Storage Stability of Dietary Nitrate and Phenolic Compounds in Beetroot (Beta vulgaris) and Arugula (Eruca sativa) Juices

Nitrate and polyphenols from the diet may enhance the production and bioavailability of nitric oxide, a radical signaling molecule critical for cardiovascular health. Understanding the stability of these bioactives in beetroot and arugula juices is important for their functions. In this study, the stability of nitrate and phenolics in beetroot and arugula juices was measured for 32 days at different temperatures (25, 4, -20, and -80 degrees C). The levels of nitrate were measured by reversed-phase HPLC and initial levels were found to be 4965.34 +/- 72.69 mu g/mL for beetroot and 6310.20 +/- 24.79 mu g/mL for arugula. Interestingly, nitrate degradation started within 24 hr at 25 degrees C and after 4 days at 4 degrees C. At-20 degrees C and -80 degrees C, nitrate levels remained stable for one month. Total phenolics and free radical scavenging activity varied significantly during storage conditions. Beetroot juice at 25 degrees C, significant decrease in total phenolics and antioxidant activity was observed, whereas at 4, -20 and -80 degrees C, the levels remained relatively stable. By contrast, arugula juice at 25 and 4 degrees C, an increase in total phenolics and antioxidant activity were observed after one month. Furthermore, UPLC-HR-QTOF-MS analysis demonstrated that flavonoid glucosides were converted to their aglycones and lower phenolics, resulting in higher total phenolics and antioxidant activity during storage. In conclusion, beetroot and arugula juices required frozen conditions for long-term storage to prevent degradation of nitrate and to maintain their nutritional value.