The challenges of unloading Distillers Dried Grains with Solubles (DDGS), the major co-product of corn ethanol production, from hopper cars or ship cargo holds at their points of destination have been a major logistical problem facing the marketing of DDGS.
Product caking in hopper cars is primarily the major problem when hopper cars turn up “hard” with nonflowable product (Figure 1). This problem has led to two Class I railroad carriers announcing that they will not permit railroad-owned hopper cars to be used to haul DDGS. Also, a third Class I carrier does not permit their cars to be used for hauling DDGS, even though the carrier did not make this explicit statement in its tariff.
When product cakes up in hopper cars, it takes time and money to dislodge, increasing the cost of shipment. Additionally, customers or suppliers are discouraged from shipping products to those markets, limiting potential buyers. DDGS typically begins as a flowable dried bulk product, but could end up as a caked mass during storage or shipment (Figure 2). The causes of caking and conditions at which caking occurs is the primary goal of this article.
Why DDGS cakes
In general, bulk solids are dried to very low moisture for safe storage and handling. But, the fluctuations in environmental humidity and temperature induce moisture adsorption on the particle surface or moisture absorption into the bulk. During transportation and storage, DDGS is exposed to various temperature and humidity environments. Exposure of DDGS to adverse conditions promotes chemical and physical instability leading to caking and quality loss. Especially, moisture interaction with free-flowing DDGS transforms them into a caked mass. The higher the equilibrium moisture content (EMC) of DDGS at a given environmental relative humidity (RH), the higher will be the propensity for caking to occur. Knowledge of moisture sorption isotherm is important for transportation to longer distances and moisture changes during storage, and can be used to understand the storage stability of powdered food and feed components. Based on the prevailing weather conditions, sorption isotherms can be used to establish critical moisture levels that may be encountered during storage and transportation.
A 2008 study by Ganesan et al indicated that higher environmental temperature increases the sorption capacity of DDGS indicating the possibility of occurrence of mold growth during storage. Kingsly and Ileleji noted considerable increase in DDGS moisture content after 60% RH with a steep rise above 70% RH. For storage stability of DDGS, optimum conditions with equilibrium moisture content 15% corresponding to less than 50 to 60% RH can be considered safe in the temperature range of 20 to 30 C. However, drying to about 10% moisture would be a prudent approach to reduce the propensity of caking to occur in warm and high RH environment.
High variability in chemical composition of DDGS can alter its safe moisture level for storage. A recent study conducted by Kingsly et al indicated that the chemical composition of DDGS is highly related to the blend ratio of wet distillers grains (WDG, also known as wet cake) and condensed distillers solubles (CDS, also known as syrup) during drying.
The variability in physical and chemical composition from plant to plant and even from batch to batch makes it difficult to fix a standard moisture content for DDGS storage and transportation, since solid-moisture interaction depends on inherent chemical composition.
In DDGS, the amount of CDS added during production of DDGS can influence the moisture sorption behavior and decrease in CDS will decrease its affinity for water. Although some of the chemical components present in DDGS like protein, sugars, oil and glycerol are nutritious and energy dense, they have a high attraction to moisture. Mostly the protein and glycerol components have been shown to have the most effect on the moisture sorption of DDGS.