2009_Book_FoodChemistry

336 4 Carbohydrates
Fig. 4.42. Reductive depolymerization of a permethylated galactomannan (according to Kiwitt-Haschemie et al.,
1996) 1. Reductive cleavage with triethylsilane and trimethylsilylmethanesulfonate/boron trifluoride 2. Acetylation
with acetic anhydride and N-methylimidazole
spectrometry. In the example presented here,
the derivative 4-O-acetyl-1,5-anhydro-2,3,6-tri-
O-methyl-D-mannitol (a in Fig. 4.42) results
from the 1,4-linked D-mannose, the structural
unit of the main chain. The derivative 4,6-di-Oacetyl-1,5-anhydro-2,3-di-O-methyl-D-mannitol
(b) indicates the structural unit which forms
the branch and the derivative 1,5-anhydro-
2,3,4,6-tetra-O-methyl-D-galactitol (c) indicates
the terminal D-galactopyranose of the side
chain. The derivative 1,5-anhydro-2,3,4,6-tetra-
O-methyl-D-mannitol (d) produced in small
amounts shows the end of the main chain formed
by D-mannopyranose. The appearance of glucose
in the structural analysis indicates glucans or
modified glucans, e. g., modified starches or
celluloses. The identification of thickening
agents of this type is achieved by the specific
detection of the hetero-components, e. g., acetate
or phosphate.
4.4.6.2 Dietary Fibers
Gravimetric methods are the methods of choice
for the determination of dietary fibers (cf.
15.2.4.2). In the defatted sample, the digestible
components (1,4-α-glucans, proteins) are enzymatically
hydrolyzed (heat-stable α- amylase,
glucoamylase, proteinase). After centrifugation,