Investigating the effect of in vitro gastrointestinal digestion on the stability, bioaccessibility, and biological activities of baobab (Adansonia digitata) fruit polyphenolics

Abstract

Baobab (Adansonia digitata) fruit had received growing attention for its myriad nutritional and medicinal benefits, including those from its polyphenol-rich profile and powerful antioxidant activity. The current study evaluated the bioaccessibility of phenolic constituents and antioxidant capacity of baobab fruit pulp (BFP) and its byproduct, the baobab fruit shell (BFS), upon in vitro digestion. In general, the in vitro digestion reduced phenolic contents and antioxidant capacity; however, several flavonoids, particularly quercetin, proanthocyanidin, proanthocyanidins B₁ and B₂ were highly bioaccessible. Specifically, a significant increase in the bioaccessibility of proanthocyanidins (173%) in BFS was observed following gastric digestion, possibly due to hydrolysis of proanthocyanidin isomers. Moreover, a significantly higher bioaccessibility of proanthocyanidin B₂ (170%) and quercetin (304%) in BFP, and proanthocyanidin (363%) in BFS was also observed following intestinal digestion probably due to pancreatin effect on the complex food matrix or the depolymerisation of insoluble proanthocyanidin and quercetin conjugates induced by the increase in pH. A considerable α-amylase and α-glucosidase inhibition in all samples (>50% inhibition) were observed following the in vitro digestion. Hence, both BFP and BFS are good sources of bioaccessible polyphenolics that could be utilised as ingredients in functional foods.

Introduction

In recent times, there has been an increase in awareness regarding the importance of diet thanks to the rising incidences of chronic diseases such as diabetes mellitus, cancer, and cardiovascular diseases, particularly in developed countries (Yahia, 2010). Several studies have shown the negative correlation between the consumption of phytochemical-rich plant foods and the occurrence of the aforementioned diseases (Liao, Greenspan, & Pegg, 2019; Williams et al., 2013).

The baobab (Adansonia digitata) is Africa's majestic tree revered by the indigenous people where every part of it is reported to be useful either as a source of food, medicine, or means of livelihood. The baobab fruit, commonly named “monkey's bread,” has an oblong-cylindrical shape and comprises black seeds embedded in a chalky white pulp (Ismail, Pu, Guo, Ma, & Liu, 2019a). More than 300 nutritional and medicinal uses of baobab fruit and its parts have been well documented in numerous African countries (Kamatou, Vermaak, & Viljoen, 2011; Rahul et al., 2015). Hence, this led to a dramatic increase in its demand by the food processing industries throughout the globe. The baobab fruit pulp (BPF) was accepted as a novel food in Europe since 2008 and had achieved the GRAS (generally recognised as safe) status in 2009 (Li et al., 2017). BFP is commonly used in various food formulations and for making yoghurts, fruit juices, and sauces. Moreover, its production yield by-products comprised the baobab fruit shell (BFS) rich in phenolic compounds. The BFS that hitherto was treated as waste with no economic value has recently been discovered to contain significant amounts of invaluable compounds with the potential of being processed into value-added products or as functional food ingredients (Ismail et al., 2019b).

BFP has been regarded as an excellent source of phenolic antioxidants of pharmacological importance, including gallic acid, quercetin, rutin, catechin, proanthocyanidins, etc (Braca et al., 2018; Ismail et al., 2019a, 2020; Tembo et al., 2017). Several bioactive properties, including anticancer, antimicrobial, antioxidant, antidiabetic, and anti-inflammatory activities, have been reported in BFP. Specifically, the antioxidant capacity of BPF linked to the rich phenolic profile is higher than the values reported in many fruits, including apples, kiwis, strawberries, and oranges (Prior & Gu, 2005).

Phenolic compounds are a diverse group of plant secondary metabolites that are of interest mainly for their antioxidant potentials and the positive impact that they have on numerous chronic diseases, including diabetes, obesity, cardiovascular diseases, and neurodegenerative disorders. Moreover, the inhibitory ability of certain phenolic compounds such as quercetin, rutin and proanthocyanidins against hyperglycaemic α-amylase and α-glucosidase enzymes could potentially provide a novel therapeutic option for treating type 2 diabetes (Pereira et al., 2020).

Despite the potential benefits of phenolic compounds, their bioactive capacity depends greatly on their bioaccessible and bioavailable properties, which play a critical role in their gastrointestinal and systemic functions (Fernandes de Araújo et al., 2021). Hence, in vitro bioaccessibility studies through simulated digestion methods are useful for studying the stability, bioaccessibility, and bioactivity of phenolics under gastrointestinal conditions (Pereira et al., 2020). This method is often favoured over in vivo methods due to its myriads of advantages, including high throughput, simplicity, suitability for mechanistic studies and hypothesis building, and lacking ethical restrictions (Mariela, Susan, & Valeria, 2020; Minekus et al., 2014; Thakur et al., 2020).

In this regard, several studies have demonstrated the bioactive potentials of BFP and its by-products (Braca et al., 2018; Coe, Clegg, Armengol, & Ryan, 2013; Ismail et al., 2019a, 2019b). However, to our knowledge, there has not been any literature reporting the effects of in vitro gastrointestinal digestion on the stability, bioaccessibility, and antioxidant capacity of baobab phenolics. Hence, this study was carried out to investigate the phenolic profile and bioactive potentials of BFP and its by-products following in vitro digestion.