By Dr. Jerry Shurson, University of Minnesota Department of Animal Science, © 2019 Feedstuffs. Reprinted with permission from Vol. 91, No. 05, May 6, 2019
Compared to other feed ingredients, corn dried distillers grains plus solubles (DDGS) have some unique physical and chemical characteristics that affect storage and handling characteristics.
The use of DDGS in animal feeds has created challenges for handling and unloading from railcars, containers and bulk vessels, especially during humid summer months, plus challenges with transport using conventional feeder screws as well as flowability and discharge from feed silos and storage bins at commercial feed mills.
Proper feed ingredient storage is essential for preserving nutritional value and preventing spoilage. The original condition of a feed ingredient is the most important factor affecting quality preservation during storage and is influenced by moisture content, relative humidity and temperature (Mills, 1989).
Moisture within a feed ingredient ultimately reaches equilibrium with the air within and between particles over time and, depending on the conditions, may lead to the growth of molds and other deleterious microorganisms (Mills, 1989). Maximum acceptable moisture concentrations of grains have been established but vary among types of grains and length of storage period. Furthermore, maximum relative humidity levels have been established for grains to prevent mold growth (less than 70%), bacteria growth (less than 90%) and storage insects (less than 60%).
However, it is important to remember that moisture and relative humidity interact with temperature in the storage environment. High temperatures of grain and feed ingredients when loading a storage bin can be maintained for many months if the mass is aerated. Temperature and moisture content determine the extent of enzymatic and biological activities of grains and other ingredients, and temperature differences within the stored mass can increase the risk of mold growth through moisture migration (Mills, 1989).
Unfortunately, no studies have been conducted to determine optimal storage conditions to maintain DDGS quality and prevent spoilage over extended storage periods of time or under various climatic conditions. As a result, it is generally assumed that drying DDGS to less than 12% moisture is acceptable to minimize the risk of spoilage when stored under moderate temperatures and humidity.
If the bin volume, hopper configuration and feeder screw design are not suitable for the new ingredient, other options must be explored (Behnke, 2007). When deciding feed ingredient allocation to storage bins, one of the most important considerations is determining the expected diet inclusion rates that will be used in all feeds manufactured so the daily or monthly usage rate and frequency of use can be calculated. Per- haps the second most important consideration is related to the ingredient’s physical properties, such as bulk density and flow characteristics.
Flowability is defined as “the relative movement of a bulk of particles among neighboring particles or along the container wall surface” (Peleg, 1977). Unfortunately, some DDGS sources have poor flowability and handling characteristics (Bhadra et al., 2008), which has prevented routine use of railcars for transport and has led to the development of specially designed unloading equipment for bulk vessels and containers. Poor flowability is a key factor that has limited DDGS use in livestock and poultry diets because of bridging in bulk storage containers.
Many factors affect the flow of a bulk ingredient (Peleg, 1977), and no single measurement adequately describes flowability (Bhadra et al., 2008). However, while DDGS moisture content and the relative humidity of the environment are the major contributing factors to bridging, caking and poor flowability, other factors such as particle size, proportion of condensed solubles added to the grain fraction before drying, dryer temperature, moisture content at dryer exit and others have also been attributed to this problem (Ganesan et al., 2008a,b,c).
The moisture content of DDGS is generally 10-12% to avoid spoilage due to mold growth during long-term storage. However, DDGS is also hygroscopic and can gradually increase in moisture content during exposure to humid conditions over a long storage period (Ganesan et al., 2007). The hygroscopic properties of DDGS can lead to bridging, caking and reduced flowability during transport and storage (Rosentrater, 2007).
Because ethanol plants have limited storage capacity for DDGS, it is sometimes loaded into transport vessels within a few hours of exiting the dryer before moisture equilibrates. When this occurs, DDGS will harden and become a solid mass in trucks, railcars and containers, making it very difficult to unload. However, if warm DDGS is allowed to cool so the moisture can equilibrate before loading, flowability is greatly improved.
Currently, most ethanol plants have implemented a minimum 24-hour “curing,” or moisture equilibration, period before loading to avoid bridging and caking to prevent railcar damage and repair costs resulting from attempts to dislodge DDGS while unloading. Ideally, holding DDGS for five to seven days is considered ideal for allowing complete moisture equilibration to occur so that the liquid bridges formed in the cooled mass can be broken, which minimizes further handling difficulties (Behnke, 2007).
Unfortunately, the majority of ethanol plants have only about two to three days of storage capacity during continuous operations, resulting in an inability to provide five to seven days for adequate moisture equilibration.
The equilibrium relationship between moisture content and environmental relative humidity for bulk solids is affected by sorption isotherms. A sorption isotherm indicates the corresponding water content at a specific, constant temperature at a specific humidity level. Therefore, as the relative humidity in the storage environment increases, the sorption increases and causes the formation of liquid bridges between particles (Mathlouthi and Roge, 2003).
Adsorption (the ability to hold water on the outside or inside surface of a material) and desorption (the release of water through or from a surface) of moisture under humid conditions are complex and affected by the carbohydrate, sugar, protein, fiber and mineral concentrations of a feed ingredient (Chen, 2000). Understanding this relationship for DDGS is important for determining the critical moisture and relative humidity levels that may cause bridging and caking during transport and storage.
Kingsly and Ileleji (2009) showed that liquid bridges formed in DDGS when the relative humidity reached 60%. At 80% relative humidity, DDGS reached maximum moisture saturation, and at 100% relative humidity, the liquid bridge formed by moisture adsorption hardened and led to the formation of a solid bridge as humidity was reduced. These results indicate that increased relative humidity during transport and storage causes irreversible bridging between DDGS particles and leads to particle aggregation (clumping), caking and reduced flowability.
Pelleting DDGS is another approach that a few ethanol plants have attempted to use to improve bulk density and flowability. Research at Kansas State University evaluated the use of various conditioning temperatures and pellet die sizes on ease of pelleting, physical properties and flow characteristics of DDGS, and results showed that almost any combination of pelleting conditions improved flowability (Behnke, 2007).
However, this approach has not been implemented in the U.S. ethanol industry for several reasons. First, it would require an additional cost for existing ethanol plants because of the need to purchase, install and operate expensive boilers and pellet mills, would require additional personnel training and labor costs and would require additional storage space. Furthermore, some DDGS customers may be reluctant to purchase pelleted DDGS because it may be perceived to be adulterated with other “fillers,” it may have reduced amino acid and nutrient digestibility due to thermal treatment during the pelleting process and because of the added cost of regrinding the pellets before adding the DDGS to other ingredients when manufacturing complete feeds.
As previously discussed, extended storage time for more complete moisture equilibration and pelleting DDGS are currently not viable options for preventing handling and flowability challenges. Therefore, several new unloading equipment designs have been developed and are being used to facilitate DDGS discharge from railcars and containers. For example, stationary devices located above a railcar pit use a steel spear to break the hardened mass before unloading.
Although these methods reduce the unloading time, they also increase labor and equipment costs. Furthermore, many commercial feed mills have chosen to use flat storage rather than bin or silo storage of DDGS to avoid flowability and transfer problems. The main advantages of flat storage are minimized flowability problems and less short-term capital investment required compared with constructing silos. However, use of flat storage requires more labor and front-end loading equipment to move the material, increases the risk of contamination with other ingredients in the storage facility and increases “shrink” losses.
Ganesan et al. (2008a) evaluated the effects of adding calcium carbonate to DDGS with variable moisture and condensed distillers solubles content in a laboratory setting and showed no benefits for improving flowability.
Johnston et al. (2009) evaluated flow- ability after adding 2.5 kg per metric ton of DMX-7 (Delst Inc.), 2% calcium carbonate (ILC Resources Inc.) or 1.25% clinoptilolite zeolite (St. Cloud Mining Co.) to DDGS containing either 9% or 12% moisture. After flow agents were added and mixed with DDGS at the ethanol plant, trucks were loaded, traveled 250 km, were parked and motionless for 60 hours and then traveled an additional 250 km back to the ethanol plant to be unloaded, and flowability measurements were obtained.
Outdoor temperatures on each of four days over a two-month period during this study ranged from 12.9°C to 27.8°C, and outdoor relative humidity ranged from 34% to 67%. Average particle size of the DDGS source ranged from 584 to 668 µm. Adding zeolite (558 kg per minute) versus DMX-7 (441 kg per minute) improved the flow rate during unloading of each truckload of DDGS, but these treatments did not differ from the control (no flow agent, at 509 kg per minute) and the calcium carbonate-treated (512 kg per minute) DDGS loads. The flowability score (1 = free flowing, and 10 = badly bridged) was improved when zeolite was added to DDGS (4.0) compared with the control (6.0), DMX-7 (6.5) and calcium carbonate (5.5).
Moisture content at the time of loading was the most important predictor (explaining 70% of the variation) of DDGS flow rate, with each 1% increase in moisture content from 9% decreasing the unloading rate by 100 kg per minute.
Ganesan et al. (2008b) reported similar results in which increasing DDGS moisture content reduced flowability. They also reported that as the Hunter b* score (yellowness of color) of DDGS increased, the flow rate also increased but only accounted for 4% of the variation in flow rate.
These results indicate that the most effective criteria for improving flow rate are to dry DDGS to less than 10% moisture content while adding DMX-7, calcium carbonate or zeolite provided no benefits for improving flowability during unloading from trucks.
Clementson and Ilelejie (2010) suggested that differences in bulk density observed during railcar loading may be due to particle segregation. This is likely to occur because DDGS is a granular bulk solid with particles of various sizes, densities and morphological characteristics found in the structural components of corn grain (Ileleji et al., 2007). Particle segregation was shown to occur during handling and gravity discharge of DDGS (Ileleji et al., 2007; Clementson et al., 2009).
Clementson and Ilelejie (2010) con- ducted a study to evaluate bulk density variation of DDGS when filling and emptying hoppers to simulate railcar loading at an ethanol plant and showed that the variation in bulk density that occurred as DDGS was loaded and emptied was mainly attributed to particle segregation. The researchers showed that after filling, the finer, smaller and denser particles were concentrated in the center of the hopper, while the larger, coarser and less-dense particles were concentrated on the sides of the hopper. This phenomenon not only causes variation in bulk density during transloading of DDGS but also should be considered when sampling DDGS for nutrient analysis because the location of sampling can influence the mixture of segregated particles and ultimately can affect the analytical results (Clementson et al., 2009).
Storage bin design can be a significant cause or a potential solution to the flowability problems with feed containing DDGS. Hilbrands et al. (2016) conducted a study to evaluate feed flow from three commercially available feed storage bins whose designs consisted of: (1) a galvanized steel, smooth-sided, seamless bin with a 60-degree round discharge cone (Steel60), (2) a galvanized, corrugated steel bin with a 67-degree round discharge cone (Steel67) and (3) a white polyethylene bin with a 60-degree round discharge cone (Poly60). The bin styles were chosen to represent differences in slopes of the sides of discharge cones as well as different construction materials in the bin walls. Diets used in this study contained 55% corn, 35% soybean meal, 40% DDGS and 2% minerals and vitamins and were ground to an average particle size of 736-1,015 microns.
The study was conducted in two experiments during the summer and fall. In the summer season, daily high and low temperatures ranged from 30.9°C to 16.6°C, and daily relative humidity ranged from 39.4% to 100%. During the fall season, daily temperatures were 2.9- 23.7°C, and the daily relative humidity range was 23.3-92.7%.
Feed flow rate out of bins was faster from Poly60 than Steel60 bins, with the Steel67 bin discharge rate being intermediate (Table 1). However, it was interesting that, although the Steel60 bins had the slowest flow rate, they required the fewest taps on bins to keep feed flowing during discharge. The presence of a passive agitator increased feed flow rate among all bin designs compared with bins without agitators, but the presence of agitators resulted in a greater feed flow rate in Poly60 bins than in steel bins. Unlike results in experiment 1, though, there was no difference in the number of taps required to keep feed flowing among the six bin design combinations.
Most field fungi do not survive during storage, but some species can continue to grow under appropriate storage conditions (Sanchis et al., 1982). Storage fungi also originate from the field and can replace field molds that infected corn grain prior to harvest (Reed et al., 2007). As shown in Table 2, storage fungi require relative humidity of more than 70% and moisture content greater than 12% for corn grain (Montross et al., 1999).
Additional fungal species may also be introduced after harvest and include Fusarium spp., Rhizopus spp. and Tilletia spp. (Williams and MacDonald, 1983; Barney et al., 1995). Because DDGS is produced from corn grain, it is reasonable to assume that these same molds may be present in DDGS.
However, due to the unique physical and chemical properties of DDGS, it is unknown if these relative humidity and moisture conditions apply similarly as for corn grain. In fact, DDGS may be more susceptible to mold growth than corn grain because mechanical damage of corn grain during and after harvest can provide entry for fungal spores (Dharmaputra et al., 1994), and broken corn kernels and foreign material promote growth of storage molds (Sone, 2001).
The corn oil present in DDGS consists primarily of polyunsaturated fatty acids, particularly linoleic acid (C18:2, 58%), which is highly susceptible to oxidation (Frankel et al., 1984). Heating lipids at relatively high temperatures produces large quantities of secondary lipid oxidation products, including aldehydes, carbonyls and ketones (Esterbauer et al., 1991). The drying temperatures used to produce DDGS can be up to 500°C, which makes the oil in DDGS susceptible to lipid oxidation. All of the pro-oxidation conditions (heat, oxygen, moisture and transition minerals) are present in ethanol plants that produce DDGS, and DDGS may be further exposed to these factors during the transport, storage and manufacturing of complete feeds in commercial feed mills. Therefore, there is some concern about the extent of oxidation in DDGS and its effects on pig growth performance and health.
Feeding oxidized lipids to pigs and broilers has been shown to reduce growth performance and increase oxidative stress. Hung et al. (2017) conducted a meta-analysis using swine and poultry data from 29 publications that showed average reductions of 5% in average daily gain, 3% in average daily feed intake, 2% in gain:feed and 52% in serum of plasma vitamin E while increasing serum thiobarbituric acid reactive substances (TBARS) 120% across all studies.
Recent reviews by Kerr et al. (2015) and Shurson et al. (2015) provide a comprehensive summary of the biological effects of feeding oxidized lipids to swine and poultry, along with the challenges of measuring lipid oxidation and interpreting the results. Hence, some swine feeding trials (Song et al., 2013; Song et al., 2014; Hanson et al., 2015a) have shown inconsistent growth performance responses from feeding pigs a highly oxidized DDGS diet.
While there are currently no standards or guidelines for measuring lipid oxidation in feed ingredients, Wang et al. (2016) suggested that 4-hydroxynonenal and a ratio of select aldehydes provide better estimates of the actual extent of oxidation in vegetable oils. Unfortunately, these analytical procedures are not commonly used in commercial laboratories.
Peroxide value is used to estimate the extent of peroxidation during the initiation phase of the oxidation process. The value of the DDGS samples was highly variable, with a minimum value of 4.2 mEq and a maximum value of 84.1 mEq/ kg of oil, for a coefficient of variation (CV) of 97.5%.
The TBARS value is used as an estimate of the extent of lipid oxidation during the propagation phase of oxidation, which is when the majority of aldehydes are produced. There was less variability (CV = 43.6%) in TBARS values among DDGS sources than peroxide values and ranged from 1.0 to 5.2 ng malondialdehyde (MDA) equivalents per milligramof oil. Peroxide values and TBARS were both greater in DDGS samples than the corn reference values. This was expected because of the thermal processing involved in producing DDGS.
Moderate negative correlations were observed for colorometric measures between L* and peroxide value (r = -0.63) and b* and peroxide value (r = -0.57), with slightly greater negative correlations between L* and TBARS (r = -0.73) and b* and TBARS (r = -0.67). These results suggest that darker-colored and less-yellow DDGS samples may be more oxidized. However, DDGS color is affected by many factors and should not be used as a definitive measure of the ex- tent of corn oil oxidation in DDGS.
Subsequent studies involving the most oxidized DDGS source fed to wean/finish pigs (Song et al., 2014) and sows and their offspring through the nursery phase (Hanson et al., 2016) showed no detrimental effects on growth performance. The lack of growth performance responses in these studies may have been a result of the naturally high concentrations of antioxidant compounds (tocopherols, ferulic acid, lutein and zeaxanthin) present in DDGS (Shurson, 2017) and conversion of sulfur compounds into endogenous antioxidants.
Only one study has been published evaluating the effectiveness of adding synthetic antioxidants to high-oil (13% crude fat) and low-oil (5% crude fat) DDGS (Hanson et al., 2015b). Samples of these two DDGS sources contained either no added synthetic antioxidants (control), 1,000 mg/kg of TBHQ (Rendox, Kemin Industries) or 1,500 mg/kg of ethoxyquin and TBHQ (Santoquin, Novus International). After antioxidants were added, samples were stored at 38°C and 90% relative humidity in a controlled environmental chamber for 28 days. Sub-samples were collected on days 0, 14 and 28 to determine the extent of lipid oxidation at each time point.
Results of this study showed that significant lipid oxidation occurred under these storage conditions (Table 4). Oxidation increased during the 28-day storage period, and the extent of oxidation was greatest in the high-oil DDGS source. However, the addition of either Rendox or Santoquin to either the high-oil or low-oil DDGS sources reduced oxidation by about 50%. Therefore, the results show that the addition of either commercially available antioxidant is effective in reducing lipid oxidation in DDGS when stored up to 28 days in hot, humid conditions. Further, the moisture content of DDGS sources increased from 10.2% to 21.4% during the 28-day storage period, which led to significant mold growth in all samples.
Compared to other feed ingredients, corn dried distillers grains plus solubles (DDGS) have some unique physical and chemical characteristics that affect storage and handling characteristics.
The use of DDGS in animal feeds has created challenges for handling and unloading from railcars, containers and bulk vessels, especially during humid summer months, plus challenges with transport using conventional feeder screws as well as flowability and discharge from feed silos and storage bins at commercial feed mills.
Proper feed ingredient storage is essential for preserving nutritional value and preventing spoilage. The original condition of a feed ingredient is the most important factor affecting quality preservation during storage and is influenced by moisture content, relative humidity and temperature (Mills, 1989).
Moisture within a feed ingredient ultimately reaches equilibrium with the air within and between particles over time and, depending on the conditions, may lead to the growth of molds and other deleterious microorganisms (Mills, 1989). Maximum acceptable moisture concentrations of grains have been established but vary among types of grains and length of storage period. Furthermore, maximum relative humidity levels have been established for grains to prevent mold growth (less than 70%), bacteria growth (less than 90%) and storage insects (less than 60%).
However, it is important to remember that moisture and relative humidity interact with temperature in the storage environment. High temperatures of grain and feed ingredients when loading a storage bin can be maintained for many months if the mass is aerated. Temperature and moisture content determine the extent of enzymatic and biological activities of grains and other ingredients, and temperature differences within the stored mass can increase the risk of mold growth through moisture migration (Mills, 1989).
Unfortunately, no studies have been conducted to determine optimal storage conditions to maintain DDGS quality and prevent spoilage over extended storage periods of time or under various climatic conditions. As a result, it is generally assumed that drying DDGS to less than 12% moisture is acceptable to minimize the risk of spoilage when stored under moderate temperatures and humidity.
Storage bin space
When a commercial feed mill uses a new feed ingredient for the first time, appropriate storage space must be made available or constructed, because it is unusual for a feed mill to have an open bin or unused storage space to accommodate a new ingredient. While a simple solution is to discontinue using an existing ingredient and designate that storage bin for the new ingredient, it is very difficult to do this without disrupting the feed manufacturing process (Behnke, 2007).If the bin volume, hopper configuration and feeder screw design are not suitable for the new ingredient, other options must be explored (Behnke, 2007). When deciding feed ingredient allocation to storage bins, one of the most important considerations is determining the expected diet inclusion rates that will be used in all feeds manufactured so the daily or monthly usage rate and frequency of use can be calculated. Per- haps the second most important consideration is related to the ingredient’s physical properties, such as bulk density and flow characteristics.
Flowability of DDGS
One of the greatest challenges for handling DDGS is its propensity for bridging, caking and causing poor flowability when unloading it from railcars, containers and bulk vessels.Flowability is defined as “the relative movement of a bulk of particles among neighboring particles or along the container wall surface” (Peleg, 1977). Unfortunately, some DDGS sources have poor flowability and handling characteristics (Bhadra et al., 2008), which has prevented routine use of railcars for transport and has led to the development of specially designed unloading equipment for bulk vessels and containers. Poor flowability is a key factor that has limited DDGS use in livestock and poultry diets because of bridging in bulk storage containers.
Many factors affect the flow of a bulk ingredient (Peleg, 1977), and no single measurement adequately describes flowability (Bhadra et al., 2008). However, while DDGS moisture content and the relative humidity of the environment are the major contributing factors to bridging, caking and poor flowability, other factors such as particle size, proportion of condensed solubles added to the grain fraction before drying, dryer temperature, moisture content at dryer exit and others have also been attributed to this problem (Ganesan et al., 2008a,b,c).
The moisture content of DDGS is generally 10-12% to avoid spoilage due to mold growth during long-term storage. However, DDGS is also hygroscopic and can gradually increase in moisture content during exposure to humid conditions over a long storage period (Ganesan et al., 2007). The hygroscopic properties of DDGS can lead to bridging, caking and reduced flowability during transport and storage (Rosentrater, 2007).
Because ethanol plants have limited storage capacity for DDGS, it is sometimes loaded into transport vessels within a few hours of exiting the dryer before moisture equilibrates. When this occurs, DDGS will harden and become a solid mass in trucks, railcars and containers, making it very difficult to unload. However, if warm DDGS is allowed to cool so the moisture can equilibrate before loading, flowability is greatly improved.
Currently, most ethanol plants have implemented a minimum 24-hour “curing,” or moisture equilibration, period before loading to avoid bridging and caking to prevent railcar damage and repair costs resulting from attempts to dislodge DDGS while unloading. Ideally, holding DDGS for five to seven days is considered ideal for allowing complete moisture equilibration to occur so that the liquid bridges formed in the cooled mass can be broken, which minimizes further handling difficulties (Behnke, 2007).
Unfortunately, the majority of ethanol plants have only about two to three days of storage capacity during continuous operations, resulting in an inability to provide five to seven days for adequate moisture equilibration.
The equilibrium relationship between moisture content and environmental relative humidity for bulk solids is affected by sorption isotherms. A sorption isotherm indicates the corresponding water content at a specific, constant temperature at a specific humidity level. Therefore, as the relative humidity in the storage environment increases, the sorption increases and causes the formation of liquid bridges between particles (Mathlouthi and Roge, 2003).
Adsorption (the ability to hold water on the outside or inside surface of a material) and desorption (the release of water through or from a surface) of moisture under humid conditions are complex and affected by the carbohydrate, sugar, protein, fiber and mineral concentrations of a feed ingredient (Chen, 2000). Understanding this relationship for DDGS is important for determining the critical moisture and relative humidity levels that may cause bridging and caking during transport and storage.
Kingsly and Ileleji (2009) showed that liquid bridges formed in DDGS when the relative humidity reached 60%. At 80% relative humidity, DDGS reached maximum moisture saturation, and at 100% relative humidity, the liquid bridge formed by moisture adsorption hardened and led to the formation of a solid bridge as humidity was reduced. These results indicate that increased relative humidity during transport and storage causes irreversible bridging between DDGS particles and leads to particle aggregation (clumping), caking and reduced flowability.
Pelleting DDGS is another approach that a few ethanol plants have attempted to use to improve bulk density and flowability. Research at Kansas State University evaluated the use of various conditioning temperatures and pellet die sizes on ease of pelleting, physical properties and flow characteristics of DDGS, and results showed that almost any combination of pelleting conditions improved flowability (Behnke, 2007).
However, this approach has not been implemented in the U.S. ethanol industry for several reasons. First, it would require an additional cost for existing ethanol plants because of the need to purchase, install and operate expensive boilers and pellet mills, would require additional personnel training and labor costs and would require additional storage space. Furthermore, some DDGS customers may be reluctant to purchase pelleted DDGS because it may be perceived to be adulterated with other “fillers,” it may have reduced amino acid and nutrient digestibility due to thermal treatment during the pelleting process and because of the added cost of regrinding the pellets before adding the DDGS to other ingredients when manufacturing complete feeds.
Effects of DDGS oil content on flowability.
Physical properties of conventional high-oil (Rosentrater, 2006), reduced-oil (Ganesan et al., 2009) and low-oil (Saunders and Rosentrater, 2007) DDGS have been evaluated. Ganesan et al (2009) showed that reduced-oil DDGS may have improved flow properties compared to conventional high-oil DDGS, but both types were classified as having “cohesive” properties, suggesting that, regardless of oil content, DDGS is prone to bridging and caking problems during long-term storage. Furthermore, the researchers suggested that chemical composition and particle surface morphology (roughness, size and shape) may have a greater effect on DDGS flowability than oil content.As previously discussed, extended storage time for more complete moisture equilibration and pelleting DDGS are currently not viable options for preventing handling and flowability challenges. Therefore, several new unloading equipment designs have been developed and are being used to facilitate DDGS discharge from railcars and containers. For example, stationary devices located above a railcar pit use a steel spear to break the hardened mass before unloading.
Although these methods reduce the unloading time, they also increase labor and equipment costs. Furthermore, many commercial feed mills have chosen to use flat storage rather than bin or silo storage of DDGS to avoid flowability and transfer problems. The main advantages of flat storage are minimized flowability problems and less short-term capital investment required compared with constructing silos. However, use of flat storage requires more labor and front-end loading equipment to move the material, increases the risk of contamination with other ingredients in the storage facility and increases “shrink” losses.
Effects of additives on DDGS flowability.
The addition of various flow agents is another approach that has been attempted to improve DDGS flowability, but only a few studies have been conducted to evaluate the effectiveness.Ganesan et al. (2008a) evaluated the effects of adding calcium carbonate to DDGS with variable moisture and condensed distillers solubles content in a laboratory setting and showed no benefits for improving flowability.
Johnston et al. (2009) evaluated flow- ability after adding 2.5 kg per metric ton of DMX-7 (Delst Inc.), 2% calcium carbonate (ILC Resources Inc.) or 1.25% clinoptilolite zeolite (St. Cloud Mining Co.) to DDGS containing either 9% or 12% moisture. After flow agents were added and mixed with DDGS at the ethanol plant, trucks were loaded, traveled 250 km, were parked and motionless for 60 hours and then traveled an additional 250 km back to the ethanol plant to be unloaded, and flowability measurements were obtained.
Outdoor temperatures on each of four days over a two-month period during this study ranged from 12.9°C to 27.8°C, and outdoor relative humidity ranged from 34% to 67%. Average particle size of the DDGS source ranged from 584 to 668 µm. Adding zeolite (558 kg per minute) versus DMX-7 (441 kg per minute) improved the flow rate during unloading of each truckload of DDGS, but these treatments did not differ from the control (no flow agent, at 509 kg per minute) and the calcium carbonate-treated (512 kg per minute) DDGS loads. The flowability score (1 = free flowing, and 10 = badly bridged) was improved when zeolite was added to DDGS (4.0) compared with the control (6.0), DMX-7 (6.5) and calcium carbonate (5.5).
Moisture content at the time of loading was the most important predictor (explaining 70% of the variation) of DDGS flow rate, with each 1% increase in moisture content from 9% decreasing the unloading rate by 100 kg per minute.
Ganesan et al. (2008b) reported similar results in which increasing DDGS moisture content reduced flowability. They also reported that as the Hunter b* score (yellowness of color) of DDGS increased, the flow rate also increased but only accounted for 4% of the variation in flow rate.
These results indicate that the most effective criteria for improving flow rate are to dry DDGS to less than 10% moisture content while adding DMX-7, calcium carbonate or zeolite provided no benefits for improving flowability during unloading from trucks.
Particle segregation
Maintaining consistent bulk density of DDGS when loading railcars and containers has been a challenge for both marketers and buyers because of the desire to achieve consistent freight weights in sequentially loaded railcars and containers to minimize shipping costs (Ileleji and Rosentrater, 2008). Bulk density varies among DDGS sources and has been reported to range from 391 to 496 kg/cu. m (Rosentrater, 2006) and from 490 to 590 kg/cu. m (Bhadra et al., 2009).Clementson and Ilelejie (2010) suggested that differences in bulk density observed during railcar loading may be due to particle segregation. This is likely to occur because DDGS is a granular bulk solid with particles of various sizes, densities and morphological characteristics found in the structural components of corn grain (Ileleji et al., 2007). Particle segregation was shown to occur during handling and gravity discharge of DDGS (Ileleji et al., 2007; Clementson et al., 2009).
Clementson and Ilelejie (2010) con- ducted a study to evaluate bulk density variation of DDGS when filling and emptying hoppers to simulate railcar loading at an ethanol plant and showed that the variation in bulk density that occurred as DDGS was loaded and emptied was mainly attributed to particle segregation. The researchers showed that after filling, the finer, smaller and denser particles were concentrated in the center of the hopper, while the larger, coarser and less-dense particles were concentrated on the sides of the hopper. This phenomenon not only causes variation in bulk density during transloading of DDGS but also should be considered when sampling DDGS for nutrient analysis because the location of sampling can influence the mixture of segregated particles and ultimately can affect the analytical results (Clementson et al., 2009).
Storage design
Feed storage bin design.
DDGS flowability is an issue not only during loading, transport, storage and feed manufacturing but can also create challenges on swine farms when DDGS diets are fed in meal form. Suboptimal feed flow can reduce the feed delivery rate to feeders as well as bridge in feeders, leading to out-of-feed events that can increase stress and the likelihood of gut health problems and reduced growth performance in pigs (Hilbrands et al., 2016). This problem is a greater concern when there is an economic incentive to increase dietary inclusion rates of DDGS to 30% or more, especially when pigs are fed meal diets with a small particle size to improve their feed conversion.Storage bin design can be a significant cause or a potential solution to the flowability problems with feed containing DDGS. Hilbrands et al. (2016) conducted a study to evaluate feed flow from three commercially available feed storage bins whose designs consisted of: (1) a galvanized steel, smooth-sided, seamless bin with a 60-degree round discharge cone (Steel60), (2) a galvanized, corrugated steel bin with a 67-degree round discharge cone (Steel67) and (3) a white polyethylene bin with a 60-degree round discharge cone (Poly60). The bin styles were chosen to represent differences in slopes of the sides of discharge cones as well as different construction materials in the bin walls. Diets used in this study contained 55% corn, 35% soybean meal, 40% DDGS and 2% minerals and vitamins and were ground to an average particle size of 736-1,015 microns.
The study was conducted in two experiments during the summer and fall. In the summer season, daily high and low temperatures ranged from 30.9°C to 16.6°C, and daily relative humidity ranged from 39.4% to 100%. During the fall season, daily temperatures were 2.9- 23.7°C, and the daily relative humidity range was 23.3-92.7%.
Feed flow rate out of bins was faster from Poly60 than Steel60 bins, with the Steel67 bin discharge rate being intermediate (Table 1). However, it was interesting that, although the Steel60 bins had the slowest flow rate, they required the fewest taps on bins to keep feed flowing during discharge. The presence of a passive agitator increased feed flow rate among all bin designs compared with bins without agitators, but the presence of agitators resulted in a greater feed flow rate in Poly60 bins than in steel bins. Unlike results in experiment 1, though, there was no difference in the number of taps required to keep feed flowing among the six bin design combinations.
These results indicate that feed bin design affects the flow rate during discharge of meal diets containing 40% DDGS. The Poly60 bin provided the best feed flow and highest discharge rates compared with the steel bin designs evaluated, and installing passive agitators increased feed flow in all bin designs.
To further evaluate the effects of DDGS particle size on ME content and nutrient digestibility for growing pigs, Liu et al. (2012) determined the ME content and nutrient digestibility of the same source of DDGS ground to three particle sizes: coarse (818 µm), medium (594 µm) and fine (308 µm). They also evaluated the flowability of diets containing 30% DDGS.
As expected, the ME content of DDGS improved as the particle size was reduced: Each 25 µm reduction in average particle size (between 818 and 308 µm) increased the ME content of the diet by 13.5 kcal/kg of dry matter. However, there were no effects of DDGS particle size on nitrogen and phosphorus digestibility. Diet flowability was reduced in the 30% DDGS diets compared with the control corn/soybean meal diet and was lowest in the diet containing finely ground DDGS (determined by measuring the drained angle of repose). When the flowability of these diets was determined using the poured angle of repose as the measurement criteria, there were no differences in flowability between the control and 30% DDGS diets nor among diets containing different DDGS particle sizes.
Particle size.
Particle size among DDGS sources is highly variable, averaging 660 µm and with a standard deviation of 440 µm (Liu, 2008). Particle size not only contributes to DDGS flow properties (Ganesan et al., 2008a,b,c) but also affects metabolizable energy (ME) content and nutrient digestibility (Mendoza et al., 2010).To further evaluate the effects of DDGS particle size on ME content and nutrient digestibility for growing pigs, Liu et al. (2012) determined the ME content and nutrient digestibility of the same source of DDGS ground to three particle sizes: coarse (818 µm), medium (594 µm) and fine (308 µm). They also evaluated the flowability of diets containing 30% DDGS.
As expected, the ME content of DDGS improved as the particle size was reduced: Each 25 µm reduction in average particle size (between 818 and 308 µm) increased the ME content of the diet by 13.5 kcal/kg of dry matter. However, there were no effects of DDGS particle size on nitrogen and phosphorus digestibility. Diet flowability was reduced in the 30% DDGS diets compared with the control corn/soybean meal diet and was lowest in the diet containing finely ground DDGS (determined by measuring the drained angle of repose). When the flowability of these diets was determined using the poured angle of repose as the measurement criteria, there were no differences in flowability between the control and 30% DDGS diets nor among diets containing different DDGS particle sizes.
Minimizing molds, mycotoxins
Toxigenic fungal species of molds can develop on grains while growing in fields before harvest as well as during storage after harvest (Suleiman et al., 2013). Consequently, fungal species are often classified as field fungi or storage fungi (Barney et al., 1995). Field fungi can infect corn grains and produce mycotoxins before harvest at moisture contents between 22% and 33%, at relative humidity exceeding 80% and over a wide range of temperatures of 10-35°C (Williams and MacDonald, 1983; Montross et al., 1999).Most field fungi do not survive during storage, but some species can continue to grow under appropriate storage conditions (Sanchis et al., 1982). Storage fungi also originate from the field and can replace field molds that infected corn grain prior to harvest (Reed et al., 2007). As shown in Table 2, storage fungi require relative humidity of more than 70% and moisture content greater than 12% for corn grain (Montross et al., 1999).
However, due to the unique physical and chemical properties of DDGS, it is unknown if these relative humidity and moisture conditions apply similarly as for corn grain. In fact, DDGS may be more susceptible to mold growth than corn grain because mechanical damage of corn grain during and after harvest can provide entry for fungal spores (Dharmaputra et al., 1994), and broken corn kernels and foreign material promote growth of storage molds (Sone, 2001).
Lipid oxidation
Effects of feeding oxidized lipids to pigs.
Corn DDGS contains one of the highest lipid concentrations among the most common feed ingredients used in animal diets around the world. Lipid oxidation is a complex chemical chain reaction induced by heat, oxygen, moisture and transition metals (e.g., copper and iron), where free radicals are converted to toxic aldehydes and other oxidation compounds (Shurson et al., 2015).The corn oil present in DDGS consists primarily of polyunsaturated fatty acids, particularly linoleic acid (C18:2, 58%), which is highly susceptible to oxidation (Frankel et al., 1984). Heating lipids at relatively high temperatures produces large quantities of secondary lipid oxidation products, including aldehydes, carbonyls and ketones (Esterbauer et al., 1991). The drying temperatures used to produce DDGS can be up to 500°C, which makes the oil in DDGS susceptible to lipid oxidation. All of the pro-oxidation conditions (heat, oxygen, moisture and transition minerals) are present in ethanol plants that produce DDGS, and DDGS may be further exposed to these factors during the transport, storage and manufacturing of complete feeds in commercial feed mills. Therefore, there is some concern about the extent of oxidation in DDGS and its effects on pig growth performance and health.
Feeding oxidized lipids to pigs and broilers has been shown to reduce growth performance and increase oxidative stress. Hung et al. (2017) conducted a meta-analysis using swine and poultry data from 29 publications that showed average reductions of 5% in average daily gain, 3% in average daily feed intake, 2% in gain:feed and 52% in serum of plasma vitamin E while increasing serum thiobarbituric acid reactive substances (TBARS) 120% across all studies.
Recent reviews by Kerr et al. (2015) and Shurson et al. (2015) provide a comprehensive summary of the biological effects of feeding oxidized lipids to swine and poultry, along with the challenges of measuring lipid oxidation and interpreting the results. Hence, some swine feeding trials (Song et al., 2013; Song et al., 2014; Hanson et al., 2015a) have shown inconsistent growth performance responses from feeding pigs a highly oxidized DDGS diet.
Lipid oxidation among DDGS sources.
Song and Shurson (2013) evaluated measures of lipid oxidation and color for 31 corn DDGS sources obtained from ethanol plants in nine U.S. states and compared these values with a reference sample of corn (Table 3). Peroxide value and TBARS are two common measures of lipid peroxidation the feed industry has used for many years. However, these oxidation indicators have several limitations — like all other measures of oxidation — and, therefore, are not always reflective of the true extent of oxidation of lipids (Hung et al., 2017; Shurson et al., 2015).
Peroxide value is used to estimate the extent of peroxidation during the initiation phase of the oxidation process. The value of the DDGS samples was highly variable, with a minimum value of 4.2 mEq and a maximum value of 84.1 mEq/ kg of oil, for a coefficient of variation (CV) of 97.5%.
The TBARS value is used as an estimate of the extent of lipid oxidation during the propagation phase of oxidation, which is when the majority of aldehydes are produced. There was less variability (CV = 43.6%) in TBARS values among DDGS sources than peroxide values and ranged from 1.0 to 5.2 ng malondialdehyde (MDA) equivalents per milligramof oil. Peroxide values and TBARS were both greater in DDGS samples than the corn reference values. This was expected because of the thermal processing involved in producing DDGS.
Moderate negative correlations were observed for colorometric measures between L* and peroxide value (r = -0.63) and b* and peroxide value (r = -0.57), with slightly greater negative correlations between L* and TBARS (r = -0.73) and b* and TBARS (r = -0.67). These results suggest that darker-colored and less-yellow DDGS samples may be more oxidized. However, DDGS color is affected by many factors and should not be used as a definitive measure of the ex- tent of corn oil oxidation in DDGS.
Subsequent studies involving the most oxidized DDGS source fed to wean/finish pigs (Song et al., 2014) and sows and their offspring through the nursery phase (Hanson et al., 2016) showed no detrimental effects on growth performance. The lack of growth performance responses in these studies may have been a result of the naturally high concentrations of antioxidant compounds (tocopherols, ferulic acid, lutein and zeaxanthin) present in DDGS (Shurson, 2017) and conversion of sulfur compounds into endogenous antioxidants.
Use of commercial antioxidants to minimize lipid oxidation.
Synthetic antioxidants are commercially available and are used to minimize oxidation in feed fats and oils (Valenzuela et al., 2002; Chen et al., 2014). The most commonly used synthetic antioxidants include t- butyl-4-hydroxyanisole, 2,6-di-t-butyl- hydroxytoluene, t-butylhydroquinone (TBHQ), ethoxyquin and 2,6-di-ter-bu- tyl-4-hydroxymethyl-phenol (Guo et al., 2006).Only one study has been published evaluating the effectiveness of adding synthetic antioxidants to high-oil (13% crude fat) and low-oil (5% crude fat) DDGS (Hanson et al., 2015b). Samples of these two DDGS sources contained either no added synthetic antioxidants (control), 1,000 mg/kg of TBHQ (Rendox, Kemin Industries) or 1,500 mg/kg of ethoxyquin and TBHQ (Santoquin, Novus International). After antioxidants were added, samples were stored at 38°C and 90% relative humidity in a controlled environmental chamber for 28 days. Sub-samples were collected on days 0, 14 and 28 to determine the extent of lipid oxidation at each time point.
Results of this study showed that significant lipid oxidation occurred under these storage conditions (Table 4). Oxidation increased during the 28-day storage period, and the extent of oxidation was greatest in the high-oil DDGS source. However, the addition of either Rendox or Santoquin to either the high-oil or low-oil DDGS sources reduced oxidation by about 50%. Therefore, the results show that the addition of either commercially available antioxidant is effective in reducing lipid oxidation in DDGS when stored up to 28 days in hot, humid conditions. Further, the moisture content of DDGS sources increased from 10.2% to 21.4% during the 28-day storage period, which led to significant mold growth in all samples.
Conclusions
The physical and chemical characteristics of DDGS can cause challenges in handling and storage. The particle size among DDGS sources is highly variable (660 µm + 440 µm), which contributes to its flow properties while also affecting ME content and nutrient digestibility.
Reducing the moisture content to less than 10% seems to create the greatest improvement in DDGS flow rate, while adding flow agents (DMX-7, calcium carbonate and zeolite) appears to provide no benefits. Feed bin design affects flow rate during discharge of meal diets containing 40% DDGS, and installing passive agitators increases feed flow in all bin designs.
Bulk density varies from 391 to 590 kg/cu. m among DDGS sources. Particle segregation occurs during filling, where the finer, smaller and denser particles are concentrated in the center of the hopper, while the larger, coarser and less-dense particles are concentrated on the hopper sides. This particle distribution should also be considered when sampling DDGS for nutrient analysis, because the location of sampling can influence the mixture of segregated particles and affect the analytical results.
The hygroscopic properties of DDGS cause it to accumulate moisture over time, which could encourage mold growth and mycotoxin production during extended storage periods under humid conditions.
The extent of heating during the drying process used to produce DDGS can cause lipid oxidation, which may lead to reductions in pig growth performance, but studies have shown inconsistent responses. However, adding commercial antioxidants can reduce oil oxidation in DDGS during storage under high temperature and humidity conditions.
Reducing the moisture content to less than 10% seems to create the greatest improvement in DDGS flow rate, while adding flow agents (DMX-7, calcium carbonate and zeolite) appears to provide no benefits. Feed bin design affects flow rate during discharge of meal diets containing 40% DDGS, and installing passive agitators increases feed flow in all bin designs.
Bulk density varies from 391 to 590 kg/cu. m among DDGS sources. Particle segregation occurs during filling, where the finer, smaller and denser particles are concentrated in the center of the hopper, while the larger, coarser and less-dense particles are concentrated on the hopper sides. This particle distribution should also be considered when sampling DDGS for nutrient analysis, because the location of sampling can influence the mixture of segregated particles and affect the analytical results.
The hygroscopic properties of DDGS cause it to accumulate moisture over time, which could encourage mold growth and mycotoxin production during extended storage periods under humid conditions.
The extent of heating during the drying process used to produce DDGS can cause lipid oxidation, which may lead to reductions in pig growth performance, but studies have shown inconsistent responses. However, adding commercial antioxidants can reduce oil oxidation in DDGS during storage under high temperature and humidity conditions.
References
References are available upon request from tim.lundeen@farmprogress.com.
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