Glucosinolate content and myrosinase activity evolution in three maca (Lepidium meyenii Walp.) ecotypes during preharvest, harvest and postharvest drying
Research highlights
► Glucosinolate (GL) profiles, contents and myrosinase activity are presented. ► Six different GLs that increased during 90 days before harvest were identified. ► During post-harvest drying, a significant increase in GLs was observed. ► Total GLs losses of 20–52% were observed during post-harvest drying. ► Traditional post-harvest drying should be revised to obtain higher GL content.
Introduction
Maca (Lepidium meyenii Walp.) is a traditional Andean crop belonging to the Brassicaceae family that grows at altitudes between 3500 and 4500 m above sea level in the central Andean region of Peru. This area is characterised by barren and rocky terrains with intense sunlight, strong winds and freezing temperatures. Few crops belonging to the Solanaceae family can survive in such harsh conditions (Flores, Walker, Guimarăes, Bsid, & Vivanco, 2003).
The nutritional value of this crop has been largely studied (Dini et al., 1994, Valentová et al., 2006) and stimulant effects have been observed. In the Peruvian Andes, folklore use of this crop includes treatment of infertility, increase of mental and physical energy and treatment of menopause. Maca meal nutritional properties and apparent effects on reproductive and sexual performance in rats and humans have been extensively documented (Gonzales et al., 2001, Gonzales et al., 2001, Gonzales et al., 2004). However, studies related to the improvement of sexual performance are still controversial (Wang, Wang, McNeil, & Harvey, 2007). Maca, in addition, showed anti-proliferative functions and slowed down the prostate weight increase induced by testosterone treatment (Gonzales et al., 2005, Gonzales et al., 2006). Rats that were orally administered red maca showed beneficial effects in the treatment of prostatic hyperplasia (BPH) experimentally induced by testosterone (Gasco, Villegas, Yucra, Rubio, & Gonzales, 2007). Proposed mechanisms for this anti-proliferative function included its ability to scavenge free radicals and cytoprotection under oxidative stress conditions (Lee et al., 2005, Sandoval et al., 2002). The presence of alkaloids and sterols in maca may also have contributed to its proposed anti-cancer activity (Wang et al., 2007) as well as its glucosinolates and derivatives (Fahey, Zalcmann, & Talalay, 2001). The presence of phytosterols and other secondary metabolites in maca are related to its anti-postmenopausal osteoporosis function (Wang et al., 2007).
Glucosinolates (GLs) are the most important secondary metabolites in maca (Jones, 1981) being most of them of the aromatic type (Dini et al., 2002, Flores et al., 2003, Li et al., 2001), of which glucotropaeolin is the most abundant. According to Clément et al. (2009) 80–90% of the total GLs in maca corresponded to glucotropaeolin. Maca GLs are present in different organs of the plant (e.g., seeds, sprouts). GLs in maca vary in content and type depending on plant age and hypocotyl colour (Clément et al., 2010). Glucosinolate content in fresh maca is about 1%, which is about 100 times more than that found in cruciferous crops, such as cabbage, cauliflower and broccoli (Li et al., 2001). The highest total glucosinolate content in maca is found in fresh hypocotyls, followed by seeds, sprouts, dried hypocotyls and fresh leaves (Li et al., 2001). Myrosinase is involved in the breakdown of GLs into isothiocyanate, thiocyanate and nitriles (Fahey et al., 2001). In intact cells, GLs and myrosinase are in different compartments. Thus, cell damage favours GL breakdown by myrosinase. The effects of variety, cultivation practices, harvest time, climate and processing on GL content have been extensively documented (Kushad et al., 1999, Mrkic et al., 2009, Rosa et al., 1996, Verkerk et al., 2001). However, to our knowledge, there are no studies related to the GL content and fate of maca during post-harvest drying.
The functional and nutraceutical potential of maca has resulted in a significant increase in its production, resulting from a sustainable increase in exports (from 189 to 602 Tm between 1999 and 2009). The main export products are: maca flour (dried and milled), gelatinised maca flour (dried, extruded and milled), plain maca or encapsulated and hydroalcoholic extracts. The main export destinations are USA and Japan (Aliaga & Espinoza, 2007).
Regardless of the maca product, post-harvest drying of maca is carried out in an artisanal way in the places of production, before other types of processing (extrusion, milling, etc.) are applied. Briefly, environmental drying is carried out for approximately 90 days in extreme temperature conditions ranging from −10 to 15 °C. These extreme environmental conditions together with the handling procedures during harvest and post-harvest can have a significant effect on the final glucosinolate content in maca.
Thus, the main objective of this work was to understand the effect of the series of events happening during pre-harvest, harvest and post-harvest drying of three maca ecotypes on glucosinolate profile, content and myrosinase activity. Understanding the events of GL breakdown in maca during pre-harvest, harvest and post-harvest drying would suggest better alternatives to reduce GL losses and result in maca products with a higher added value.
Section snippets
Plant material
Three different maca ecotypes (yellow, red and black) were selected from a commercial plantation. Cultivation was carried out in November in the community of San Pedro de Cajas at 4200 m above sea level (Junín, Perú). Maca tubers were harvested in July. Random samples were taken as follows: (a) samples from 10 plants were taken (∼2 kg) 90, 45, 30 and 15 days before harvest time, (b) at harvest and (c) after 15, 30, 45, 60, 75 and 90 days of post-harvest drying. During post-harvest handling,
Identification and quantification of GLs at harvest in three maca ecotypes
Six different glucosinolates were identified in the three coloured maca ecotypes (Fig. 1). These glucosinolates corresponded to three aromatic: 4-hydroxybenzyl (glucosinalbin), benzyl (glucotropaeolin) and 3-methoxybenzyl (glucolimnanthin); one aliphatic: 5-methylsulfinylpentyl (glucoalyssin) and two indolic: 4-hydroxy-3-indolylmethyl (4-hydroxyglucobrassicin) and 4-methoxy-3-indolylmethyl (4-methoxyglucobrassicin) (Table 1). The presence of glucoalyssin was confirmed with the white cauliflower
Conclusions
Maca is an important source of glucosinolates mainly of the aromatic type (glucotropaeolin). In total, six glucosinolates were identified in the yellow, red and black ecotypes studied. These glucosinolates corresponded to 5-methylsulfinylpentyl, 4-hydroxybenzyl, benzyl, 3-methoxybenzyl, 4-hydroxy-3-indolylmethyl and 4-methoxy-3-indolylmethyl.
The glucosinolate profile and content for the three analysed ecotypes were similar. They gradually and significantly increased 90 days before harvest.
Acknowledgements
The authors are thankful to Ecoandino s.a.c for the maca material. This research was supported by the Consejo Nacional de Ciencia y Tecnología (CONCYTEC), Peru.
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Cited by (0)
- 1
Present address: Facultad de Ingenieria en Industrias Alimentarias de la Universidad Nacional del Centro del Perú, Huancayo, Peru.
- 2
Present address: Food Safety and Quality Unit, Institute for Reference Materials and Measurements, Joint Research Centre, European Commission, Geel, Belgium.