Physicochemical Properties and Metabolomic Profiles of Marula Juice Extracted from Thermal and Non-Thermal Pre-Treated Ripe Marula Fruit (Sclerocarya birrea)

Abstract

Marula fruit (Sclerocarya birrea) is a seasonal, underutilised indigenous crop prevalent in South Africa, renowned for its substantial vitamin C and phytochemical content. Marula fruit is a good source of bioactive compounds which may contribute to human health. Despite that, Marula fruit juice processing results in loss of sensitive compounds in the finished product. This study evaluated the effects of thermal (blanching and steaming) and non-thermal (ultrasonication) pre-treatments of Marula fruit prior to juicing on the physicochemical properties (such as pH, soluble solids and titratable acidity) and metabolomic profile of the resulting juice. The marula fruits were blanched manually, with a handheld thermometer used to monitor the blanching water temperature. The fruit (Marula) was steamed with a locally fabricated steamer, and the fruit was also ultrasonicated with an ultrasonication machine (LABOTEC, Model 705, South Africa). The extracted juice was stored in sterilised bottles and stored at 4°C (refrigerated) and analysed within 24 h. The physicochemical properties were analysed following AOAC protocols and established methods reported in the literature. Metabolites were analysed by liquid chromatography quadrupole time-of-flight mass spectrometry (LC-qTOF-MS), followed by multivariate statistical analysis. Compound annotation of biomarkers associated with pre-treatments was performed using the in silico fragmentation predictor SIRIUS. The results showed that ultrasonication enhanced juice extraction efficiency (22.81%) compared with steaming (21.26%), blanching (20.66%), and the control (17.50%). Both ultrasonication and steaming contributed to an enhanced vitamin C content (77.79 and 81.02 mg/100g respectively) when compared to the control (64.28 mg/100g). In a similar vein, increased levels of calcium (0.37 and 0.32 mg/g) and potassium (3.98 and 3.44 mg/g) were documented in the juices extracted from the ultrasonication and steaming, respectively. The total phenolic content and antioxidant activities (ABTS and FRAP) were higher in the juices obtained by ultrasonication and steaming, with total phenolic content of 4.67 and 3.40 μg GAE/ml and ABTS inhibition rates of 98.69% and 97.73%, respectively. Ultrasonication significantly enhanced the colour characteristics, including L* (60.82), b* (14.23), ΔE (6.88), and chroma (14.57), compared with the control juice. Metabolomic analysis revealed a heterogeneous array of compounds, encompassing phenolics, flavonoids, vitamins, esters, and amides, characterised by an elevated relative abundance subsequent to ultrasonication. Results showed that ultrasonication enhanced the release of bioactive compounds and preserved physicochemical properties presumably through acoustic cavitation. Conversely, thermal treatments predominantly led to reduced polyphenol concentrations, reduced antioxidant activity, and metabolite levels. While steaming yielded higher levels of vitamin C, total soluble solids, and titratable acidity, ultrasonication significantly increased overall juice yield. Metabolites were mostly detected under ultrasonicated Marula fruit juice as compared to thermal treated juice. The results may be attributed to limited thermal degradation on ultrasonication treatment. In conclusion, ultrasonication improved the efficiency of juice extraction while maintaining the nutritional integrity, antioxidant capacity, and metabolomic diversity of Marula fruit juice. These results underscore the potential of ultrasonication as an innovative processing technique. While demonstrated here at bench scale, further validation might be needed before industrial application.