Impact of cooking and fermentation by lactic acid bacteria on phenolic profile of quinoa and buckwheat seeds

Highlights

• Quinoa and buckwheat cooked seeds were fermented with lactic acid bacteria.

• Autochthonous Lactobacillus paracasei and Pediococcus pentosaceus were selected.

• An increase of total phenolics and antioxidant potential was observed after cooking.

• Lactobacillales fermentation increased phenolic acids and tyrosols.

• Flavonoids were the best discriminants between different processing.

Abstract

In this work, quinoa and buckwheat cooked seeds were fermented by two autochthonous strains of lactic acid bacteria isolated from the corresponding seeds, namely Lactobacillus paracasei A1 2.6 and Pediococcus pentosaceus GS·B, with lactic acid chemically acidified seeds as control. The impact of cooking and fermentation on the comprehensive phenolic profile of quinoa and buckwheat seeds was evaluated through untargeted ultra-high-pressure liquid chromatography coupled to quadrupole-time-of-flight mass spectrometry (UHPLC-QTOF-MS). Samples were analyzed also for in vitro antioxidant capacity (as FRAP and ORAC assays) and total phenolic content (TPC). The in vitro spectrophotometric assays highlighted that the microbial fermentation was more efficient in increasing (p < .05) the TPC and in vitro antioxidant potential in quinoa cooked seeds. However, an increase (p < .05) in TPC and ORAC radical scavenging was observed in both pseudocereals after the different cooking processes (i.e., boiling or toasting).

The untargeted phenolic profiling depicted the comprehensive phenolic composition in these matrices. Raw seeds of both pseudocereals possessed a similar phenolic content (4.4 g kg⁻¹ equivalents; considering free and bound fractions). Besides, the metabolomics-based approach showed that all treatments (i.e., cooking and fermentation) induced the release of specific classes, namely phenolic acids and tyrosols. The PLS-DA multivariate approach identified in flavonoids the best markers allowing to discriminate the different treatments considered (i.e., cooking, chemical acidification and microbial fermentation). These findings support the use of cooking and microbial fermentation to ensure the health-promoting properties of non-wheat grains, such as buckwheat and quinoa.

Introduction

In recent years, the exploitation of quinoa and buckwheat seeds in human nutrition is gaining a strong interest. This trend could be linked to the excellent nutrient profile and chemical composition of these pseudocereals (Alvarez-Jubete, Arendt, & Gallagher, 2010). To date, different studies have reported that common gluten-free (GF) foods are lower in nutritional quality when compared with gluten-containing counterparts (Padalino, Mastromatteo, Lecce, Cozzolino, and Del Nobile, 2013). Therefore, quinoa and buckwheat flours, that are naturally GF, could be valid alternative ingredients in GF diet for people suffering celiac disease and gluten-related disorders (Kupper, 2005).

Other than the nutritional quality and composition, these seeds are also rich in several bioactive compounds such as saponins, phytosterols, fagopyritols, and polyphenols (Wijngaard & Arendt, 2006; Alvarez-Jubete et al., 2010). In particular, polyphenols can be divided in five main classes, namely flavonoids, phenolic acids, lignans, stilbenes and other phenolics (including tyrosols and alkylphenols). These compounds possess different molecular weight and chemical structure and are widespread into vegetables as free and/or bound forms (González Aguilar, Blancas Benitez, & Sayago-Ayerdi, 2017). From a nutritional standpoint, bound phenolics are particularly interesting due to their ability to establish interactions with the food matrix, then being released into the small or large intestine, where they become available for bacterial microflora and promote an antioxidant environment (Rocchetti et al., 2018). Buckwheat is one of the best grain sources of polyphenols, being rich in flavonoids such as glycosidic forms of quercetin (a flavonol) followed by glycosidic forms of apigenin and luteolin (two of the main flavones) and phenolic acids (Alvarez-Jubete et al., 2010). Quinoa seeds are also abundant in phenolics such as flavonoids (i.e., glycosidic forms of the flavonols kaempferol and quercetin) and phenolic acids (Alvarez-Jubete et al., 2010; Rocchetti et al., 2017).

Buckwheat is usually eaten as a cooked grain, porridge or baked into pancakes in many countries (Moroni et al., 2012), while quinoa grains are commonly used to produce bread (Bender & Bender, 1999). Besides, alternative non-wheat grains and flours are commonly used to prepare fermented products featured by an improvement in both nutritional properties and shelf-life. For example, it has been shown that the fermentation of pseudocereal flours promoted by lactic acid bacteria (LAB) could improve both the sensory and baking quality, providing in this way food with attractive flavour and texture (Hur et al., 2014). Therefore, the improvement of fermentation technology on these matrices appears to be a very good opportunity to formulate novel functional and healthy foods (Coda, Di Cagno, Gobbetti, and Rizzello, 2014). The selection of adequate starter cultures for seeds' fermentation requires strains that are highly adapted to the cereal substrate (Sekwati-Monang, Valcheva, Ganzle, 2012). However, the choice of starter cultures has a critical impact on the final quality, with the characterization of LAB representing a crucial step in order to improve the final product from both nutritional and technological point of views (Coda et al., 2014). It is well known that fermentation by sourdough LAB is able to enrich the food matrix with organic acids, indirectly stimulating the hydrolysis of proteins through the production of endogenous proteases (Gobbetti et al. 2014). Therefore, the final fermented matrix is characterized by a better nutritional profile thanks to both the release of small peptides, amino acids, and bioactive compounds, and to the reduction of anti-nutritional factors (Gobbetti et al. 2014; Rizzello, Di Cagno, Monemurro & Gobbetti, 2016). To date, few studies have identified the specific contribution of cereal/pseudocereal enzymes and defined starter culture on the conversion of free and bound phenolics during fermentation processes (Gänzle, 2014). A presumable mechanism for fermentation-induced enhancement of phenolic and antioxidant activities in cereal grains is related to degrading enzymes present in secretes of fermenting microbes, which results in structural breakdown of cell wall matrix and increased accessibility of bound/conjugated phenolic compounds to enzymatic attack (Dordević, Siler-Marinković, & Dimitrijević-Branković, 2010). In this regard, LAB characterizing cereal grains are able to promote the enzymatic conversion of phenolic compounds, mainly by means of five class of enzymes, namely glycosyl hydrolases, esterases, tannin hydrolases, reductases and decarboxylases (Gänzle, 2014).

However, to the best of our knowledge, a comprehensive evaluation of the effect of fermentative activity promoted by autochthonous LAB on the entire phenolic and in vitro antioxidant profile of quinoa and buckwheat cooked seeds is still scarce (Carciochi, Galvan-D'Alessandro, Vandendriessche & Chollet, 2016). Therefore, in this work, a metabolomics-based approach (UHPLC-ESI-QTOF mass spectrometry) was used for the first time to investigate the impact of both cooking and microbial fermentation on the phenolic composition of quinoa and buckwheat seeds.