Effects of extrusion processing on pomegranate peel polyphenols in dry pet food

Pomegranate (Punica granatum L.) peel represents a biowaste from juice extraction, which constitutes approximately 43% w/w of the fruit. It is a source of bioactive compounds, including flavonoids, complex polysaccharides, and hydrolysable tannins, such as punicalagin (PUN), ellagic acid and derivatives (EAD), and gallic acid, with a high nutraceutical value and used as an additive or ingredient by the petfood industry, even if the biological activity of its active compounds can be influenced by the extrusion production parameters which potentially reduce the beneficial effects of the pomegranate. Aim of this study was to evaluate the effect of the extrusion process on the polyphenols (PPs) of pomegranate peel extract (PPE) in a dog food. A control formula (CO diet) was developed. PPE (Total Phenol Content/TPC: 23 g/100g; EAD: 13 g/100g; PUN: 10 g/100g) was added to the CO diet at an inclusion level of 1% (PPE diet). Foods were extruded in a single screw extruder using the same production parameters (temperature, pressure, steam). Four samples were collected for each phase at ten-minute intervals. The 1st sampling step was post-mixing, the 2nd post-grinding, the 3rd post-steam conditioning, the 4th post-expansion, the 5th post-drying, and the last step on the finished product. A total of 48 extractions, in duplicate, were analyzed for TPC using Folin-Ciocalteau assay by spectrophotometry, and for PPs by liquid chromatography. A one-way ANOVA model was performed to determine the differences between extrusion phases. At the 1st sampling step of the extrusion process, the TPC in the CO diet was 0.09 g/100g with 0.08 g/100g of EAD and 0.01 g/100g of PUN whereas, the TPC in the PPE diet was 0.17 g/100g, with 0.08 g/100g of EAD and 0.08 g/100g of PUN. At the 3rd sampling step of the extrusion process, the greatest loss of PPs was observed in the PPE diet when the TPC was 0.08 g/100g (-53%; p <0.05), with 0.07 g/100g of EAD (-12.5%) and 0.01 g/100g of PUN (-87.5%; p <0.05) while, in the CO diet, a slight loss (p >0.05) was observed for TPC 0.07 g/100g (-22%), with 0.06 g/100g of EAD (-25%) and 0.01 g/100g of PUN. In the following steps of the extrusion process, TPC, EAD and PUN remained constant in both diets. Data show that steam conditioning represents the main critical point for PPs loss. Future research should aim at improving the stability of pomegranate PPs during the production processes to ensure added value in the utilization of this organic waste.