Brassica-enriched wheat bread: Unraveling the impact of ontogeny and breadmaking on bioactive secondary plant metabolites of pak choi and kale
Introduction
While humans get older, they have to deal with an increasing variety of non-communicable, age-related diseases that are often induced by oxidative stress and inflammation (World Health Organisation, 2019). To counteract these unfavorable processes, a vegetable-rich diet is recommended (World Health Organisation, 2019). Vegetables contain a wide variety of secondary plant metabolites (SPMs), which are bioactive and can exert antioxidant and anti-inflammatory effects (Andersen and Markham, 2006, Manach et al., 2009). Unfortunately, the current overall vegetable consumption in a typical Western diet is comparatively low (World Health Organisation, 2019). Hence, a strategy to overcome this dietary problem might be to add supplementary bioactive, health-promoting SPMs to food products that are highly consumed in a Western diet such as bread (Krems, Walter, Heuer, & Hoffmann, 2013).
Brassica species, in particular, are a valuable source of health-promoting SPMs. For example, kale is known for its high lutein concentration, a carotenoid that can prevent age-related ophthalmologic disorders and that is positively associated with cancer prevention and cardiovascular health (Buscemi et al., 2018). In addition to high concentrations of carotenoids, chlorophylls, and phenolic compounds, Brassica species also contain glucosinolates (GLSs), typical to plants from the order Brassicales (Wittstock, Kurzbach, Herfurth, & Stauber, 2016). Upon cell disruption, GLSs are broken down by the enzyme myrosinase to secondary products (GLS breakdown products) such as isothiocyanates (ITCs), nitriles, and epithionitriles (EPTs) (Wittstock et al., 2016). Especially, ITCs are thought to exert antimicrobial, anti-inflammatory, and even anticancerogenic effects (Oliviero, Verkerk, & Dekker, 2018). Due to the variety of structurally different SPMs, the regular consumption of Brassica vegetables is linked to health-promoting effects, such as antioxidant, anti-inflammatory, and anticancer activities (Francisco et al., 2017). Previous studies on added ingredients to bread have involved the addition of purified substances, e.g., quercetin, or lyophilized plant powders of, e.g., quinoa leaves (Lin and Zhou, 2018, Świeca et al., 2014). Thus, adding fresh or gently processed vegetative plant tissues, for instance microgreens (developed cotyledons) or leaves of the Brassica vegetables, to bread could be a promising, innovative, health-promoting strategy to increase the consumption of bioactive SPMs in a typical Western diet. It is noteworthy that the composition and concentration of a plant’s SPMs profile, e.g., GLSs breakdown products, carotenoids, chlorophylls, and phenolic compounds, is species-specific and also highly dependent on the ontogenetic stage (Heinze et al., 2018). For example, while Brassica sprouts/microgreens form higher concentration of GLSs, young leaves contain higher levels of carotenoids, chlorophylls, and phenolic compounds (Heinze et al., 2018). We therefore hypothesized differences in the profiles of SPMs not only between pak choi and kale, but also between microgreens and leaves of both plant species. Thus, we first characterized the composition and concentration of GLS breakdown products, carotenoids, chlorophylls, and phenolic compounds in pak choi (Brassica rapa subsp. chinensis) and kale (Brassica oleracea var. sabellica) at two ontogenetic stages, namely 7 day old microgreens and 14 day old leaves, in order to reveal the optimal profile of SPMs in terms of health-promoting substances.
Besides pre-harvest factors (e.g., developmental stage), post-harvest factors (e.g., processing) also have an impact on composition and concentration of SPMs. For example, during the breadmaking process, plant tissue is exposed to both mechanical impacts and thermal processing that can affect the composition and concentration of the desired metabolites (Klopsch et al., 2018). The influence of food processing on SPMs is thus dependent on the specific kind of processing (e.g., cutting, cooking, steaming, frying, baking) as well as various other parameters such as plant genotype, ontogenetic stage, overall plant composition (‘matrix’), and the chemical structure of the specific SPM (Chaaban et al., 2017, Klopsch et al., 2018). We therefore hypothesized that fresh leafy plant material from Brassica is suitable to be used as an added ingredient for enhancing bioactive, health-promoting SPMs in a typical Western diet in food products that are regularly consumed such as bread. Thus, we also evaluated how stable the SPMs are during the breadmaking process.
Section snippets
Chemicals and reagents
Ammonium acetate, ferulic acid, kaempferol-3-O-glucoside, methanol, methylene chloride (for GLS breakdown product analysis), quercetin-3-O-glucoside, sinapic acid, and tert-butyl methyl ether were obtained from Carl Roth GmbH + Co. KG (Karlsruhe, Germany). Acetic acid, methylene chloride (for carotenoid and chlorophyll analysis) and 2-propanol were bought from Merck KGaA (Darmstadt, Germany). Acetonitrile and 3-indoleacetonitrile were obtained from J. T. Baker (Fisher Scientific GmbH,
Results and discussion
The impact of ontogeny on the composition and concentration of bioactive SPMs in two Brassica species (pak choi and kale) was investigated. For this purpose, pak choi and kale were grown under controlled conditions in a climate chamber and harvested after 7 days (microgreens) and 14 days (leaves). These plants were analyzed for their SPM composition and concentration: GLS breakdown products, carotenoids, chlorophylls, and phenolic compounds. To study how suitable pak choi and kale are for
Conclusion
Microgreens as well as leaves of both Brassica species (pak choi, kale) possess the potential to provide the consumer with a variety of health-promoting bioactive SPMs such as GLS breakdown products, carotenoids, chlorophyll metabolites, and phenolic compounds. The total concentration of lipophilic carotenoids and chlorophylls was higher in the Brassica leaves compared to their corresponding microgreens, whereas hydrophilic GLS breakdown products, hydroxycinnamic acid derivatives, and
Declaration of Competing Interest
None.
Acknowledgements
This research was developed by and embedded in the competence cluster NutriAct – Competence Cluster Nutrition Research Berlin‐Potsdam funded by the Federal Ministry of Education and Research (FKZ: 01EA1408A‐G). We would also like to thank Elke Büsch for her excellent technical support in carrying out the growth experiments.
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