Organic, inorganic oxidizers control bacteria in corn mash fermentation

Ethanol plant case study examines effectiveness of antimicrobial alternative, writes Reed Semenza. This contribution appears in the January issue of EPM.
By Reed Semenza | December 21, 2015

Ethanol plants strive to control lactic and acetic acid bacteria in corn mash fermentation tanks to minimize competition with ethanol-producing yeast for the sugars, plus, acid forming bacteria can create low pH conditions that tend to inhibit the growth of the yeast. In order to control bacteria growth, many ethanol plants utilize antibiotics, while some are trialing new approaches. One of those new approaches utilizes an organic oxidizer, peracetic acid (PAA), in combination with an inorganic oxidizer, hydrogen peroxide, in the corn mash.

Mist Chemical has worked with several ethanol producers to test DeLasan CMT, a 22 percent PAA distributed by DeLaval, and hydrogen peroxide (HP). The combination, even when used in minute quantities, has proven to have sufficient antimicrobial effect such that lactic acid and acetic acid concentrations in the fermenter were maintained at acceptable levels.  An unexpected benefit was a significant increase in final fermenter ethanol percentage.

This case study reports the test done at a 65 MMgy Delta T-design ethanol plant in California. The management desired to reduce or eliminate the use of antibiotics in an effort to produce distillers grains that are free of antibiotics.  A second objective was to increase ethanol yield. The primary objective of the test was to determine the most economical dose of PAA and HP and to determine the relationship between lactic and acetic bacteria reduction and dose. Secondary objectives included measurement of final fermentor ethanol percentage as well as the decay rate of the PAA and HP in order to determine residual levels in the distillers grains.

In the plant’s process flow, the corn mash enters the stage 1 cooler at 180 degrees Fahrenheit and is cooled to 140 F.  It then enters the stage 2 cooler and is cooled to 90 F.  Flow through the cooler is 600 gallons per minute (gpm).  From the stage 2 cooler, the mash is sent to the fermenter. Once in the fermenter, the corn mash is recirculated through a plate-and-frame mash cooler. The fermenter takes about 18 hours to fill. Thirty minutes into the filling process, yeast is added to the recirculating corn mash. Once filled, fermentation begins to accelerate, releasing heat, which is dissipated in the fermenter cooler. About 50 hours after the fermenter is filled, the fermenter mash is sent to distillation. The system parameters for the 1,500-gallon mash cooler include a recirculation rate of 600 gpm, pH range of 5.5 to 6.5 and temperatures of 180 F in and 90 F out. Parameters for the 850,000-gallon fermenter include a recirculation rate of 1,000 gpm, pH range of 4 to 5.5 and temperatures of 95 F in and 90 F out.

During fermentation, carbohydrates, ethanol, and organic acids are monitored in order to insure that the fermentation process is occurring normally and to insure that undesirable bacteria are kept under control. Also, the mash temperature is kept in a range of 90 F to 95 F. The typical process parameters include the following:

• pH starts at 6.0 then slowly drops to 4.2 after 18 hours.  Toward the end of fermentation, pH rises to 4.6.

• Lactic acid percent starts at 0.1 percent and stays mostly below 0.2 percent when lactic acid bacteria are under control.  When lactic acid bacteria are present in larger numbers, the lactic acid percentage can rise to as high as 0.8 percent. 

• Acetic acid generally starts in a range of 0.03 to 0.05 percent and rises to 0.1 percent.  A rise above 0.15 percent results in reduced final ethanol and indicates that bacteria counts are out of control.

• Typical end fermenter process parameters include 16 percent ethanol, 0.20 lactic acid, 0.05 acetic acid, pH of 4.6, cell count of 250 and viability count of 97 percent.

Test Procedure
In the test, the 22 percent PAA  and 31 percent hydrogen peroxide were added at a rate of 3 and 1.5 gallons per hour respectively to a stainless steel pipe that carried the mixture to a header located between the stage 1 and stage 2 mash coolers.  At the point of injection, the mash temperature was 140 F and the mash flow rate was 600 gpm. Diaphragm pumps fitted with Teflon liquid ends were used to pump both solutions. The dose was 15 ppm PAA and 40 ppm HP. However, they were fed for only eight hours of the 18-hour fermenter fill time, making the overall dose 7.0 ppm of PAA and 22 ppm of HP.

Samples were taken every hour at the mash cooler exit and fermentation tank and tested using a Hach DPD total chlorine test. A 3-milliliter sample was added to a test tube, then one DPD powder pillow was added and stirred gently for five seconds when a pink color was observed at the bottom of the test tube. Peracetic acid concentration was estimated based on the hue of pink color after 30 seconds.  Hydrogen peroxide concentration was estimated after six minutes. Normal corn mash tests on carbohydrates, ethanol, and organic acids were taken by plant personnel every six hours.  Results were compared with normal historical averages when antibiotics were used.

Seven days of test results indicated the following:

• Both lactic and acetic acids stayed within normal ranges.  Lactic acid generally ranged from 0.1 to 0.2 percent.  Acetic acid generally ranged from 0.05 to 0.10 percent.  Overall, the organic acids at drop were the same concentration with DeLasan CMT as with antibiotics (see Chart Comparison 1).

• Glycerol is a stress indicator for yeast.  Higher stress results in higher glycerol.  The glycerol level was 7 percent lower (see Chart 2) with the alternative program vs. antibiotics.  The significantly lower glycerol indicates the PAA-HP combination stresses the yeast less than antibiotics or that bacteria competing with the yeast were better controlled.

• Total sugars were lower at drop with the program indicating more efficient conversion of sugars to ethanol (see Chart 3).

• Ethanol production was 0.44 percent higher with DeLasan CMT vs antibiotics. (see Chart 4).  This result was not expected, as alcohol production is thought to be inversely correlated with organic acids.  Higher organic acid concentration usually results in lower ethanol concentration. The most logical reason for the ethanol increase was a decrease in glycerol and residual sugars. Reduced glycerol and sugar levels resulted in increased available glucose and the extra glucose was converted into ethanol.

Overall test results were extremely positive, and are comparable to tests done at other ethanol plants.  The tests indicated that the alternative program can effectively replace the use of antibiotics in preventing ethanol loss due to the formation of organic acids.  No negative effects of the program were noted based on the balance of carbohydrates, ethanol, and organic acids.  Also, yeast cell counts and viability tests were all in the normal range.

In addition to helping the plant achieve antibiotic-free distillers grains, the program was cost-effective, due to the low dose used. There was a net increase in ethanol production of 0.44 percent. In a 65 MMgy plant, that amounts to increased profits of approximately $700,000 per year. The PAA-HP mixture is easy to feed and test for. It is added directly from totes into the corn mash pipe with no mixing other than the natural turbulence in the pipe required.  The control test is a modified total chlorine test and is taken at the mash cooler discharge. Unlike antibiotics and sodium chlorite, the DeLasan does not contribute antibiotic residue or inorganic salts to the distillers grains or backset.

Author: Reed Semenza
Business and Technical Manager, DeLaval     

Co-author: Michael Welker
Technical Service, Mist Chemical