Polyphenol composition and in vitro antioxidant activity of amaranth, quinoa buckwheat and wheat as affected by sprouting and baking
Introduction
The pseudocereals amaranth, quinoa and buckwheat have attracted much interest in recent years. One of the reasons for this renewed interest is their excellent nutrient profile. In addition to being one of the important energy sources due to their starch content, these pseudocereals provide good quality protein, dietary fibre and lipids rich in unsaturated fats (Alvarez-Jubete, Arendt, & Gallagher, in press). Moreover, they contain adequate levels of important micronutrients such as minerals and vitamins and significant amounts of other bioactive components such as saponins, phytosterols, squalene, fagopyritols and polyphenols (Berghofer and Schoenlechner, 2002, Taylor and Parker, 2002, Wijngaard and Arendt, 2006). In addition, amaranth, quinoa and buckwheat seeds are also naturally gluten-free and thus, they are currently emerging as healthy alternatives to gluten-containing grains in the gluten-free diet (Kupper, 2005).
Much research has been conducted over the past ten years on the polyphenol composition of foods, and also on polyphenol bioavailability, metabolism and biological effects (Manach, Scalbert, Morand, Remesy, & Jimenez, 2004). The increased interest in polyphenols in the past decade has been brought about by results from epidemiological studies linking the consumption of diets rich in plant foods with decreased risk of diseases associated with oxidative stress, such as cancer and cardiovascular disease (Scalbert, Manach, Morand, Remesy, & Jimenez, 2005).
Polyphenols are secondary plant metabolites that play a role in the protection of plants against ultraviolet radiation, pathogens and herbivores (Harborne & Williams, 2000). Several hundred molecules with polyphenol structure (i.e., benzene rings with one or more hydroxyl groups) have been identified in edible plants (Manach et al., 2004). Fruit and beverages, such as tea, red wine, and coffee, are the main sources of polyphenols, however, vegetables, cereals and leguminous plants are also good sources (Manach et al., 2004). Current estimated intake of polyphenols is >100 mg d−1. This is in contrast with the intake of important vitamins such as vitamin E (8.5 mg d−1), vitamin C (80 mg d−1) and β-carotene (1.9 mg d−1). This suggests that these compounds represent an important part of the total dietary intake of biologically active compounds (Hooper & Cassidy, 2006).
Published data on the antioxidant properties and polyphenol composition of the pseudocereals amaranth and quinoa is limited, however, substantially more has been reported on the antioxidant properties of buckwheat. However, to properly assess their relevance as potential sources of dietary antioxidants, information on the impact of processing on these compounds is also essential, as they are generally processed before consumption.
Processing can modify the polyphenol content of foods in several ways (Manach et al., 2004). In particular, sprouting has been reported as a means of increasing the polyphenol content of buckwheat seeds (Kim, Kim, & Park, 2004). Conversely, studies have shown that thermal processing of fruits, vegetables and cereals can have a detrimental effect on flavonoid compounds (Dietrych-Szostak & Oleszek, 1999). The extent of which flavonoid loss was due to processing has been shown to be highly dependent on factors such as the type of substrate and the processing conditions, mainly the length and temperature of the process (Sensoy, Rosen, Ho, & Karwe, 2006). For example, in a recent study, extrusion did not affect the antioxidant activity of buckwheat, whereas roasting caused a slight decrease in antioxidant activity (Sensoy et al., 2006). Thermal processing of cereals, such as baking, can also result in the synthesis of substances with antioxidant properties, such as some Maillard reaction products in bread crust (Lindenmeier & Hofmann, 2004).
Baking still represents one of the most common ways of processing cereals; however, to date no studies have been published on the impact of baking on the antioxidant properties and polyphenol composition of amaranth, quinoa and buckwheat. Identifying foods rich in antioxidants, as well as processing methods for preserving or enhancing these high levels may ultimately result in the manufacture of food products rich in these compounds, with their associated potential health protective properties.
In a previous study (Alvarez-Jubete et al., in press), the use of amaranth, quinoa and buckwheat as potential healthy ingredients for improving the nutrient content of gluten-free breads was evaluated. Results showed that these pseudocereals represent feasible ingredients in the manufacture of nutrient-rich gluten-free products. The nutritional benefits of pseudocereals were also highlighted, not only for celiac patients, but also for the general population. In the present study, another aspect of amaranth, quinoa and buckwheat seeds nutritional quality is examined: their in vitro antioxidant properties and polyphenol composition, and how these properties are affected by the sprouting and baking processes.
The objectives of the present study were the following:
- (1)
To determine the antioxidant capacity, total phenol content and polyphenol composition of amaranth, quinoa, buckwheat and wheat seeds.
- (2)
To determine the effects of sprouting and breadmaking on these properties.
- (3)
To aid in contributing to the formulation of nutritionally enhanced gluten-free breads.
Section snippets
Seed materials
Amaranth seeds (Amarantus caudatus, harvested in Peru) and quinoa seeds (Chenopodium quinoa, grown in Bolivia) were obtained from Ziegler & Co., Germany. Buckwheat seeds (Fagopyrum esculentum Möench, grown in China) were sourced by Munster Wholefoods, Ireland. Wheat grains (Triticum aestivum L.), variety Raffles, grown in Ireland were provided by Gold Crop, Ireland.
All seeds were delivered cleaned from dust and any other contaminants.
Quinoa seeds were pre-processed by the manufacturer to
Total phenols and antioxidant capacity of the seeds
The results for the total phenol content assay (by FCR) and antioxidant capacity assays (DPPH and FRAP assays) of amaranth, quinoa, buckwheat and wheat seeds are presented in Table 2.
Total phenol content among the pseudocereal seed methanolic extracts differed greatly and was highest in buckwheat (323.4 mgGAE/100 g), followed by quinoa and amaranth (p < 0.01). The total phenol content of wheat was significantly higher than amaranth, lower than buckwheat (p < 0.01), and statistically not different
Conclusions
The total phenol content and antioxidant activity amongst the pseudocereal seed extracts was highest in buckwheat (p < 0.01) and decreased in the order buckwheat > quinoa > amaranth. Analysis by using HPLC–DAD showed that quinoa and buckwheat represent the best sources of polyphenols among the studied seeds and consist predominantly of quercetin and kaempferol glycosides in quinoa, and catechin and quercetin glycosides in buckwheat.
Sprouting resulted in an overall increase in the total phenol content
Acknowledgements
The authors would like to thank Mr. Florian Hubner for carrying out the sprouting of the seeds. This study is financially supported by Enterprise Ireland.
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