Effect of Lactic Acid on Protein Solubilization and Starch Yield in Corn Wet‐Mill Steeping: A Study of Hybrid Effects

To better understand the role of lactic acid (LA) in corn wet‐milling, steeping studies were performed on different yellow dent corn hybrids using four different solutions containing LA, sulfur dioxide (SO₂), a combination of LA and SO₂, or no added chemicals. Although there was variation in protein solubilization among the hybrids, protein release was consistently higher when LA was included in the steepwater than when it was excluded (both with and without SO₂). Several groups have reported that starch recoveries are improved when steepwater contains LA. To explore the relationship between protein solubilization and starch yield as effected by LA, several yellow dent hybrids were steeped in 0.20% SO₂ and 0.50% LA‐0.20% SO₂ solutions and milled to recover starch by a 100‐g laboratory corn wet‐milling procedure. In all instances, both starch yields and protein solubilization were enhanced in solutions containing LA. These results support the hypothesis that direct dissolution of the endosperm protein matrix by LA contributes to the improved starch recoveries.     

Variability in Starch Acetylation Efficiency from Commercial Waxy Corn Hybrids

Raw material variability is common for starch processors and is responsible for increased processing costs. In this study, variability of starch acetylation due to hybrid influence was quantified. Six waxy corn (maize) hybrids from 1998 and five waxy corn hybrids from 1999 were wet‐milled in the laboratory. Starch obtained from each hybrid was modified according to a laboratory‐scale acetylation procedure. To evaluate reaction efficiency, reaction rate, acetyl content, pH, and amount of NaOH used were recorded for each reaction. After modification, a Rapid Visco Analyser (RVA) was used to characterize modified starches and determine differences in modified starches from different hybrids. Using the same acetylation protocol, reaction efficiencies were observed at 47–73%. Reaction efficiencies were significantly lower for 1998 hybrids (50.0%) compared with the efficiency observed for the same hybrids grown in 1999 (62.7%). Acetylated starch from 1999 had increased peak, trough and final viscosities and increased reaction efficiency as compared with acetylated starch from 1998. Differences in setback were observed among 1998 hybrids for acetylated samples. Differences in trough and final viscosity were observed among 1999 hybrids for acetylated and native (unmodified) samples. Differences in breakdown among 1999 hybrids also were observed for native samples.     

Yield,Protein Content,and Viscosity of Starch from Wet‐Milled Corn Hybrids as Influenced by Environmentally Induced Changes in Test Weight

Forty‐three yellow dent corn samples of five different hybrids varying in test weight and moisture content were obtained from 14 different locations in 1993. The locations for acquired samples were selected randomly to cover a wide range of test weights based on preliminary data from eight states of the corn belt where 94% of the U.S. corn crop was produced in 1993. Samples were wet‐milled using a 100‐g standard laboratory‐scale wet‐milling procedure. Protein content in starch and starch viscosity were determined. Starch yield, protein content in starch, and starch viscosity were not affected significantly by test weight.     

Hybrid-Dependent Effect of Lactic Acid on Corn Starch Yields

The effect of lactic acid on starch yields of different corn hybrids was determined by wet-milling 18 commercial corn hybrids at three levels of lactic acid. All 18 hybrid samples tested had higher starch yields when lactic acid was added to the steep solution, although the magnitude of the increased starch yields varied between 2.9 and 12.0%. The optimal lactic acid concentration for maximum starch recovery was found to be between 0.55 and 1.67% lactic acid, by wet-milling nine of the same 18 corn hybrids with seven levels of added lactic acid. Between 0.83 and 1.67% lactic acid, the starch yields of eight of the nine hybrids were constant (within ±0.5%). Results showed that the average starch yield across all hybrids decreased with a lactic acid concentration <0.55% and a lactic acid concentration >1.67%.     

Batch Steeping of Corn: Effects of Adding Lactic Acid and Sulfur Dioxide at Different Times on Starch Yields,Protein Contents,and Starch Pasting Properties

The effect of adding lactic acid and sulfur dioxide at different times from the start of batch steeping on corn starch yields was studied. Five commercial hybrids were steeped with 0.5% lactic acid or 0.2% sulfur dioxide added over the first 15 hr of steeping and wet-milled following a 100-g corn wet-milling procedure. No significant differences were observed in starch yields when lactic acid was added to the steep solution (SO₂ and water) from 0 hr (start of steeping) to 15 hr. Addition of SO₂ to the steep solution (lactic acid and water) resulted in significantly higher average starch yields when SO₂ was added between 5 and 15 hr compared with addition at 0 hr (SO₂ and lactic acid for full 24 hr of steeping). Based on the results of the first experiment, a second experiment was done in which one of five original hybrids was steeped for 24 hr, during which lactic acid or SO₂ was added until 23.9 hr (i.e., 5 min before milling) after the start of steeping. Similar results were found in the second experiment. Residual protein in starch samples did not exceed 0.85%. Steepwater protein content decreased with delays (16–20 hr) in adding either chemical to the steep solution. A significant effect on starch pasting properties of chemicals and duration of chemicals in steep-water was observed. Testing these findings using a larger scale (1,000 g) corn wet-milling procedure produced results similar to those obtained with the 100-g corn wet-milling procedure.     

A 10‐g Laboratory Wet‐Milling Procedure for Maize and Comparison With Larger Scale Laboratory Procedures

A very small scale laboratory procedure (≈10 g) is needed to test wet‐milling characteristics of corn when amounts of corn available for testing are quite limited. The objective of this study was to downscale 100‐g laboratory wet‐milling methods already widely used to measure wet‐milling properties of 10 g of corn. A Standard 100‐g procedure, a Modified 100‐g procedure, and an Experimental 10‐g procedure were compared using three corn hybrids with known differences in wet‐milling properties. All three procedures ranked most fraction yields (all except for germ) of the three hybrids the same. Germ separation was conducted differently for each procedure and probably accounts for these differences. Flotation and screening methods were likely affected by germ density and germ size, and hand‐picking the germ was efficient in recovering a pure germ fraction. The two 100‐g procedures were performed very similarly except for fiber recovery. The Modified 100‐g procedure was more efficient in recovering fiber because of intensive washing. Hybrid effects on the starch/gluten separation were more pronounced when the Experimental 10‐g procedure was used, which may allow for more discrimination among hybrids. Although most fraction yields are too small to run replicates for analytical tests, the Experimental 10‐g procedure will be useful in measuring milling efficiency of early generations of corn hybrids where limited samples are available, such as when valuable recombinant proteins are expressed for therapeutics and industrial enzymes.     

Comparison of Alkali and Conventional Corn Wet-Milling: 1-kg Procedures

An alkali corn wet-milling process was developed to evaluate the process as a method to produce high purity corn starch and coproducts with added value. Using a single hybrid (R1064 × LH59), the effects of alkali concentration (0.18–0.82% NaOH), time (29–61 min), and temperature (36–75°C) were investigated. Starch yield was not affected by steep time or temperature. Starch yield was optimal at 65.2% using 0.5% alkali. Increasing the concentration of alkali to 0.82% or decreasing it to 0.18% caused a decrease in starch yield of 8–10 percentage points. Other wet-milling products (fiber, germ, and gluten) also were affected. Steep conditions of 0.5% NaOH, 60 min, and 45°C gave optimal starch yield. Comparisons between alkali and sulfur dioxide wet-milling processes, using 1-kg sample size, were performed on 10 commercial yellow dent corn hybrids. The alkali process averaged 1.7 percentage points more starch than the sulfur dioxide process. Each hybrid had a higher starch yield when wet-milled with the alkali method. Alkali wet-milling produced pure corn starch with <0.30% protein (db).     

Development of an Ethanol Yield Procedure for Dry‐Grind Corn Processing

In 2008, the United States produced ethanol at a rate of 39.5 billion L/year; an additional 8.5 billion L/year capacity was under construction. Kernel composition and physical properties are not correlated with ethanol yield. A procedure that measured the potential of hybrids to produce ethanol would benefit corn seed companies, corn producers, and ethanol processors. The objective was to develop a laboratory procedure to measure ethanol yield from corn samples and evaluate the developed procedure for accuracy and precision. To determine parameters for routine analyses, effects of mill type, dry solids, and yeast addition were investigated separately followed by effects of fermentation time (T_f), glucoamylase dose, and yeast addition. Measurement of ethanol using HPLC and gravimetric (change in weight due to CO₂ loss) methods were compared. Using the procedure developed, ethanol yields for five diverse hybrids (dent, waxy, white, high oil, and high amylose) were measured. Effects of mill type, dry solids, T_f, glucoamylase dose, and yeast addition were significant (P < 0.05). The gravimetric method estimated higher yields (428 ± 10 L/tonne) than HPLC (405 ± 15 L/tonne) and had a higher level of precision. Both methods had coefficients of variations of <4% and gave similar conclusions. In the final procedure, we used corn (25 g/batch) liquefied with α‐amylase (60 min at 90°C) in 75 mL of distilled water. Simultaneous saccharification and fermentation was used (64 hr at 32°C) with glucoamylase and yeast. Gravimetric and HPLC methods measured differences in ethanol yield for the five hybrids (158–435 L/tonne). The method is suitable for routine testing of ethanol yield potential and as a reference method for verifying more rapid measurement techniques.     

Wet‐Milling and Dry‐Milling Properties of Dent Corn with Addition of Amylase Corn

A transgenic corn (amylase corn) has been developed that produces an endogenous α‐amylase that is activated in the presence of water and elevated temperature (>70°C). Wet‐ and dry‐milling characteristics of amylase corn were evaluated using laboratory wet‐ and dry‐milling procedures. Different amounts of amylase corn (0.1–10%) were added to dent corn (with the same genetic background as the amylase corn) as treatments. Samples were evaluated for wet‐ and dry‐milling fraction yields using 1‐kg laboratory procedures. Milling yields for all amylase corn treatments were compared with the control treatment (0% amylase corn or 100% dent corn). No significant differences were observed in wet‐ and dry‐milling yields between the control and the 0.1, 1, and 10% amylase corn treatments. Most of the amylase activity (77%) in wet‐milling fractions was detected in the protein fraction. In dry‐milling, amylase activity (68.8%) was detected in endosperm fractions (fines, small grits, and large grits).     

Evaluation of Waxy Grain Sorghum for Ethanol Production

The objective of this research was to investigate the fermentation performance of waxy grain sorghum for ethanol production. Twenty‐five waxy grain sorghum varieties were evaluated with a laboratory dry‐grind procedure. Total starch and amylose contents were measured following colorimetric procedures. Total starch and amylose contents ranged from 65.4 to 76.3% and from 5.5 to 7.3%, respectively. Fermentation efficiencies were in the range of 86.0–92.2%, corresponding to ethanol yields of 2.61–3.03 gallons/bushel. The advantages of using waxy sorghums for ethanol production include easier gelatinization and low viscosity during liquefaction, higher starch and protein digestibility, higher free amino nitrogen (FAN) content, and shorter fermentation times. The results showed a strong linear relationship between FAN content and fermentation rate. Fermentation rate increased as FAN content increased, especially during the first 30 hr of fermentation (R² = 0.90). Total starch content in distillers dried grains with solubles (DDGS) was less than 1% for all waxy varieties.     

Effect of Chemical Pretreatments and Lactic Acid on the Rate of Water Absorption and Starch Yield in Corn Wet-Milling

The relative effectiveness of dehulling, potassium hydroxide dipping (alkali concentrations 0.2, 0.5, and 1.0 %), and ethyl oleate spraying (aqueous emulsion 1%, v/v) to increase the rate of water absorption by dent and flint corn during steeping was compared with untreated corn samples. These pretreatments increased the water absorption rate of both hybrids when compared with the untreated control samples. To evaluate the observed increase, the diffusion coefficients of pretreated and untreated corn samples were estimated. Corn grains steeped in SO₂ aqueous solution and variable lactic acid concentrations (0.2, 0.5, and 1.0%, v/v) were performed. Absorption rates for lactic acid concentrations were ≈0.5% higher than those steeped only in SO₂ solution. This effect was more marked for dent than for flint corn. Corn samples pretreated with potassium hydroxide had lower starch yields than the control. However, the presence of lactic acid in steepwater increased the starch yield of dent and flint corn, particularly for the samples treated with alkaline solution. This procedure was particularly beneficial for flint corn. An effective release of the starch granules was achieved within 24 hr of steeping.     

Wet-Milling Characteristics of Selected Yellow Dent Corn Hybrids as Influenced by Storage Conditions

Three yellow dent corn hybrids (FR1064×LH59, FR600×FR1087, and FR618×LH123HT) from the 1994 crop, one hybrid (FR1064×LH59) from the 1995 crop, and two hybrids (FR1064×LH59 and FR618×LH123HT) from the 1996 crop were used to study the effect of storage time and storage temperature on starch yields. Samples of all of the corn hybrids were stored under one of two conditions: in a 4°C cold room or under a shed exposed to ambient conditions. The hybrids from the 1994, 1995, and 1996 crops were stored for up to 24, 12, and 3 months, respectively. No significant differences were found between starch yields of the hybrids with respect to storage time. However, there was a significant difference in hybrids from the 1994 samples. Starch yields of two of the three corn hybrids (from the 1994 crop) stored in the 4°C cold room were higher when compared to the starch yields of the same hybrids stored at ambient conditions.     

Dry‐Grind Processing of Corn with Endogenous Liquefaction Enzymes

An amylase corn has been developed that produces an α‐amylase enzyme that is activated in the presence of water at elevated temperatures (>70°C). Amylase corn in the dry‐grind process was evaluated and compared with the performance of exogenous amylases used in dry‐grind processing. Amylase corn (1–10% by weight) was added to dent corn (of the same genetic background as the amylase corn) as treatments and resulting samples were evaluated for dry‐grind ethanol fermentation using 150‐g and 3‐kg laboratory procedures. Ethanol concentrations during fermentation were compared with the control treatment (0% amylase corn addition or 100% dent corn) which was processed with a conventional amount of exogenous α‐amylase enzymes used in the dry‐grind corn process. The 1% amylase corn treatment (adding 1% amylase corn to dent corn) was sufficient to liquefy starch into dextrins. Following fermentation, ethanol concentrations from the 1% amylase corn treatment were similar to that of the control. Peak and breakdown viscosities of liquefied slurries for all amylase corn treatments were significantly higher than the control treatment. In contrast, final viscosities of liquefied slurries for all amylase corn treatments were lower than those of the control. Protein, fat, ash, and crude fiber contents of DDGS samples from the 3% amylase corn treatment and control were similar.     

Compositional,Physical, and Wet‐Milling Properties of Accessions Used in Germplasm Enhancement of Maize Project

Forty‐nine accessions used in the Germplasm Enhancement of Maize (GEM) project, two commercial hybrids (Pioneer Brand Hybrids 3394 and 3489), and two Corn Belt inbreds (B73 and Mo17) were evaluated for compositional, physical, and wet‐milling properties. GEM accessions had lower starch contents (65.9–69.1% vs. a mean of 72.2% for the commercial hybrids) and greater protein contents (12.0–14.4% vs. a mean of 8.2% for the commercial hybrids) than did the improved Corn Belt material. Absolute densities were consistently higher for the GEM accessions compared with the commercial hybrids (1.320 vs. 1.265 g/cm³, respectively). The wet‐milling characteristics of the GEM accessions were not nearly as good as for the commercial hybrids. Mean starch yields were only 54.3% for the GEM accessions versus 64.8% for the commercial hybrids. Residual protein levels in the starches recovered from the GEM accessions were much greater (0.45–2.03%) than for commercial corn hybrids (<0.3%).     

Effects of Granular Structures on the Pasting Behaviors of Starches

Japonica (Tainung 67 [TNu67]) and waxy (Taichung 70 [TCW70]) rice, normal and waxy corn, and cross-linked waxy rice and corn starches were used in an investigation of the influence of the granular structure on the pasting behavior of starch, using small amplitude oscillatory rheometry. Both normal corn and normal rice (TNu67) starches had the highest storage moduli (G′), followed by their cross-linked versions; native waxy corn and rice starches had the lowest. Native waxy starches showed paste characteristics (G′ < 500 Pa; tan δ > 0.2) at concentrations of up to 35%. However, cross-linked waxy starches exhibited gel behavior at 10% concentration (cross-linked TCW70) or higher (cross-linked waxy corn starch). The degrees of swelling power were in the order: TCW70 > native waxy corn > TNu67 ≅ cross-linked TCW70 ≅ normal corn ≅ cross-linked waxy corn starches. Solubilities were in the order: normal corn > TNu67 > native waxy > cross-linked waxy starches. The addition of 2% purified amylose from indica rice (Kaohsiung Sen 7) did not induce gelation of waxy corn starch. Swelling powers of normal corn, TNu67, and crosslinked waxy starches were similar, but normal corn and TNu67 had much higher G′ value. Such results implied that the formation of gel structure was governed by the rigidity of swollen granules and that the hot-water soluble component could strengthen the elasticity of the starch gel or paste.     

Laboratory Measurement of Yield and Composition of Dry‐Milled Corn Fractions Using a Shortened,Single‐Stage Tempering Procedure

The objective was to describe a laboratory‐scale dry‐milling procedure that used single‐stage tempering and determine the effect of hybrid on yields and fraction compositions in milled corn. Samples of 11 commercially available hybrids were processed through a laboratory dry‐milling procedure that used 1 kg samples of corn to produce milling fractions of large grits, small grits, fines, germ, and pericarp. Compositions of milling fractions (protein, neutral detergent fiber, ash, and crude fat) were determined. The procedure used a single‐stage tempering step that increased corn moisture from 15 to 23.5% wb during an 18‐min tempering period. Germ were separated from endosperm particles using a roller mill followed by screening over a sieve with 1.68‐mm openings. Coefficients of variability were small, indicating acceptable repeatability. Overall yield means were 39.2, 25.3, 13.8, 78.2, 14.3, and 6.8 g/100 g (db) for large grits, small grits, fines, total endosperm, germ, and pericarp, respectively. There were effects due to hybrid (P < 0.05) on fraction yields and compositions of milling fractions. Correlations (r) among endosperm fractions (large grits, small grits, and fines) ranged from 0.54 to |–0.92|. Correlations among endosperm fractions and germ and pericarp were <0.68. The developed dry‐milling method estimated milling yields among hybrids with low standard deviations relative to the means and should be a useful tool for research and industry in measuring dry‐milling characteristics.     

Evaluation of Nebraska Waxy Sorghum Hybrids for Ethanol Production

To evaluate the ethanol production performance of waxy sorghum hybrids and the effects of location and harvest year on ethanol yield, samples of four waxy sorghum hybrids collected from two Nebraska locations (Mead and Lincoln) in both 2009 and 2010 were tested for ethanol production in a dry‐grind process. No significant difference (P = 0.216) in starch contents was observed among the four hybrids, but starch contents of the hybrids were significantly affected by growth location (P = 0.0001) and harvest year (P = 0.0258). Location, hybrid, and harvest year all had significant effects on ethanol fermentation efficiency in the dry‐grind process. Lincoln sorghum samples showed higher (P = 0.022) ethanol fermentation efficiency (90.4%) than did Mead sorghum samples (90.0%). Sorghums harvested in 2010 had higher (P < 0.001) ethanol fermentation efficiency (91.1%) than those harvested in 2009 (89.3%). The 2009 sorghum flours had more amylose‐lipid complexes than the 2010 samples did, and amylose‐lipid complexes as previously reported had adverse effects on ethanol fermentation. Residual starch contents in distillers dried grains with solubles (DDGS) were significantly affected by hybrid and harvest year (P < 0.0001), but we observed no difference in protein content in DDGS from the four hybrids.     

Comparison of Physicochemical Properties and Structures of Sugary‐2 Cornstarch with Normal and Waxy Cultivars

Starches from normal, waxy, and sugary‐2 (su2) corn kernels were isolated, and their structures and properties determined. The total lipid contents of normal, waxy, and su2 corn starches were 0.84, 0.00, and 1.61%, respectively. Scanning electron micrographs showed that normal and waxy corn starch granules were spherical or angular in shape with smooth surfaces. The su2 starch granules consisted of lobes that resembled starch mutants deficient in soluble starch synthases. Normal and waxy corn starches displayed A‐type X‐ray patterns. The su2 starch showed a weak A‐type pattern. The chain‐length distributions of normal, waxy, and su2 debranched amylopectins showed the first peak chain length at DP (degree of polymerization) 13, 14, and 13, respectively; second peak chain length at DP 45, 49, and 49, respectively; and highest detectable DP of 80, 72, and 76, respectively. The su2 amylopectin showed a higher percentage of chains with DP 6–12 (22.2%) than normal (15.0%) and waxy (14.6%) amylopectins. The absolute amylose content of normal, waxy, and su2 starches was 18.8, 0.0, and 27.3%, respectively. Gel‐permeation profiles of su2 corn starch displayed a considerable amount of intermediate components. The su2 corn starch displayed lower gelatinization temperature, enthalpy change, and viscosity; a significantly higher enthalpy change for melting of amylose‐lipid complex; and lower melting temperature and enthalpy change for retrograded starch than did normal and waxy corn starches. The initial rate of hydrolysis (3 hr) of the corn starches followed the order su2 > waxy > normal corn. Waxy and su2 starches were hydrolyzed to the same extent, which was higher than normal starch after a 72‐hr hydrolysis period.     

Laboratory Wet Milling of Corn: Milling Fraction Correlations and Variations Among Crop Years

Several coproducts result from fractionating corn in the wet‐milling process. Because small changes in product composition and milling characteristics can have a major impact on coproduct yields and values, testing is done to anticipate final product yields. Using small sample size and controlled conditions, a laboratory wet‐milling method proved to be a useful tool for wet milling and genetics industries. A wet‐milling process (100‐g batches) was used for data collection. Data collected during 11 years (1994–2004) were observed for samples used as benchmarks to verify process precision and accuracy and determine correlations among wet‐milling yields. More than 400 milling tests were performed on benchmark samples. Data from benchmark samples also were pooled. Coefficients of variation were low (<6%) for mean yields; year‐to‐year standard deviations of benchmark sample yield means were homogenous and implied precision of the procedure. Some differences were detected in mean yields among years (P ≤ 0.05) for benchmark data due to combined effects of hybrid and environment. A negative correlation (r = –0.58) was observed between starch and gluten yield for pooled benchmark data. Four years (2002–2005) of milling data from commercially available hybrids were analyzed using the milling procedure. For pooled commercial data, the correlation between starch and fiber yield was (r = –0.80); correlation between starch and gluten was (r = –0.76).     

Ethanol Production from Modified and Conventional Dry‐Grind Processes Using Different Corn Types

Different corn types were used to compare ethanol production from the conventional dry‐grind process to wet or dry fractionation processes. High oil, dent corn with high starch extractability, dent corn with low starch extractability and waxy corn were selected. In the conventional process, corn was ground using a hammer mill; water was added to produce slurry which was fermented. In the wet fractionation process, corn was soaked in water; germ and pericarp fiber were removed before fermentation. In the dry fractionation process, corn was tempered, degerminated, and passed through a roller mill. Germ and pericarp fiber were separated from the endosperm. Due to removal of germ and pericarp fiber in the fractionation methods, more corn was used in the wet (10%) and dry (15%) fractionation processes than in the conventional process. Water was added to endosperm and the resulting slurry was fermented. Oil, protein, and residual starch in germ were analyzed. Pericarp fiber was analyzed for residual starch and neutral detergent fiber (NDF) content. Analysis of variance and Fisher's least significant difference test were used to compare means of final ethanol concentrations as well as germ and pericarp fiber yields. The wet fractionation process had the highest final ethanol concentrations (15.7% v/v) compared with dry fractionation (15.0% v/v) and conventional process (14.1% v/v). Higher ethanol concentrations were observed in fractionation processes compared to the conventional process due to higher fermentable substrate per batch available as a result of germ and pericarp fiber removal. Germ and pericarp yields were 7.47 and 6.03% for the wet fractionation process and 7.19 and 6.22% for the dry fractionation process, respectively. Germ obtained from the wet fractionation process had higher oil content (34% db) compared with the dry fractionation method (11% db). Residual starch content in the germ fraction was 16% for wet fractionation and 44% for dry fractionation. Residual starch in the pericarp fiber fraction was lower for the wet fractionation process (19.9%) compared with dry fractionation (23.7%).