Physical Chemistry of Foods

Question 2

What is the zero shear rate viscosity of 0.25%aqueous solutions of

Dextran
Na-alginate at high ionic strength
Locust bean gum
Xanthan at high ionic strength
Xanthan at low ionic strength

What would the viscosities be at a shear rate of 10 s^1?

Answer

Table 6.5 gives the critical concentration for chain overlapcand the slope of the
log–log relation forc>cfor systems 1–4. With the help of Figure 6.15 or a newly
drawn graph, the specific viscosity at zero shear rate can be read off. Assuming the
viscosity of the solvent, water, to be 1 mPa?s^1 , the viscosity directly follows. For
example, for Na-alginate,c¼ 0 :2g=dL. Sincec½ZŠ 0 ¼4,½ZŠ 0 ¼20 andc½ZŠ 0 ¼5.
The logarithm of 5¼ 0 :7, and we read from Figure 6.15 that logðZspÞ¼ 1 :33 or
Zsp¼21, leading to Z¼22 mPa?s, all at zero shear rate. There are some
complications. For dextran,c<c, and the lower slope curve should be used. For
locust bean gum, Figure 6.15 shows ac½ZŠ 0 value of 2.7. For xanthan, the slope is
different, i.e., 3.9. The results are in the Table.

Polysaccharide

c*

g=dl c*½ZŠ 0 c½ZŠ 0

Z 0

mPa?s

Za; 10

mPa?s

Dextran 2.5 4 0.4 1.5 1.2

Na-alginate, highI 0.2 4 5 22 10

Locust bean gum 0.2 2.7 3.4 17 8

Xanthan, highI 0.1 4 10 300 20

Xanthan, lowI 0.05 4? 20 104 50

Those for xanthan, lowI(about 1 mmolar), have been obtained by using the earlier
given value ofc*& 0 :05 g=dL. The value forZ 0 obtained agrees well with the result
shown in Figure 6.6. It can also be read from that graph thatZa; 10 , i.e., the apparent
viscosity at a shear rate of 10 s^1 , is about 50 Pa?s. For the other systems, obtaining
values forZa; 10 is almost guesswork, and such results are given in the table. It may be
noticed that the range of viscosities is very much wider forZ 0 (a factor 7000) than for
Za; 10 (a factor 40).