineral bioavailability have been established.
The first two studies of this thesis focused on the techno-functional properties of protein-enriched rice bran and rapeseed press cake fractions. Phytic acid degradation by phytase improved protein solubility at acidic pH, whereas heat-induced gelation properties of the materials were altered only at alkaline pH. The resulting gels showed increased strength and water holding capacity. The phenomenon was at least partly linked to phytase-mediated liberation of cations, as supported by the observation that extrinsic cation addition resulted in similar techno-functional changes.The release of cations was hypothesized to contribute to enhanced gel-forming ability, particularly within the soluble DF fraction.
The third study of this thesis aimed at elucidating protein nutritional quality of the rice bran and rapeseed press cake fractions in relation to phytic acid. No clear impact of phytic acid on protein digestibility in vitro was established. This aligns with earlier studies showing inconsistent results across raw materials and analytical approaches, with phytic acid generally exhibiting at most a minor impact on protein digestibility.
Taken together, the findings of this thesis indicate that protein-enriched rice bran and rapeseed press cake fractions are potential structure-forming ingredients for food applications. Despite differences in raw material origin and processing history, both side stream materials exhibited similar responses to phytase treatment with respect to techno-functional properties. Leveraging this mechanism through a feasible enzymatic treatment provides a novel and practical tool for tailoring the functional properties of plant-based side stream ingredients. In parallel, phytase treatment is expected to improve mineral bioavailability in the materials and in food matrices incorporating these ingredients, indicating a dual advantage of phytase in food manufacturing.