title / Mineral and nutritional content of produce
/ DEFRA
project code / HH3720SX
Department for Environment, Food and Rural Affairs CSG 15
Research and Development
Final Project Report
(Not to be used for LINK projects)
Two hard copies of this form should be returned to:Research Policy and International Division, Final Reports Unit
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Project title / Mineral and nutritional content of produce
DEFRA project code / HH3720SX
Contractor organisation and location / Horticulture Research International
Wellesbourne
Warwick CV35 9EF
Total DEFRA project costs / £ 88,030
Project start date / 01/06/03 / Project end date / 30/03/04
Executive summary (maximum 2 sides A4)
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CSG 15 (Rev. 6/02) 3
Projecttitle / Mineral and nutritional content of produce
/ DEFRA
project code / HH3720SX
This contract comprised two distinct one-year projects. The first project assessed the evidence for the statement that the mineral content of edible horticultural and potato crops is declining, and the second project determined ways to enhance the gamma-linolenic acid (GLA) content of blackcurrant seeds.
Project 1. Various authors have stated explicitly that the mineral concentrations in vegetables and fruit consumed in the UK have decreased since the 1930s. Here we address their claim through a desk-based study. First, we observe that insufficient data exist in the primary literature to test the hypothesis that the mineral concentrations in any individual vegetable, fruit or nut available in the UK has declined since the 1930s. However, values for the mineral concentrations in "produce in general" can be compared across this period, with an estimate of variation being derived from the changes in mineral content of many different crops. Thus, we tested the null hypothesis that the mineral concentrations in horticultural produce available in the UK had not declined between the 1930s and the 1980s. We conclude that the average concentrations of Cu, Mg and Na in vegetables and the average concentrations of Cu, Fe and K in fruit available in the UK have decreased significantly between the 1930s and the 1980s. The same hypothesis was tested with comparable data from the USA, whose historical horticultural and consumer practices have paralleled those of the UK. Data from the USA confirmed that the average Cu concentrations in the dry matter of vegetables and that the average concentrations of Cu, Fe and K in fruit had decreased significantly since the 1930s. To halt the decline in the mineral concentrations of horticultural crops, research towards developing appropriate genotypes and improved cultural practices is recommended.
Project 2: Gamma-linolenic acid (GLA) is a naturally occurring fatty acid found in the seed oils of borage, evening primrose and blackcurrant. GLA is nutritionally very important to humans and has been shown to have a number of therapeutic applications. However, its use in the pharmaceutical industry is dependent on a continuous and reliable source. Blackcurrant seed is an under-exploited source of GLA because the quality of the seed oil is not consistent year to year (suggesting environmental influences) and, crucially, GLA must be over 15% of the total normalised fatty acids in the seed to ensure a premium market value. Previously it was observed that blackcurrant varieties differ substantially in GLA content of their seeds, ranging from 11.6 to 24.6 weight % of total fatty acids. Here we investigated the effect of the environment on GLA content in seeds of 'Baldwin' blackcurrants. We noted first that 'Baldwin' blackcurrants showed relatively high GLA content, and average to above-average contents of two other therapeutic pharmaceuticals (alpha-linolenic acid and stearidonic acid) compared to the blackcurrant genotypes analysed previously. However, since there were no significant effects of agronomic manipulations (manipulation of nitrogen supply, manipulation of crop load, and manipulation of soil drying) on fatty acid content and composition of blackcurrant seed oil, we concluded that an approach based on selective breeding is more likely to improve the blackcurrant seed fatty acid content and profile than crop husbandry. We also report the establishment of in vitro cultures of blackcurrant. The purpose of these cultures was twofold: (1) to produce a source of uniform plant material from which to raise future stock plants and (2) to have available clonal blackcurrant tissue that could be used to investigate the manipulation of biochemical pathways leading to the production of GLA.
CSG 15 (Rev. 6/02) 3
Projecttitle / Mineral and nutritional content of produce
/ DEFRA
project code / HH3720SX
Scientific report (maximum 20 sides A4)
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CSG 15 (Rev. 6/02) 3
Projecttitle / Mineral and nutritional content of produce
/ DEFRA
project code / HH3720SX
This contract comprised two distinct one-year projects that were combined within HH35: Sustainable Crop Nutrition. The first project assessed the evidence for the statement that the mineral content of edible horticultural and potato crops is declining, and the second project determined ways to enhance the gamma-linolenic acid content of blackcurrant seeds.
Project 1: Is the mineral content of edible horticultural crops declining?
Abstract
Various authors have stated explicitly that the mineral concentrations in vegetables and fruit consumed in the UK have decreased since the 1930s. Here we address their claim through a desk-based study. First, we observe that insufficient data exist in the primary literature to test the hypothesis that the mineral concentrations in any individual vegetable, fruit or nut available in the UK has declined since the 1930s. However, values for the mineral concentrations in "produce in general" can be compared across this period, with an estimate of variation being derived from the changes in mineral content of many different crops. Thus, we tested the null hypothesis that the mineral concentrations in horticultural produce available in the UK had not declined between the 1930s and the 1980s. We conclude that the average concentrations of Cu, Mg and Na in vegetables and the average concentrations of Cu, Fe and K in fruit available in the UK have decreased significantly between the 1930s and the 1980s. The same hypothesis was tested with comparable data from the USA, whose historical horticultural and consumer practices have paralleled those of the UK. Data from the USA confirmed that the average Cu concentrations in the dry matter of vegetables and that the average concentrations of Cu, Fe and K in fruit had decreased significantly since the 1930s. To halt the decline in the mineral concentrations of horticultural crops, research towards developing appropriate genotypes and improved cultural practices is recommended.
Introduction
It has been repeatedly reported in the food-science literature [1], in promotional materials of companies supplying prophylactic mineral supplements [e.g. 2,3] and in the popular press [e.g. 4-6] that the mineral concentrations in edible portions of horticultural crops consumed in the UK has decreased since the 1930s. In these articles, the decrease in the mineral nutrient concentrations in raw fruit and vegetables is attributed to modern breeding and cultural practices. Since these claims are potentially detrimental to the UK horticulture industry, it is important to ascertain their validity. If they prove to be correct, then there are immediate opportunities to improve crop genotypes and cultural practices to ameliorate the problem.
Data on Mineral Concentrations in Produce Available in the UK in the 1930s and 1980s
McCance and colleagues undertook the first systematic analyses of the mineral concentrations in UK produce between 1934 and 1935. They are contained in their laboratory notebooks, which are held at the Wellcome Library for the History and Understanding of Medicine [7]. The summary data (Table 1) are summarised in a 1938 publication of the Medical Research Council [8]. Data are available for potassium (K), calcium (Ca), magnesium (Mg), iron (Fe), copper (Cu), sodium (Na), chlorine (Cl) and phosphorus (P). In dietary terms, the UK population consumes more per head per day than the recommended daily intake of sodium and chlorine [9]. So, the delivery of Na or Cl to our diet through vegetables, fruit and nuts should not be an issue. Furthermore, since table salt (NaCl) can be replaced by KCl, a decline in K content of produce is not a dietary issue either. The most recent analyses of UK produce are contained in the 6th Summary Edition of McCance and Widdowson's The Composition of Foods published by the Food Standards Agency in 2002 [10]. However, the data on mineral concentrations in raw vegetables, fruits and nuts in this publication were taken from the supplements to the fourth and fifth editions published in 1991 and 1992 by Holland and colleagues [11,12], which are mostly derived from analyses they undertook between 1984 and 1987 (Table 2). The data from both these sources take the form of "representative" values of mineral concentrations for different types of produce. Occasionally, data are given for different varieties of a vegetable or fruit, but this is uncommon. Thus, genetic effects on the mineral concentration within a crop species cannot be assessed. Furthermore, the experimental procedures do not allow even the biological variation within a particular crop to be assessed. The sampling procedure for each vegetable or fruit was to purchase a representative sample from several sources and to bulk these together prior to mineral analysis. Analyses of the composite samples were then performed in duplicate, and the variation between duplicate determinations was generally minor.
Unfortunately, without knowledge of biological variability it cannot be determined whether the representative values of mineral concentrations cited for a particular vegetable, fruit or nut differ significantly between the 1930s and the 1980s. Thus, there is insufficient data to test the null hypothesis that the mineral content of specific horticultural crops has not declined from these data. However, the values for "produce in general" can be compared between the 1930s and 1980s, and an estimate of variation can be derived from the changes in mineral content of many different crops. A similar analysis was previously undertaken by Mayer [1] using the values published in 1960 by McCance and Widdowson [13] and in 1991 by Holland et al. [14]. From an analysis of 20 fresh vegetables and 20 fresh fruit, she concluded "There are statistically significant reductions in the levels of Ca, Mg, Cu and Na in (UK) vegetables and Mg, Fe, Cu and K in fruit". However, she expressed mineral concentrations on a fresh weight basis (and there was a reduction in the dry matter of vegetables and a significant reduction in the dry matter of fruit) and she did not use the original 1930s data.
Have Mineral Concentrations in Produce Available in the UK Generally Declined since the 1930s?
To test whether the mineral concentrations in vegetables, fruit and nuts had declined, we erected the null hypothesis that the mineral concentrations had not declined between the 1930s the 1980s for the mineral elements Na, Cl, Ca, Mg, Fe, Cu, K and P. We expressed all data on a dry weight basis, to remove the vagaries of tissue hydration. On a dry weight basis, true fruit generally have lower Ca and Fe concentrations than leaf or root vegetables, but the K, Mg and Cu concentrations are similar in all crops, with the exception of mushrooms, which have extremely high Cu concentrations (data not shown).
We compared the original data of McCance and colleagues (Table 1) with the analyses of Holland and colleagues (Table 2). A "new/old" quotient was calculated for each crop and mineral element (Table 3). Quotients less than unity indicate that the mineral concentration in a crop may have declined between the 1930s and the 1980s. However, it is noteworthy that these quotients varied considerably, with the concentrations of some minerals in some crops declining and the concentrations of some minerals in some crops increasing. The natural logarithm of the "new/old" quotient was taken and a "one sample, two-tailed, t-test" was performed with the null hypothesis that the quotient was equal to zero (Table 4). A quotient less than zero indicates a decline in mineral concentration, a mean quotient greater than zero indicates an increase in mineral concentration. All analyses were performed using Genstat 5 (Genstat 5 Committee, [15]).
Statistical analyses indicated that the average concentrations of Cu, Mg and Na in vegetables and the average concentrations of Cu, Fe and K in fruit had decreased significantly between the 1930s and the 1980s (Table 4). This is consistent with the observation of Mayer [1] that the concentrations of some mineral elements had declined in UK fruit and vegetables between the 1930s and the 1980s, most notably Cu. It is, perhaps, noteworthy that (1) the analytical methods for Cu of McCance and colleagues differed from those of Holland and colleagues, (2) the amounts of Cu in produce are small and, therefore, subject to experimental errors, (3) modern fertilisers have less contaminating metals and (4) there has been a reduction in the use of Cu-containing pesticides. Interestingly, although the average Mg concentration in dried fruit had decreased, the average Cu concentration had increased. A significant increase in the average Cu concentration in nuts was also observed.