Lynne Wong's PhD thesis

CHAPTER 6. PROPERTIES OF THE SORBED WATER
IN SUGAR CANE FIBRES
In a review on moisture sorption isotherm characteristics of food products, Al-Muhtaseb et
al. (2002) quoted that all types of water in a biological system exist as free or bound water.
With type II isotherm as exhibited by the sugar cane component parts (Section 5.6.4.3),
most authors (Singh et al., 2006, Arslan and Toğrul, 2005) divide the isotherm into three
regions. The first, low water activity a w region (0.05 – 0.20) is indicative of strongly
bound water, at the intermediate water activity region (0.20 – 0.60), water molecules which
are less firmly bound, and in the region of high water activity (a w ≥ 0.6) excess water is
present in macro-capillaries or as part of the fluid phase in high moisture materials. This
water exhibits nearly all the properties of bulk water and thus is capable of acting as a
solvent.
The water in these three regions has also been termed non-freezable bound water, freezable
bound water and free water. The water in the first region is strongly held by hydrogen
bonds formed with the hydrophilic groups on the surface of the fibre and is not removable
by drying in a vacuum. This was observed in this work that despite stringent drying, some
moisture still remained in the fibres. This water is termed ‘non-freezable’ because it is
frequently impossible to observe crystallisation or melting. The second type of bound
water is less strongly held and does exhibit crystallisation and melting. Because of the
bipolar nature of the water molecule, these water molecules form the multilayers by
aligning themselves above those bonded to the hydroxyl groups of the fibres and form
hydrogen bonds to produce hydrated layers. The water in the third region exists in the
larger voids and capillaries, and essentially acts as bulk water.
Water binds to the amorphous and not the crystalline part of the cellulose, it is known that
water molecules directly attached to the hydroxyl group in the amorphous region are nonfreezing
(Hatakeyama et al., 2000). In the case of cotton cellulose, the maximum amount
of bound water of cellulose fibre corresponds to that where one hydroxyl group in the
amorphous region attracts about one water molecule. Hailwood and Horrobin (1946)
called the fraction of a polymer molecule which is inaccessible to water molecule to form a
monohydrate the “crystalline” portion of the fibres.