Effect of management practices on the composition of nutritionally relevant compounds and sensory quality of crops
Apple, onion, cabbage, lettuce, potato, and wheat are analysed for secondary metabolites that may have health beneficial effects and importance for the sensory quality of fruit and vegetables
By Eduardo A.S. Rosa, Richard N. Bennett and Alfredo Aires, UTAD Research
It is known that vegetable crops contain particular secondary metabolites, which have shown beneficial pharmacological properties such as anti-neoplasia, antioxidant, anti-allergic effects, platelet aggregation inhibition, interference with cancer promotion mechanisms and probiotic activity.
These compounds are therefore thought to be involved in the well-documented beneficial effects of vegetable consumption on health. Many of these compounds are also important for the sensory quality of fruit and vegetables, in particular the secondary metabolites, which define characteristic flavours, aromas and colours of each product.
Nutritional composition and deleterious components
Cereals and vegetables also contribute significantly to the intake of a range of undesired compounds, which are considered hazardous if consumed in too high quantities. This includes nitrate (> 60 percent of which are taken up with cereals and vegetables), heavy metals (e.g. lead and cadmium), pesticide residues and mycotoxins. Some secondary metabolites are also known to function as toxicants when they occur in too high concentrations, e.g. glycoalkaloids, glucosinolates and polyacetylenes.
Effects of organic vs. conventional farming on nutritional and sensory aspects
There is considerable sample-to-sample variation in the content of many of the compounds mentioned above. Several hypotheses have been published on how the production system, in particular organic farming as compared with conventional farming, could affect nutritional value and/or other qualities.
Several studies and reviews indicate systematic differences in sensory and technical quality, and in concentrations of minerals, essential amino acids, vitamins, secondary metabolites and mycotoxins in crops produced from different production systems (conventional, 'low input' and/or organic) or specific components (e.g. fertility practices) of such systems. Other studies report no significant differences or inconsistent results.
From the currently available information, it is therefore not possible to conclude to what extent variation in production system or in specific components of production systems may affect the content of nutritionally relevant compounds in crops.
In WP2.1.6 all selected crops will be analysed for contents of relevant minerals, vitamins and pesticide residues. These crops include:
In addition, each crop will be analysed for characteristic secondary metabolites and/or mycotoxins with known or suspected relevance for health and/or sensory quality.
Based on the results from the chemical analysis, relevant treatments will be selected for evaluation of sensory/technical quality, consisting of a sensory evaluation of fresh (where appropriate) and/or one or two processed typical foods made from the crop (WP2.1.6). Additionally, for wheat and potato, material from the selected treatments will be compared in a rat preference test (WP2.1.6).
Update on completed and ongoing research activities
1. Apple (Malus domestica L.) Fruits
All apple samples from 2004 harvest have been extracted and run on the HPLC. All samples have the typical phytochemical profile for apple fruits i.e. chlorogenic acid, flavan-3-ols (catechin, epicatechin, and oligo-/polymeric procyanidins), flavonols (quercetin 3-O-Ara, 3-O-Xyl, 3-O-Rha, 3-O-Gal, 3-O-Glc and traces of rutin), anthocyanins (predominantly cyanidin 3-O-Gal) and phloretin derivatives (phloridzin and phloridzin xyloside).
2. Onion (Allium cepa L.) Bulbs
All the onion samples have been extracted and the majority analysed by HPLC. The samples have the characteristic flavonol profile as reported by many researchers i.e. predominantly quercetin 4′-O-monoglucoside > quercetin 3, 4′-di-O-glucoside >>> minor quercetin and isorhamnetin mono- and di-O-glucosides.
3. Cabbage (Brassica oleracea var. capitata) Heads
All the cabbage samples have been run. However, there were clearly problems with sample preparation and the levels of glucosinolates and phenolics in all samples were extremely low compared with a control commercial white cabbage sample that was freeze-dried and processed at UTAD.
4. Lettuce (Lactuca sativa L.) Leaves
All of the lettuce samples have been extracted and analysed. They had the characteristic phenolic profiles as reported in the literature i.e. a mix of mono-O- and di-O-caffeoyl quinic acids (chlorogenic acids), mono-O- and di-O-caffeoyl tartaric acids (caftaric acids), and various flavonol glucosides, malonylglucosides, and glucuronides. The chlorogenic and caftaric acids were predominant in the majority of samples.
5. Potato (Solanum tuberosum L.) Tubers
All the potato samples have been extracted and run on the HPLC. They have the characteristic phenolic profile as reported in the literature i.e. chlorogenic acid is predominant with smaller amounts of free p-coumaric, caffeic, and ferulic acids.
6. Wheat (Triticum aestivum L.) Leaves
The majority of the wheat samples have been extracted and analysed by HPLC. They have a complex profile of phenolics and flavonoids (see Future Research Activities).
7. General Information
A large number of the samples have not been appropriately processed, and therefore there will be changes in the phtyochemical profiles; the biggest problem will be with the cabbage samples. Samples received at UTAD required additional freeze-drying to improve the quality of the samples prior to powdering and extraction. This means that for certain sets of data it will be necessary to consider the quality of the results very carefully.
For phytochemical quantification a number of new calibration curves will be run on the new LC-MS to ensure accurate quantification of the phytochemicals. Where possible pure commercial standards have been obtained for this purpose, in other cases phytochemicals will be expressed as “equivalents” of the structurally most closely related standard e.g. the flavonols will be expressed as quercetin 3-O-Gal equivalents.
Future Research Activities
1. Extraction and Analyses for Identification and Quantification of Other Phytochemicals
There are a few samples from 2005 that are currently being run to complete the analyses of recently received samples. This includes the wheat leaf samples, that have been found to have a complex phenolic/flavonoid profile (chlorogenic acids, C-glycosylflavones and some minor flavonols) and selected samples will need to be analysed by LC-MS.
From older and more recent literature (some profiling of wheat leaves in the 1970s and this year an LC-MS analysis of leaves/stem tissue of Triticum durum L.) the complex profile is typical of wheat leaves. There are also the Allium non-protein amino acid (S-alk(en)yl-L-Cysteines) flavour precursors, and the Allium phytoalexins (Tsibulins 1d and 2d) to be analysed.
2. Confirmation of Phytochemical Identities by LC-MSn
With the installation of a new LC-MSn (Ion-Trap MS) system in UTAD some samples can be re-evaluated. Selected samples from each of the foods will be analysed using LC-MSn methods in positive ion and negative ion modes.
Although 90 percent of the major phytochemcials in each food have been identified by retention time and UV-spectral data in relation to standards, it is sensible to re-run some samples by LC-MSn to confirm the major phytochemicals and to obtain a more global/metabolomic view of the phytochemicals and especially those that are UV opaque e.g. some amino acids and terpenoids.