The effect of cellulose crystallinity on in vitro digestibility (IVD) and fermentation kinetics was investigated in samples of meadow hay and barley, wheat and rice straws. A saturated solution of potassium permanganate was used to isolate the celluloses, and their crystallinity was evaluated in a Fourier transform infrared spectrometer as the ratio of the observed transmittance at two distinct wave frequencies . IVD was determined after 48 h of incubation, and the kinetics of fermentation was studied with fully automated gas production equipment. Despite significant differences (P < 0.05), crystallinity showed low variation (0.30–0.42) among celluloses and correlated positively with IVD (P < 0.05). Although positively correlated with maximal gas production (r = 0.91) and rates of fermentation, IVD (averaging 784 g kg−1 organic matter (OM)) was also low for all samples as compared with the usually referred values for isolated celluloses. The lowest means were observed for meadow hay and rice straw. The cumulative gas production profiles were well described by a monophasic model (r2 = 0.997, RSD (residual standard deviation) = 8.367), but all the curves had a lag phase varying from 3 to 6 h. Cellulose isolated from rice straw showed the lowest maximal gas production (366 ml g−1 OM) and highest times to reach half of the maximal gas production and maximal rate. Transmission electron microscopy (TEM) of two cellulose samples showing considerable gas production at highest rates as compared with cellulose isolated from rice straw that provided the lowest gas production at the lowest rate revealed generalised bacterial colonisation, the presence of glycocalyx fibres and cell erosion in all samples. Although samples collected after 12 h of fermentation appeared to present lower microbial attack as compared with those incubated for 24 h, TEM did not explain the observed differences in the data. Since crystallinity, IVD, gas production and rates of fermentation were shown to be associated with the nutritional availability of cellulose, further research dealing with extreme values of cellulose crystallinity may be important.
© 2003 Society of Chemical Industry