Finding alternative uses for Australian rosella (Hibiscus sabdariffa) byproducts: Nutritional potential and in vitro digestibility studies

Context: Rosella (Hibiscus sabdariffa L.) is a relatively new crop in Australia, with only a few commercial producers. The calyces are harvested for human consumption, with the seed pods often discarded as a waste byproduct. However, this material may have potential as a livestock feed supplement or as a value-added product in the food processing industry. Aims: This study aimed to investigate the nutritional potential of rosella pods for use as a feed supplement, including an in vitro assessment of their digestibility in beef cattle. Methods: Samples of rosella plants were separated into leaves, pods and calyces, then freeze-dried and ground. These products were measured in duplicate for their antioxidant capacity, as well as protein, metal, phenolic and anthocyanin content. Samples of the dried rosella pods (n = 3) underwent in vitro digestibility analysis using rumen/pepsin digestions and were compared with samples of commercial feedstock (n = 3). The resulting simulated gastrointestinal fluids from these digestions were tested for antioxidant capacity and phenolic content. Key results: The highest antioxidant and phenolic contents of plant parts were observed in the pods (14 300 mg Trolox equivalents and 2960 mg gallic acid equivalents 100 g-1, respectively), highlighting their potential for human or animal nutrition. The pods also contained moderate to high levels of minerals including calcium, potassium, sodium, magnesium, copper, iron, manganese and zinc. Although the in vitro dry matter digestibility was lower (P < 0.001) than that of a conventional grain feedstock, the rosella pod substrate released significantly higher (P = 0.02) levels of antioxidants upon digestion than the commercial feedstock. Conclusion: Although rosella pod digestibility was lower than that of a conventional grain feedstock, it was comparable to mean values for Australian hays and silages. Given the higher levels of antioxidants released during digestion, rosella waste product has potential to be used in small amounts as a feed supplement, with the added benefit of enhancing free radical quenching ability. Implications: This study provides evidence for further investigation of the use of rosella waste as an animal feed, which would improve sustainability in agriculture as well as potentially improve animal health