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.