Amorphous sucrose

Figure 2 Resulting eake struetures after freeze-drying solutions of left mannitol (crystalline) and right sucrose (amorphous). Compare with Figure 9 in Chapter 5 for NaCl crystallisation from its solid solution in an amorphous sucrose matrix. Bar length = 100 im. From Pafra Biopreservation, unpublished results...

Cane sugar is generally available ia one of two forms crystalline solid or aqueous solution, and occasionally ia an amorphous or microcrystalline glassy form. Microcrystalline is here defined as crystals too small to show stmcture on x-ray diffraction. The melting poiat of sucrose (anhydrous) is usually stated as 186°C, although, because this property depends on the purity of the sucrose crystal, values up to 192°C have been reported. Sucrose crystallines as an anhydrous, monoclinic crystal, belonging to space group P2 (2). 

The non-precipitable (that is, lower molecular weight) component of a product from thermolysis (170°C, 80 min.) of anhydrous amorphous sucrose acidified with 1% citric acid contains 19% disaccharides, predominantly di-D-fructose dianhydrides.93 Only two of these were identified, namely a-D-Fru/-1,2 2,1 - 3-D-Fru/ (5) and ct-D-Fru/-l,2 2,1 - 3-D-Frup (1) in the ratio 1 1. This result can be compared with the ratio 2 1 for the commercial caramel.94... 

The glass transition of solutes that remain amorphous during and after the freezing process can often be seen in the DSC thermogram as a shift in the baseline toward higher heat capacity. This is illustrated in the DSC thermogram of sucrose solution in Fig. 6, in which the glass transition is observed at — 34°C. 

Figure 2 represents specific surface area of composites obtained as measured by low-temperature adsorption-desorption of argon in comparison to that calculated by the additive model. Over the entire range of sucrose content, excluding the end points corresponding to individual components the measured specific area of the composites obtained exceeds the values calculated by the additive model. The higher the amorphous carbon content... 

To illustrate this more quantitatively, consider the hypothetical sucrose example discussed by Ahlneck and Zografi [80]. Assuming that all the sorbed water is taken up by the amorphous portion of material, 0.1% total moisture would correspond to approximately 20%, 10%, 4%, and 2% moisture content in the amorphous material, respectively, for 0.5%, 1%, 2.5%, and 5% of amorphous solid. The glass transition temperatures for the amorphous portions... 

During secondary drying, a small RM should be reached, since Tg of the dry product increases with decreasing water content. Tg- of amorphous, freeze dried sucrose increases from 16 °C with 8.5 % RM to 63 °C or 64 °C between 1.0 and 0.7 % RM. It should be taken into account, that RM cannot be taken only at the end of drying, but a possible increase during storage by water desorbed from the stopper has to be considered (see Section 1.3.2 and Pikal et al. [3.7]). 

Luck, W.A.P. 1981. Structures of water in aqueous systems. In Water Activity Influences on Food Quality (L.B. Rockland and G.F. Stewart, eds), pp. 407 134. Academic Press, New York. Ludescher, R.D., Shah, N.K., McCaul, C.P., and Simon, K.V. 2001. Beyond Tg Optical luminescence measurements of molecular mobility in amorphous solid foods. Food Hydro colloids 15, 331-339. Ludwig, R. 2001. Water From cluster to the bulk. Angewandte Chem. Int. Ed. 40, 1808-1827. Maclnnes, W.M. 1993. Dynamic mechanical thermal analysis of sucrose solutions. In The Glassy State in Foods (J.M.V. Blanshard and PJ. Lillford, eds), pp. 223-248. Nottingham Univ. Press, Loughborough, Leicestershire. 

Makower, B. and Dye, W.B. 1956. Equilibrium moisture content and crystallization of amorphous sucrose and glucose. J. Agric. Food Chem. 4, 72-77. 

Fig. 11.—Conventional, I.r. Spectra of Freeze-dried (FD) and Quenched-melt (QM) Sucrose, and Amorphous Maltose (MA).

Glass Transition Temperature (Tg) of Amorphous Sucrose versus Water Content... 

Saleki-Gerhardt, A. and Zogra, G. (1994). Non-isothermal and isothermal crystallization of sucrose from the amorphous stat )harm. Res., 11 1166-1173. 

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