Bridge-like structures between cellulose microfibrils in radiata pine (Pinus radiata D. Don) Kraft pulp and holocellulose

Radiata pine Kraft pulp fibre was examined using transmission electron microscopy combined with digital image processing and analysis. Bridge-like structures were observed occurring between the cellulose microfibrils and apparently linking adjacent microfibrils to one another. In places these structures appeared to be regularly spaced along the microfibril and were clearly defined in thinner areas where crowding and overlapping of the microfibrils did not obscure them. Cellulose microfibril diameter in Kraft fibre cell walls was found to be 5.7 nm +/- 0.5 nm while the bridge-like structures were slightly smaller with an average diameter of 4.8 nm +/- 0.9 nm. In untreated wood, cellulose microfibrils were found to be 3.6 nm +/- 0.5 nm. Digital image processing using a fast fourier transform combined with other techniques, was used to enhance images of the cell wall ultrastructure indicating that the bridge-like structures were numerous and located throughout the S2 region of the fibres examined. The assumptions used in this technique were checked using model images. Observations on bleached peracetic holocellulose confirm that these bridge-like structures are not lignin based and are probably hemicellulosic in nature. No evidence of bridge like connections between microfibrils could be found in native wood, possibly because they are obscured by lignin encrustation. We propose that the bridge-like structures be called interfibrillar bridges.