The Relationship of Sugar Moisture
to Relative Humidity
J. E. M A U D R U 1 A N D T. E. PAXSON2
T h e m a i n t e n a n c e o f p r o p e r relative h u m i d i t y i n sugar storage warehouses a n d b u l k storage b i n s is of great i m p o r t a n c e n o t only from t h e standp o i n t of e l i m i n a t i n g h a r d a n d l u m p y sugar b u t also from a b a g b r e a k a g e
standpoint.
T h e sugar crystal, possibly slightly hydroscopic in its p u r e form, contains m i n u t e a m o u n t s of ash a n d a t h i n c o a t i n g of even m o r e i m p u r i t i e s on
its surface w h i c h causes it to be even m o r e hydroscopic.
C h a n g e s in relative h u m i d i t y cause sugar to absorb or release m o i s t u r e
to m a i n t a i n e q u i l i b r i u m . An increase in relative h u m i d i t y of t h e air surr o u n d i n g the sugar releases m o i s t u r e . T h i s m o i s t u r e dissolves a small
a m o u n t of sugar from t h e crystal faces u n t i l a s a t u r a t e d solution of m i n u t e
thickness occurs on t h e crystal face. T h i s c o n d i t i o n is n o t o b j e c t i o n a b l e
except at h i g h h u m i d i t i e s w h e r e t h e sugar becomes crawly.
Decreasing h u m i d i t y causes h a r d sugar by reversing t h e above series
of events. W a t e r e v a p o r a t e s from t h e crystal faces a n d recrystallizes sugar
on t h e m . T h e p o i n t of contact of two t o u c h i n g crystals enlarges a n d fuses
so t h a t t h e crystals stick together. E a c h t i m e t h e h u m i d i t y is lowered t h e
foregoing process is r e p e a t e d so t h a t finally t h e sugar becomes a h a r d mass.
F i g u r e 1 shows t h e r e l a t i o n s h i p of m o i s t u r e in sugar to relative h u m i d i t y
of s u r r o u n d i n g air. C o n d i t i o n s for sugar of low (.0010%) ash, m e d i u m
( . 0 1 0 1 % ) , a n d h i g h (.0285%) ash are shown. T h e h i g h ash sugar starts
g a i n i n g m o i s t u r e a t s o m e t h i n g m o r e t h a n 4 0 % h u m i d i t y while the low ash
sugar m a i n t a i n s a c o n s t a n t m o s i t u r e c o n t e n t to m o r e t h a n 6 0 % . All t h r e e
sugars increase r a p i d l y i n m o i s t u r e above 7 5 % a n d a t 8 5 % h u m i d i t y start
t o liquefy. T h i s d a t a was collected b y d e t e r m i n i n g m o i s t u r e o n small samples
of sugar after they h a d b e e n allowed to reach e q u i l i b r i u m in a dessicator in
which air of c o n t r o l l e d h u m i d i t y was p r e s e n t . H u m i d i t y was c o n t r o l l e d by
t h e m e t h o d of Stokes 8c R o b i n s o n 3 in which sulfuric acid of v a r i o u s conc e n t r a t i o n s was p l a c e d in t h e b o t t o m of t h e dessicator. L a r g e q u a n t i t i e s of
t h e acid s o l u t i o n a n d small samples of sugar wTere e m p l o y e d so t h a t changes
in m o i s t u r e of t h e sugar w o u l d n o t c h a n g e a p p r e c i a b l y t h e c o n c e n t r a t i o n
of acid. Samples r e m a i n e d in the dessicator 4 days to assure e q u i l i b r i u m .
Relative
Humidity
20
40
60
70
75
80
85
1
2
3
% Anhydrous H^SO^
by weight
57.76
47-71
38.35
33.09
30.14
26.79
22.88
General Chemist, Holly Sugar Corp., Colo. Springs, Colo.
Chief Chemist, Holly Sugar Corp., Delta, Colo.
Stokes, R. H., & Robinson, Ind. Eng. Chem. 41, 2013 (1949)
PROCEEDINGS—-SIXTH GENERAL MEETING
0.3
539
Fig. I
T h e straight line graph on Figure 2 shows moisture content of paper from
a natural craft 5 ply, 250-lb. basis, 100-lb. paper bag again plotted against
relative humidity. We have it on good authority that paper becomes dangerously brittle, causing excessive breakage below 7% moisture, which corresponds to about 40% relative humidity. We must then store sugar between
40 and 60% relative humidity to minimize breakage due to paper brittleness
on the one hand and prevent hard sugar on the other, by drying out after
the sugar leaves the warehouse.
T h e western climate is characterized by great changes in humidity.
T h e humidity on a cold winter day may be as low as 10%. If the outside
temperature is —20° F. and if the air is heated in the warehouse to +50° F.
the relative humidity becomes almost 0. If later a warm thaw occurs, the
outside temperature may become +40° F. with 60% humidity. Upon being
heated to 50° F. the humidity is only reduced to 40%. Thus, the humidity
varies from almost 0% to 40% without changing the warehouse temperature.
540
AMERICAN SOCIETY OF SUGAR BEET TECHNOLOGISTS
Warehouses, then, require controlled humidification and temperatures should
be no higher than necessary.
Another source of hard sugar in connection with bin sugar occurs, during cold weather, by bringing the bulk sugar into a humid warehouse when
the sugar is below the dew point of the warehouse air. The sugar absorbs
moisture and later upon releasing the moisture to reach equilibrium it
cakes and becomes hard.
In summary, warehouse air should be controlled both as to temperature
and humidity. Relative humidities between 40 and 60% minimize breakage
and prevent hard sugar. High ash sugars are more hydroscopic than sugars
of lower ash content.
Additional investigation should be made on the factors which affect
the strength of paper with the thought that a paper might be developed
which is not so sensitive to low relative humidity.