(56c) Development of a Pilot-Scale Phytate Extraction System from Ethanol Coproducts

Commercial production of fuel ethanol involves breaking down the starch present in corn to simple sugars, followed by the bioconversion of sugar molecules to ethanol catalyzed by yeast. The downstream processing involves the recovery of ethanol by distillation and the residual coproducts are further processed to serve as animal feed. In the dry-grind process, which accounts for over 70% of the total plants in the United States, the residual components are separated after the fermentation and distillation.

When coproducts are fed as an energy source to animals (above 15 to 20% of diet), protein and phosphorus are overfed. The environmental impact of feeding protein and phosphorus-rich materials to animals is related to one of the most pervasive forms of pollution from modern agriculture, as animal manure is applied to the soil. Phytate is the primary storage form of phosphorus and inositol in plants. The bioavailability of phosphorus bound to phytate is low for non-ruminants, and thus the phytate-derived phosphate ingested by these species (e.g. poultry and swine) is largely excreted, resulting in both nutritional deficiencies and environmental pollution. On the other hand, phytate is a widely applied valuable chemical as human nutrition, pharmaceutics, cosmetics, chelating chemical, and the raw material to produce inositol (Vitamin B8). It is primarily produced from rice bran in Asia and imported to the US

This creates a great opportunity for the US ethanol industry because extracting phytate as a new product from corn ethanol coproducts, knowing that corn germ has up to 5% phytate in its composition, can create additional revenue while increasing the feeding value of coproducts and decrease the phosphorus excretion in the animal manure.

Methods

Samples from four companies in the Midwest (Iowa, Illinois, and Minnesota) were obtained. Samples are stored at -20 ºC prior to analysis and use.

Total-P and Phosphate-P are determined using P-ash method, in which digested samples are analyzed with a colorimetric assay of P in the digests. Samples are first dried at 105 ^oC until constant dry weight, and subsequently, they will be ashed at 550 ^oC. Concentrated HNO₃ (7 mol L^-1) will be used to dissolve the inorganic ash residue. Spectrophotometric assays are then performed with the molybdenum blue method. An enzymatic method derived from De Boland, Garner, & O'Dell (1975) is used for determination of phytate phosphorus.

Five macroporous resins (IRA series) and the fine resin (AG-1 series) were evaluated for the phytate adsorption capacity, high selectivity, high purity after desorption and low maintenance requirements. All of the resins selected for this study are alkali-type and have strong phytate adsorption capacity. The selection of the resin is one of the most important factors to be considered in the extraction process because all of the extraction process designs are based upon the resin selection.

The adsorption isotherms predict the maximum adsorption capacity of each resin. They are expressed in amount of substrate adsorbed in equilibrium onto the resin (qe) vs. the residual substrate in solution in equilibrium (Ce). Freundlich, Langmuir and Dubinin-Radushkevich adsorption models were used to estimate isotherm parameters.

Results and discussion

WS, containing undissolved corn fragments and residual yeast cells, present a moisture content of 87.94%. The TS, after the solid-liquid separation from WS, has the moisture content of 95.41%. The CDS, concentrated from TS through series of evaporators in order to reduce its moisture content, achieves the moisture content of 70.48%. On regards to the liquid fraction, mainly due to the increase in suspended solids, viscosity and osmotic pressure, the remaining water in CDS was difficult to remove by evaporation (Belyea et al., 2006), achieving moisture content levels close to 70%. The CDS is then mixed with the wet distillers grains to produce WDGS (M.C. 49.75%), which is then further dried to produce DDGS, at the moisture content of 14.92%. Usually, limited by cost in transportation for marketing, DDGS has a low moisture content ranging from 10% to 15%, which also prevents it to become moldy and unusable. Evaporation and drying are one of the most energy intensive steps in the corn ethanol processing.

WS has a total phosphorus concentration of 12.02 mg/g, of which 40% is found as phytate-phosphorus, at a concentration of 4.81 mg/g. After the first solid-liquid separation, the concentrations of total and phytate phosphorus (23.57 mg/g and 10.11 mg/g, respectively) in the TS are over doubled. The water removal step on TS to produce CDS maintains similar levels for total phosphorus (23.77 mg/g) and a decrease on phytate phosphorus (9.41 mg/g). WDGS and DDGS showed similar concentrations of total phosphorus (10.11 mg/g and 10.59 mg/g, respectively), whereas DDGS had higher levels of phytate than WDGS (4.50 mg/g and 3.27 mg/g, respectively). Comparing the results reported at literature (Alkan-Ozkaynak et al., 2010), similar values were found, especially for fractions of TS (19.4 mg/g), CDS (18.8 mg/g), and WS (11.0 mg/g) in means of total phosphorus.

Phytate is found most concentrated in the small-size range of particles in TS, indicating that it is possible to solubilize or aggregate these small particles. Selecting the appropriate size range of particles is crucial in order to process only the most concentrated fraction, which will allow the most efficient adsorption operation. The fractions of 0.10 mm and below account for about 90% of the total phytate found in TS. Screening experiments were conducted to evaluate the pressure drop required to process TS and 0.10 mm-filtered TS. Our results showed that having a filtration step prior to the adsorption process may be energetically feasible, since this step decreases the required pressured drop of the adsorption process by 55%.

Thin stillage is a phytate-rich ethanol co-product and is the liquid fraction that is being evaluated as the raw material in the phytate extraction. It also has significant presence of phosphate and other types of phosphorus-based groups. Reactive-phosphorus (i.e. free orthophosphate) was also adsorbed onto the resin. The characteristics of the most effective resin are those with the capability to adsorb most of the phytate and the least amount of phosphate, when determining resin selectivity. IRA-400 (0.128 mg phytate/mg resin) and IRA-900 (0.126 mg phytate/mg resin) resins adsorbed the most phytate per mass of resin, but IRA-900 (0.733 phytate phosphorus / total phosphorus) showed more selectivity compared to the IRA-400 (0.661 phytate phosphorus / total phosphorus) resin from this screening experiment.

In order to reduce the operating cost of the adsorption process and to achieve technical feasibility, the optimal resin should have an extended lifespan. A study was conducted to evaluate the regeneration capacity of the 5 macroporous (IRA series) resins using NaCl (2 M) as desorbent and NaOH (150 g/L at a loading rate of 4 %) as a regenerating agent for 4 cycles of operation. The equilibrium concentrations (q_e) of phytate-P (mg/g resin) in all the four cycles for all the resins tested, were within 2.5 standard deviations from the mean, which is an acceptable deviation for this study. The adsorption- desorption capacity and selectivity were similar for all 4 cycles.

References

Alkan-Ozkaynak, A., Karthikeyan, K., & Roa-Espinosa, A. (2010). Reducing phosphorus concentration in animal feed coproducts from the corn distilling industry. Transactions of the ASABE, 53(4), 1287-1294.

Belyea, R., Rausch, K., & Tumbleson, M. (2004). Composition of corn and distillers dried grains with solubles from dry grind ethanol processing. Bioresource Technology, 94(3), 293-298.

De Boland, A. R., Garner, G. B., & O'Dell, B. L. (1975). Identification and properties of phytate in cereal grains and oilseed products. Journal of Agricultural and Food Chemistry, 23(6), 1186-1189.