How we wish to be cited:
Folates in strawberries and
rosehips
Modern models
This is a summary of the PhD thesis (1-6), which Lena
Strålsjö defended at the Swedish University of Agricultural Sciences,
Uppsala, Sweden, the 31st of January 2003. The thesis is divided into
three parts. First the literature on folates is briefly reviewed, giving
a background to the vitamin with emphasis on berries. In the second part
research contributions regarding folates in berries are presented,
focusing on two model crops - strawberries and rosehips. A pertinent
part of the work was the evaluation and optimization of an RPBA method
for folate analysis in berries. This optimized RPBA method was used to
study folate content and folate retention in berries and berry products.
A human ileostomy model to determine the bioavailability of folate in
food was also evaluated. The third and last part consists of the basic
papers (1-5). The dissertation has bearing on current health debate on
human fertility and human vascular disease (7-10).
Objectives
The folate intake in many Western populations today is far below the daily recommendations. Therefore, the food industry, the health and food authorities and the general population alike all have a need for and interest in a critical evaluation of dietary folate sources. According to the Swedish study, ‘Riksmaten 1997-98’, almost all types of foods contribute to the daily intake of folates. Fruits and berries, with their 15% contribution, constitute one of the main sources. However, unspecific quantification methods with insufficient quality control are used to establish folate concentrations reported in food tables.
Presently, no knowledge is available surrounding the effects of folate content in fruits and berries due to cultivar, growing conditions and post-harvest handling. Even the folates are vulnerable to losses during food handling; only a few studies on folate retention after storage and processing are to be found for other foods, while none focus on fruits and berries. An evaluation of dietary folate sources requires not only knowledge about the folate content. Equally important is to what extent folates in various foods are available for absorption in the body. The main objectives of the present study were:
Results and discussion
Evaluation of a modified RPBA for food folate analysis
The optimized RPBA was demonstrated to be a useful method for folate
quantification in berries containing mainly 5-CH3-H4folate.
Results after internal evaluation regarding selectivity, background radiation,
unspecific binding, and accuracy were presented. To avoid unspecific binding,
purification of sample extracts by solid phase extraction, prior to
quantification, was included for some berries. Microbiological assay (MA) and
high performance liquid chromatography (HPLC) were used to compare obtained
folate concentrations and to characterize
individual folate forms in tested matrices. The predominating folate form in all
tested berries was 5-CH3-H4folate. Therefore, the recently
commercially available natural form (6S)-5-CH3-H4folate
was chosen as calibrant.
Folate content in berries and retention during processing
Berries rich in folate were rosehips (100-180 µg/100g fresh weight) and
strawberries (70-90 µg/100g fresh weight). These two model berries were used to
study factors affecting the folate content in berries. Significant effects on
folate content were observed from cultivar, grade of ripeness and year of
harvest (Table 1).
Folate retention in the two model berries was studied during storage, drying procedures and in commercial products. After storage of fresh strawberries in the refrigerator high folate retention was obtained, whereas storage in room temperature and at exposure to light resulted in folate losses (Table 2).
In rosehips folate retention during various drying procedures was improved by minimizing the drying time using high temperature and cutting of berries into slices. To study folate degradation during drying, samples of sliced rosehips were analyzed throughout the drying procedure. As shown in figure 1, the folate retention remained relative stable when the water activity (aw) was dropping from around 0.95 down to 0.75 after 30 minutes of drying.
Almost no folate losses occurred when strawberries were cooked as jam or stewed as strawberry desserts from frozen berries, with a folate retention of 79-103%. Commercial products from both berries contained 10-20 µg of folate per 100 g FW. Berries and semi-manufactures of berries are suitable ingredients in various food products to increase their nutritional value regarding folate intake.
Folate bioavailability – evaluation of an ileostomy model
A newly developed human ileostomy model suitable for evaluating folate
bioavailability from these products is presented. It enables to determine
absolute and relative folate absorption from foods using stomal folate excretion
and plasma AUC for comparison of dietary folate sources. Application of a
reference dose of the biologic form (6S)-5-CH3-H4folate by
i.m. injection ensures same handling by the body and an exact estimation of the
reference dose.
Conclusions
We will recommend fresh and frozen berries as well as processed berry products as rich folate sources. For instance, 150-200 g of rosehips or 250-350 g of strawberries (~200 µg folate) supplies 60-100% of the daily European intake recommendations (200-300 µg/day) or approximately 50% of the US recommendations (400 µg/day). This is relevant as folate is among the most limited of all vitamins, as regards the nutritional requirements in humans. Our studies, which show folate content in berries to be pronouncedly higher than reported food table data, indicate that fruits and berries are folate sources that contribute more to the total folate intake than previously assumed. Probably, also unreliable folate data exist for other foods.
In studying folate content in strawberries and rosehips, significant effects due to cultivar, year of harvest and ripeness were observed. By choosing cultivars high in folate and optimizing growing and storage conditions, folate-rich berries can be obtained. The high to moderate retention of folate in berries during storage and processing is probably related to the antioxidative milieu, with high contents of organic acids and phenolic compounds. Using mild drying procedures dried berry products are potentially rich folate sources (~ 500 µg/100g DM). Our observations are interesting to the food industry using berries and semi-manufactures of berries as ingredients in food products with increased nutritional value (functional foods). Furthermore, the newly developed ileostomy model can provide a tool for evaluating the folate bioavailability from these products. Increased intake of berries and berry products could be an alternative to folate supplements and fortification.
It should be emphasized that the present studies have implications for current discussion on folate in human fertility and human vascular disease (7-10).
Lena Strålsjö
Department of Food Science
Swedish University of Agricultural Sciences
P.O.Box 7051, 750 09 UPPSALA, SWEDEN
Tel: +46-18-671453,
References
Published May 26, 2003