The Challenges of DDGS Caking in Transit
Purdue Researchers offer tips on how to prevent DDGS from caking in railcars.
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.
For long-term storage or for transportation to longer distances in high RH conditions, DDGS may be prepared with less CDS without compromising the nutritional quality. New evolving dry-grind processes which extract the oil from the CDS (syrup) would definitely change the moisture sorption behavior of DDGS for the better, and thus reduce its propensity to cake.
The model which we have developed to predict the moisture sorption of DDGS based on the chemical composition could be incorporated into NIR analyzers to predict the moisture sorption of DDGS from these new processes, and can provide a useful decision tool available at ethanol plants to determine the propensity of their product to cake when being shipped to various locations.
To understand more about the moisture-DDGS interaction at particle level and its effect on caking, a microscopic study was conducted at Purdue University. The study gave a better understanding of the fundamental mechanisms of particle caking in DDGS and revealed the role and interaction of particles with moisture in humidifying and dehumidifying environments.
As shown in Figure 3, formation of liquid bridges in DDGS samples was noted above 60% RH due to adsorption of vapor from the atmosphere. In spite of the temperature difference, the humidity range at which the onset of liquid bridge started was almost similar at 5 and 10 C. Inter-particle bonding, as influenced by humidity and temperature, has a very important influence on the mechanical properties of powders. The liquid bridge formed by absorption of moisture during humidifying (wetting cycle) hardened and led to the formation of a solid bridge between the particles during dehumidifying (drying cycle).
The fluctuation of RH and temperature during storage and transportation such as wetting and drying cycles likewise will induce irreversible bridging between DDGS particles leading to particle agglomeration, and progress toward a nonflowing bulk. The induction of caking due to increase in humidity is a result of storing or transporting DDGS above the RH range of 60 to 65%.
For DDGS, a complex multicomponent bulk solid blend containing particles of different chemical components, an increase in temperature will result in greater stickiness and caking. The temperature at which stickiness increases is called the glass transition temperature. The glass transition behavior of any material depends on the chemical composition, molecular weight and the amount of plasticizing chemical such as moisture and glycerol.
The susceptibility of DDGS to environmental temperature increases with increase in moisture content. Higher levels of glycerol and sugar components in CDS reduce the transition temperature of DDGS, and thus increasing CDS levels in DDGS reduces the temperature at which the onset of caking begins.
Our studies show that stickiness of DDGS particles can initiate at an environmental temperature of 21 C for DDGS at 9% moisture having 29.7, 12.0, 5.9, 8.5 and 6.0 crude protein, crude fat, crude fiber, glycerol and total reducing sugars (% dry basis), respectively. But for DDG produced without any CDS, and having 34.4, 8.3, 8.5, 2.3 and 3.3 crude protein, crude fat, crude fiber, glycerol and total reducing sugars (% dry basis), respectively, the transition temperature increased slightly to 23 C. This means the former DDGS product will cake before the latter DDG product under increasing temperature and RH conditions. The dependence of glass transition of DDGS on the chemical composition makes it possible for control by varying its chemistry in the production process to match the climatic conditions which would prevent caking of the product during transportation to the final destination.
Storage and transportation solutions
Based on how DDGS is currently stored in flat storages and transported in railcar hoppers, it is definitely a challenge to find ways to control storage and environmental conditions in these applications.
As a first step, ensure that the product is cooled down after drying to the prevailing ambient temperature as fast as possible prior to loading into a railcar. Improper cooling — loading a warm product into hopper cars or ship cargo holds — would increase the propensity of caking to occur and the product would be potentially difficult to dislodge upon arrival at destination.
Any way by which the product can be covered (tarped) to limit moisture exchange with the environment should decrease its propensity to cake. The effectiveness of these solutions has not yet been investigated by the authors, and will be pursued with the industry in the near future. So we welcome any feedback from interested collaborators.