EDIBLE CANNA IN HAWAII 13 
The table on page 12 shows that edible canna tubers have about 
the same feeding value as other tuberous crops possessing a nutritive 
ratio that is narrower than that of cassava or taro, and wider than 
that of potatoes or sweet potatoes. As a forage crop, the tops 
compare favorably with other crops. They are succulent and have 
a comparatively narrow nutritive ratio. Compared with the grasses, 
they are lower in crude fiber and higher in protein, calculated on a 
dry-weight basis. Low and high protein are both advantageous. 
EDIBLE CANNA STARCH 
Chemically, all starches are alike, being the condensation product 
of dextrose and having the general formula (C 6 H 10 5 )n. In this 
formula, however, "n varies between wide limits, ranging from 
about 30 to 200 or more. The value of "n" and the space arrange- 
ment within the starch granule are subject to great variation and 
give rise to striking physical differences. When determining the 
character of a starch, it is necessary, therefore, to study only its 
physical properties. 
IDENTIFICATION 
Morphologically, edible canna starch is characterized by its 
exceptionally large granules. (PI. I, fig. 1.) Its identifying char- 
acteristics are as follows: Shape, irregular, ovoid; hilum, eccentric 
and annular; rings, plainly visible; length, 0.04 to 0.13 millimeters; 
with polarizer, well defined cross at hilum is visible both with and 
without selenite plates. 
Edible canna starch and potato starch are somewhat alike in 
appearance, the chief difference being in the hilum, which is annular 
and not so prominent in canna starch, and occurs as a spot in potato 
starch. In the latter the small grains are almost round, with the 
hilum central. In the potato starch both large and small granules 
occur in about equal proportion (PL I, fig. 2), whereas, in the canna 
starch the large ones greatly predominate. For purposes of com- 
parison the starches of corn and cassava are shown in Plate II, 
figures 1 and 2. 
VISCOSITY 
When a starch is heated in water to a temperature that is suf- 
ficiently high to break the granules it becomes colloidal in nature. 
The viscosity of this so-called " soluble starch" is to some extent a 
measure of its value for certain uses, as for example, in the textile 
industry. 
The viscosity of colloidal starch can be made to vary between 
wide limits. The time and method of cooking, the temperature, 
and the concentration greatly affect the total viscosity. If the 
solution is vigorously shaken while it is hot the viscosity can be 
reduced to as little as one-half of its value without such agitation. 
The viscosity increases with decreasing temperature and the starch 
hydrogel changes to a definite hydrogel if the solution is sufficiently 
concentrated. Different samples of starch from the same source 
often show pronounced differences in viscosity 
