Bioactive Carbohydrates in Vining Peas (Pisum sativum) - Variation in alpha-galactoside content and the colonic fermentation of sugar alcohols

University dissertation from Division of Applied Nutrition and Food Chemistry, P.O Box 124, 221 00 Lund

Abstract: The composition of the colonic microflora is essential for the health of the human host. To be able to survive, the microflora is dependent of a continuous supply of substrate. In this respect dietary fibre and other indigestible carbohydrates, of which the consumption in affluent societies is generally low, are of great importance. Numerous of studies have demonstrated beneficial effects of dietary fibre in the protection of welfare diseases. The main products of microbial degradation in colon are short-chain fatty acids (SCFAs; acetic, propionic and butyric acid) and some of these may explain some of the physiological effects related to increased dietary fibre consumption. Since the different SCFAs have diverse physiological implications, the proportions and total amounts of these acids present in colon are important. The formation is dependent on both the microflora and the substrate available. The present work has investigated how the content of the indigestible raffinose family oligosaccharides (RFOs) and other low molecular weight carbohydrates (LMWCs) in vining peas (Pisum sativum) is affected by choice of cultivar, cultivation conditions, harvest year, brine grade, size, maturity and losses during blanching. For this purpose an extraction procedure of the RFOs was optimized in order to be able to perform a quantitative measurement of the oligosaccharides present in vining peas. A final study examined the SCFA formation in caecum when rats were given some of the LMWCs present in peas in pure form, i.e. sorbitol, xylitol or myo-inositol. The amount of sugar alcohols reaching the hindgut was estimated by analysis of the sugars in the caecum and faeces of rats treated with antibiotics to prevent hindgut fermentation. Extraction of RFOs from vining peas was complete after 30 min in 50% ethanol at room temperature. Increasing the temperature or extension the time did not improve the yield. Extraction in 80% ethanol generally yielded only half the amount RFOs extracted with 50% ethanol. There was a positive correlation between the amount of raffinose, stachyose and sorbitol in the pea samples (n=137). High contents of these sugars were instead associated with small amounts of verbascose and contrariwise. Warm and dry weather generated a high content of raffinose and stachyose, and a lower content of verbascose, whereas the formation of verbascose was stimulated by cold and rain. The content of raffinose and stachyose accumulated (131% and 365%, respectively) during the five-day period around harvest investigated, while the content of verbascose generally decreased (47%). The differences in size and brine grade (density) appear to be related to maturity. Small and floating peas (low density) had a higher content of sucrose and a lower content of raffinose and stachyose than large and sinking peas (high density). The latter are therefore likely to be more mature. The loss of oligosaccharides following blanching was on average 32%, with verbascose showing the highest loss (56%), followed by raffinose (28%) and stachyose (16%). A remarkably high proportion of propionic acid was observed in rats given myo-inositol (29% versus 15 ± 2% for the other substrates), while sorbitol gave the highest proportion of butyric acid (13% versus 8 ± 1% for the other two substrates). All substrates, but in particularly xylitol, gave high proportions of lactic acid (21% versus 15 ± 1% for the two other substrates). The faecal bulking capacity was negligible and there was no faecal excretion of sugar alcohols. myo-Inositol had a higher osmotic effect in the caecum than sorbitol and xylitol. From the caecal pool size of short-chain fatty acids the amount of sugar alcohols was estimated to be 35 to 40% of the administered amount.

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