The hygroscopicity of carbohydrates is the ability to be able to absorb moisture from the surrounding atmosphere. This property may be desirable in some cases, e.g. cakes where the included carbohydrate attracts moisture to keep the cake moist and counters the drying out associated with such products. In most cases it is not desirable, however, a good example being boiled sweets. Control of moisture absorption is a role well suited to glucose syrups in foods.
Whether a glucose syrup or dried glucose syrup or other carbohydrate or a food product absorbs water from its surroundings depends on its equilibrium relative humidity (ERH). At a specific vapor pressure and temperature each product will neither gain nor lose moisture from its surroundings. This is the ERH of that product. Hygroscopicity is directly related to DE with the higher DE syrups being more hygroscopic. Additionally, the higher DE products, although starting as non-crystalline solids, rapidly liquefy as moisture is absorbed whilst the lower DE solids remain as such even at 100% humidity.
Products with a lower ERH than their surroundings will attract moisture, becoming sticky, and products with a higher ERH will tend to lose moisture and dry out. An increase in molecular concentration decreases the ERH and therefore a range of glucose syrups of different DE and hence molecular weight can be used to control the moisture absorbent properties of foods. Low DE products (high molecular weight = low molecular concentration) increase the ERH of a food and reduce its tendency to absorb moisture (but may lead to drying out) whilst higher DE products produce the opposite effect. It has also been reported that the lower DE syrups attain equilibrium more quickly than the higher DE products (Kearsley and Birch, 1975).
At the molecular level, many studies concerning the hygroscopic nature of individual glucose polymers have been reported in the literature (Cleland and Fetzer, 1944; Donnelly et al., 1973; Johnson and Srisuthep, 1975) as well as studies using commercial syrups (Mahdi and Hoover, 1965; Kearsley and Birch, 1975). Many conflicting results have been published but these appear largely due to experimental inconsistencies rather than any obvious differences in the syrups or glucose polymers themselves. It is known for example that the particle size of the dried glucose syrup, the method used to dry the syrup (probably resulting in different particle size ranges), and any impurities, e.g. minerals in the product, will affect its hygroscopic tendencies. The hygroscopic tendencies of individual oligomers have been reported and some interesting results obtained (Donnelly et al., 1973). In this study it was apparent that the amount of water absorbed by different glucose oligomers did not follow the predicted path of DP1>DP2>DP3>DP4 etc. At 38°C and 90% relative humidity, for example, the order was DP3>DP4=DP5=DP7>DP6>DP11>DP2.
In the case of minerals the effect appears most likely due to interactions between the carbohydrate and the inorganic material resulting in a loose complex formation and also due to inorganic material entering the helical structure of the higher molecular weight oligomers, again producing a complex product.
Chemical modification of glucose syrup, for example by hydrogenation, leads to a significant reduction in moisture absorption at very high humidities (Kearsley, 1978a).
Table table below gives typical values for moisture absorption of a range of glucose syrups after 14 days at 75% and 100% humidity and 25°C (Kearsley, 1978a). It is interesting to note that dextrose (100 DE) is less hygroscopic than even a 21 DE glucose syrup at 75% humidity. This is undoubtedly due to the fact that the anhydrous dextrose forms the monohydrate and that this is a relatively stable product. After 14 days the moisture absorbed by the dextrose exceeds that absorbed by the 65 DE product but is still less than that absorbed by the 85 DE material.
| MA (% w/w) | ||
| DE | 75% Humidity | 100% Humidity |
| 21 | 18 | 105 |
| 31 | 20 | 136 |
| 43 | 21 | 132 |
| 50 | 25 | 167 |
| 65 | 28 | 181 |
| 84 | 34 | 208 |
| 100 | 15 | 185 |
In systems containing mixtures of carbohydrates water absorption may not follow predicted patterns. For example, a theoretical consideration of boiled sweets containing sucrose and glucose syrup might predict moisture absorption. In practice, however, although a limited amount initially occurs, sucrose crystallization at the sweet surface forms a microcrystalline layer which prevents further rapid moisture absorption.