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Effect of Moisture Content and Soluble Level on the Physical, Chemical, and Flow Properties of Distillers Dried Grains With Solubles (DDGS)

August 14, 2008

By Ganesan, V Muthukumarappan, K; Rosentrater, K A

ABSTRACT Distillers dried grains with solubles (DDGS) is a bulk material that has been widely used as a protein source for ruminants and nonruminants for more than two decades. DDGS is the nonfermentable processing residue (i.e., protein, fiber, fat, and ash) from fuel ethanol manufacturing. With the exponential growth of the fuel ethanol industry in the past several years, significant quantities (

The production of ethanol from corn grain is a continually evolving process as new discoveries are made that can make the process more efficient and cost-effective. The actual production of ethanol requires only the starch portion of the grain; the other materials including protein, fiber, and oil are superfluous to the process. Distillers dried grains (DDG), condensed distillers solubles (CDS), or a combination product, distillers dried grains with solubles (DDGS), are coproducts (ethanol production yields

CDS is commonly termed syrup. In most plants, it is added back to the wet distillers grain. CDS is a tan to brownish, freeflowing to semi-solid liquid, with a digestible energy (DE) value of

Distillers dried grain with solubles (DDGS) have been widely recognized as a valuable source of energy, protein, water-soluble vitamins, and minerals for animal feeds as well. DDGS often contains 86-93% db dry matter, 26-34% db crude protein, and 3-13% db fat (Rosentrater and Muthukumarappan 2006). With the tremendous growth of the ethanol industry in the United States in the past decade, significant quantities of DDGS are now being produced. In 2006- 2007, DDGS production was forecasted to be

Furthermore, the marketing of distillers grain products can be hampered by inconsistencies in physical properties, both within a single plant over time and as well as between plants. Quantification of relevant physical and chemical properties is important because DDGS storage and flow behavior will depend to a large extent on these characteristics of DDGS, as well as external conditions such as temperature, relative humidity, and compression pressures. At times, small differences in these can cause caking and bridging of DDGS and can result in a great disparity in flowability. As there is only limited information available regarding the handling and flow behavior of DDGS, there is a critical need for this data.

Material flow is the direct consequence of a combination of the physical and chemical properties of the material, environmental conditions, equipment used in handling and transport, and storage time and type (Prescott and Barnum 2000). Some of the factors that affect flowability include moisture content, humidity, temperature, pressure, fat content, particle size, and addition of flow agents.

Moisture plays a key role in determining flow properties of a material. Most organic materials, being hygroscopic in nature, gain or lose moisture when they are exposed to various humidity conditions. This leads to physical and chemical changes in the material itself. Moisture sorption is coupled with increased cohesiveness, chiefly because of interparticle liquid bridge formation. Thus, moisture content is a critical variable that affects the cohesive strength and arching ability of bulk materials (Johanson 1978). In the past few decades, many research studies have examined the effect of moisture content on physical and flow properties of granular solids and powders (Craik and Miller 1958; Johanson 1978; Moreyra and Peleg 1981; Hollenbach et al 1983; Marinelli and Carson 1992; Duffy and Puri 1994; Yan and Barbosa 1997). Moisture is also important because it influences microbial growth and thus affects shelf life during storage.

There is a lack of not only information, but also understanding regarding DDGS flowability. This knowledge is considered to be critical to the successful marketing and use of DDGS by the livestock feed industry. Therefore, the objective of this study was to examine the effect of various moisture contents on the physical and chemical characteristics of DDGS with varying soluble levels. Keeping in mind that variations exist in the nutritional and physical properties of DDGS, the effect of four levels of solubles (10, 15, 20, and 25% db) and five levels of moisture content (10, 15, 20, 25, and 30% db) on DDGS flowability were investigated.

MATERIALS AND METHODS

Samples of CDS and DDG were obtained from the Dakota Ethanol Plant (Wentworth, SD) and were stored in sealed plastic buckets (the DDG at room temperature and the CDS at 4[degrees]C +- 1) until needed. The moisture content of each was determined by airoven drying using Approved Method 44-19 at 135[degrees]C for 2 hr (AACC International 2000). Triplicate determinations were made for each sample.

Generally, the term solubles refers to substances suspended or dissolved in a liquid. In other words, soluble matter is in solution and is present as a single phase. Thus, it cannot be physically removed without a phase change (Davis and Cornwell 1991). Suspended solids, on the other hand, can be physically removed through filtration, centrifugation, or settling. Solubles in this study were the nonwater portion of CDS that passed through a filter media, while insolubles were the nonwater portion retained on a filter. The soluble solid levels of both DDG and CDS were determined following the methodology of Ganesan et al (2006). Accordingly, DDGS with four different soluble levels (10, 15, 20, and 25% db) were prepared by blending specific amounts of CDS to DDG using a laboratory-scale mixer (model D300, Hobart Corporation, Troy, OH) for 10 min.

After preparing DDGS with the various soluble levels, the moisture content of each mixture was determined as discussed above. Target moisture levels for the DDGS were achieved by either adding appropriate quantities of water or by drying in an oven at 1000C for an appropriate length of time. After adding water, the DDGS samples were blended in the mixer for 10 min to attain uniformity. In the case where DDGS had to be dried, DDGS sample of

Particle-size distribution was measured by the S3 19.3 method (ASAE 2003) using a Rotap sieve analyzer (model RX-29, W.S. Tyler, Mentor, OH) and appropriate sieving screens. Five replicates were measured for each treatment combination.

Flow Property Measurement by Carr Indices

Carr (1965) described a number of standard procedures that permit the evaluation of flow characteristics of granular materials. These procedures also provide information that is crucial to determine and understand possible flow problems of these materials. There are two general types of observed flow behavior: free-flow and floodable flow/flushing. A free-flowing material tends to flow in a steady and consistent way, as individual particles. A nonflowing material will tend to flow en masse or as agglomerated particles. Flushing is an unstable, liquid-like flow that is caused by excess air entrainment. Floodable flow can be discontinuous, gushing, uncontrollable, and spattering, which often produces process instabilities. It can even be caused by the fluidization of a mass of particles by air. To evaluate the flowability and floodability of granular solids, Carr (1965) tested over 2,800 dry material samples and created a point score (index) system.

The point score system for flowability was divided into seven categories, ranging from very good to very bad. The floodability point score system was divided into five categories, ranging from very high to would not flush. A powder characteristic tester (model PTR, Hosokawa Micron Powder Systems, Summit, NJ) (Fig. 1) was used to measure the flow properties, also known as Carr Indices, of DDGS with various soluble levels and moisture contents. This tester follows the procedure described by Carr (ASTM D6393). The properties measured included angle of repose (AoR), compressibility, angle of spatula (AoS), uniformity, flowability index, angle of fall (AoF), angle of difference (AoD), dispersibility, and floodability index. All flowability parameters were conducted in triplicate and were tested at room temperature (25[degrees]C+- 1).

Angle of repose is defined as the angle between the horizontal and the slope of a heap of granular material dropped from a given elevation (the standard height is 6.8 cm). Angle of repose corresponds qualitatively to the flow properties of that material and is a direct indication of the potential flowability. A wide acute angle indicates a material with poor flowability, but a narrow angle indicates good flowability. Materials with an angle of repose 45[degrees] would probably not flow easily (Carr 1965).

The compressibility of a material can be computed by the equation C= 100 (P – A)IP, where C is the compressibility (%), P is the packed bulk density (PBD) (kg/cm3), and A is the aerated bulk density (ABD) (kg/cm3). The greater the compressibility of a bulk solid, the less flowable it is. The borderline between free-flowing and nonflowing is a compressibility of

AoS is a property that provides a relative angle of internal friction, or angle of rupture, for a dry material. A highly flowable material will have an obtuse AoS. It is an indirect measurement of cohesion, surface area, size, shape, uniformity, fluidity, deformability, and porosity of the material. Carr (1965) categorized the degree of flowability of materials with AoS 60[degrees] as bad.

Fig. 1. Powder characteristics tester for Carr indices.

The coefficient of uniformity is used with granular materials on which an effective surface cohesion cannot be measured. It is a ratio obtained between the width of a sieve opening that 60% of the sample will pass through, and the width of a sieve opening that only 10% will pass through. The more uniform a mass of particles is in both shape and size, the closer the ratio is to 1 .0 and the more flowable it is likely to be. The uniformity coefficient is thus an indirect measurement of size, shape, and compressibility of the material (Carr 1965). Materials with uniformity 17 have been categorized as less flowable materials.

The evaluation of the flowability characteristics of a granular material involve the use of the four properties detailed above. After measuring the individual values above, each was assigned an index value (Carr 1965). The sum of the AoR, compressibility, AoS, and uniformity indices constitute the flowability index. A maximum flowability index value of 100 indicates the material flowability is very good, while a low index indicates the material flowability is very bad.

The flowability index also partly indicates the potential floodability of a bulk material. The higher the flowability index, the higher the risk that the material may flood (Carr 1965).

The angle of fall (AoF) is the new angle of repose (AoR) formed when energy was given to the material through impact or impulse. The more floodable the material is, the lower its original AoR and its AoF. The AoF is thus an indirect measure of fluidity, shape, size, uniformity, and cohesion (Carr 1965). Materials with an AoF 40[degrees] are categorized as less floodable.

Angle of difference (AoD) is the difference between the AoR and AoF. The larger the AoD, the larger will be the potential of the material for flooding and fluidizing. It is an indirect measure of fluidity, surface area, and cohesion (Carr 1965). Materials with an AoD 17[degrees] are categorized as highly floodable.

Dispersibility and floodability are interrelated. The higher the dispersibility index of a material, the dustier and the more floodable it can be. Carr (1965) categorized materials with dispersibility 20 are categorized as highly floodable.

The evaluation of potential floodability of a material involves the use of the flowability index, AoF, AoD, and dispersibility indices. The sum of these is known as the floodability index. A maximum index value of 100 indicates that the chance of floodability is very high; a low value indicates that the material will not flush. Materials with a floodability index > 60 should easily flood, while those with a floodability index of

Chemical Properties

For chemical analysis, all DDGS samples were ground and dried at 135[degrees]C in an air oven for 2 hr following Approved Method 44- 19 (AACC International 2000). Chemical analysis consisted of fat and protein content. As protein and fat often contribute to flowability problems in granular solids (Morr 1990; Perez and Flores 1997; Fitzpatrick et al 2004), these constituents were measured in this study. Fat content was measured using the Goldfisch extraction as in AACC Approved Method 30-20. Protein content was measured using an elemental analyzer (Flash EA 1112 series, ThermoQuest, Rodano, Italy), based on dynamic flash combustion as in AACC Approved Method 46-30. Triplicate measurements were made on each sample for fat and protein analysis.

Statistical Analyses

DDGS with four soluble levels (10, 15, 20, and 25% db) and five moisture contents (10, 15, 20, 25, and 30% db) were prepared, which resulted in a total of 20 treatment combinations (a 4 x 5 factorial design) for the study. Each treatment was analyzed in triplicate for all physical and chemical properties, which resulted in a total of 60 experimental runs for each property. Statistical analyses on the collected data were performed using general linear models (GLM), LSD, and correlation analysis (SAS Institute, Cary, NC) using a Type I error rate (a) of 0.05 to find the significant differences between the treatments.

RESULTS AND DISCUSSION

Physical and Chemical Property Analysis

Results obtained for the DDGS physical and chemical properties are provided in Table I. Statistical analysis showed that differences did exist between treatments (i.e., soluble level and moisture content). From the results, we observed that there were significant differences in geometric mean diameter (GMD) and geometric standard deviation (GSD) as the soluble levels and moisture contents varied. Specific trends, however, were not readily evident. The GMD values ranged from 0.67 mm (10% solubles and 10% moisture) to 1.17 mm (25% solubles and 25% moisture). The GSD values, on the other hand, ranged from 0.08 mm (25% solubles and 25% moisture) to 0.13 mm (15% solubles with 10 and 15% moisture).

Visually the DDGS samples seemed to get darker in appearance as CDS level was increased; this observation was supported by the resulting data. From the chromameter results, it was observed that for all moisture content levels, the brightness (L*) of the DDGS decreased with an increase in soluble levels. Some were significant and some were not. With an increase in moisture content, the L* values did not show any noticeable trends. The redgreen (a*) value increased with an increase in soluble level, except for the 15% db solubles with 10% db moisture content. On the other hand, an increase in moisture content did not show any effect on a* values. The blue-yellow (b*) value did not show any obvious trends with an increase in soluble or moisture content levels, even though there were statistically significant differences between the treatments. The differences in L* and a* values of DDGS were probably due to the addition of CDS, which is brown in color. Fat content of DDGS did not show any significant trends as soluble level increased, but the protein content decreased with an increase in soluble levels (Table I). This occurred because of the lower protein content of the CDS. The amount of crude protein in DDGS declines slightly as more solubles are blended with DDG to produce DDGS. The fat content of the solubles (20 and 25% db) are substantially higher than the 10 and 15% db solubles contained in the DDG. Even so, there was no significant difference in fat content between 10 and 15% db solubles, nor between 20 and 25% db solubles. There was no significant difference in the protein content between 10 and 15% db solubles. Nutritional properties of CDS can vary greatly because the soluble content will differ between batches, operators, and production plants (Rosentrater and Muthukumarappan 2006).

Flow Property Analysis

The statistical analyses of all collected Carr indices for the DDGS samples are shown in Table II. AoR of the DDGS samples in the study did not show any noticeable trends with an increase in soluble or moisture content levels. A few of the treatment combinations were statistically significant but many were not. Angle of repose values for all the treatment combinations fell in the 4146[degrees] range. None of the treatment combinations had AoR

Aerated bulk density (ABD) values ranged from 0.507 g/cm3 (31.65 lb/ft3) to 0.601 g/cm3 (37.52 lb/ft3). These correspond fairly well with other published values (24.3-31.3 lb/ft3) for commercial DDGS samples (Rosentrater 2006). In general, it appears that as moisture content increased, the ABD decreased somewhat for most of the soluble levels; many of these changes were significant. In fact, there was an 18.5% difference between the lowest and highest ABD values. Packed bulk density (PBD) results exhibited similar trends but were 2.5-6.1% higher.

Compressibility, which accounts for both ABD and PBD, increased with an increase in moisture content level for most of the treatment combinations. But an increase in soluble content did not show many consistent, significant effects on the compressibility values. Materials with a compressibility value of 25% are categorized as less flowable. The mean value of compressibility for each treatment combination was

In this study, moisture content and soluble level did not have any significant effect on AoS. The mean AoS values were 55.9062.63[degrees], which fall into the normal category of flowable material.

There was no clear trend found with an increase in either soluble or moisture content. But a few treatment combinations were statistically significant. In this study, the uniformity values varied from 2.0 to 2.83, which makes DDGS fall into the category of very good flowability.

We observed that soluble and moisture content levels had no effect on AoF. None of the levels were statistically significant. The mean AoF values were 35.03-37.77[degrees], which falls into a fairly high floodable category.

AoD of DDGS did not show any obvious trends with an increase in soluble or moisture content. Some of the treatment combinations did have a statistically significant effect on AoD. The mean AoD values were 4.60-10.40[degrees], which fall between the floodability categories of would not flush and tend to flush. Flushing of bulk solids is characterized by an uncontrollable discharge. Bulk solids which fall under would not flush category will not require any flush prevention measures. On the other hand, bulk solids of tends to flush category will sometimes require a rotary seal to prevent flushing.

TABLE I

Effect of Moisture Content and Soluble Level on Physical and Chemical Characteristics of DDGS”

For dispersibility, it does appear that the flowability generally declined significantly (P

Flowability index of DDGS was affected by both soluble and moisture content levels. According to flowability index, it does appear that DDGS flowability generally declined significantly (P

The mean values of flowability index were 74-80, which fall between the flowability categories of good and fairly good. This indicates the DDGS prepared with various levels of solubles and moisture contents was not necessarily a good flowing material, but it was not necessarily a badly flowing material either. So, sometimes vibrations would be required to break the clumps and bridges.

In this study, soluble and moisture content levels affected the floodability indices of DDGS. According to floodability index, it does appear that the flowability generally declined significantly (P

Property Relationships

Correlation analysis was then used to find the potential linear relationships between all measured color, chemical, and flow properties of DDGS.

TABLE II

Effect of Moisture Content and Soluble Level on Distillers Dried Grains with Solubles (DDGS) Carr Indices”

Table III shows the results obtained from this analysis. Nine variable combinations had a correlation coefficient (r) > |0.60|, 7 were > |0.70|, 3 were > |0.80|, and 5 were > |0.90|. There was no linear relationship between soluble level and moisture content. Some of the highly correlated relationships, such as AoR and AoD, ABD and PBD, AoF and AoD, L* and a*, L* and b*, L* and soluble level, moisture and compressibility, and moisture and dispersibility were expected before analysis. AoD is the difference between the values of AoR and AoF, which explained the high correlation between them. The high correlation between ABD and PBD was due to the fact that both of these parameters measure the mass of DDGS per volume of a sample container. As discussed above, the higher soluble level decreased the brightness of the DDGS. Therefore, a high negative correlation was observed between L* and soluble level. In the mean time, a higher soluble level would increase the redness (a*) of the DDGS, which resulted in the high positive correlation between them. In general, increasing the moisture content would increase the compressibility and decrease the dispersibility of any material. So there was a fairly high correlation between moisture, compressibility, and dispersibility of the DDGS. The higher the compressibility of the DDGS, the lower should be its flowability. Thus, there was a fairly high negative correlation between the compressibility and flow index of the DDGS.

CONCLUSIONS

To date, much research work has focused on chemical and nutrient analysis of distillers grains, but not on the physical or flow properties. To improve the use of these coproduct feed materials, there is an urgent need to study flow behavior, as well as storage and handling characteristics. This information can be used to help optimize the utilization of these materials and thus improve process economics for ethanol producers, use by livestock feeders, and ultimately the rural economy of the country. By determining the Carr indices, we have shown that increases in soluble and moisture content levels affect the flowability of DDGS, which was determined to be not a highly flowable or floodable material. Soluble level had a significant effect on the color (L*a*b*) values of DDGS. Color values were not influenced by the moisture content alone, but rather the interaction between moisture and soluble levels. Protein content of DDGS decreased with an increase in soluble levels. Fat content of DDGS did not show any effect due to the increase in soluble levels. Soluble level and moisture content had significant effects on several physical parameters, including ABD, PBD, and compressibility of DDGS. Although there was little significant effect due to soluble level or moisture content on AoR, AoF, and AoD, according to dispersibility, flowability index, and floodability index, DDGS flowability generally declined with an increase in moisture content for most of the soluble levels under consideration. This is the first study of its kind to quantify flowability of DDGS. All in all, the results indicated that the flowability of DDGS was not extremely bad, nor was it extremely good, for the DDGS samples used in this study. These results do point to the need for follow-up investigations that should target consolidation behavior (i.e., compaction over time) as well as shear properties between the DDGS particles themselves. Future work should also focus on quantifying flowability on a plant scale so that laboratory scale results can be validated.

TABLE III

Correlation Results for Relationships Between Soluble Level, Moisture Content, Physical, and Chemical Properties of DDGS”

ACKNOWLEDGMENTS

We would like to thank the Dakota Ethanol Plant (Wentworth, SD) which contributed samples for the study, and the South Dakota Corn Utilization Council (SDCUC), the South Dakota Agricultural Experimental Station (AES), and the USDA-ARS for financial support. Cereal Chem. 85(4):464-470

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[Received August 29, 2007. Accepted February 4, 2008.

V. Ganesan,1 K. Muthukumarappan,2 and K. A. Rosentrater3,4

1 Graduate research assistant. Department of Agricultural and Biosystems Engineering, South Dakota State University, Brookings, SD.

2 Professor, Department of Agricultural and Biosystems Engineering, South Dakota State University, Brookings, SD.

3 Agricultural and Bioprocess Engineer, USDA-ARS. North Central Agricultural Research Laboratory, Brookings, SD. Mention of a trade name, propriety product or specific equipment does not constitute a guarantee or warranty by the United States Department of Agriculture and does not imply approval of a product to the exclusion of others that may be suitable.

4 Corresponding author. Phone: 604-693-3241. Fax: 605-693-5240. E- mail address: kurt. rosentrater(R) ars.usda.gov

doi:10.1094/CCHEM-85-4-0464

(c) 2008 AACC International, Inc.

Copyright American Association of Cereal Chemists Jul/Aug 2008

(c) 2008 Cereal Chemistry. Provided by ProQuest LLC. All rights Reserved.




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