Application Data SheetFood & BeveragepH Measurement in Corn Wet MillingBackgroundCorn wet-milling is the most common method used to processcorn in the US. In this process, corn kernels are separated into theircomponent fractions and then further treated to yield cornstarch,corn sweeteners, corn oil, and animal feed. This process consumesover 1.3 billion bushels of corn each year in the U.S. alone. Each 56pound bushel of corn requires at least 40 gallons of water, so largevolumes of water must be treated, used, and then (perhaps) treatedagain before disposal. Tight monitoring and control of all thiswater captures the maximum from every bushel — this is where pHmeasurement of the process water comes in.The ProcessThe cleaned corn kernels are “steeped” at 50 °C (120 °F) in a mild(0.1 to 0.2 %) sulfurous acid solution to loosen the hull, soften thegluten, and dissolve some constituents of the kernel. The steepingphase lasts between 24 and 48 hours at 4 pH and causes the kernelsto swell to double their normal size. Sulfur dioxide gas is added tothe water to break chemical bonds in the gluten and prevent thegrowth of bacteria in the steep tanks. Water used in steeping is laterconcentrated and used as animal feed.After steeping, the corn is coarsely ground to free the germ. Thelighter corn germ is separated from the corn slurry by cycloneseparation or by gravity in a settling tank. The germ, which contains85 % of the oil, is further washed over screens to remove anystarch that may be adhering to it. Mechanical pressure and solventextraction are used to remove the corn oil. This oil is then clarifiedand filtered before delivery as a final product.The corn/water slurry (minus the germ) is then finely ground torelease the starch and gluten from the kernel. This mixture flowsover screens that capture the fibrous kernel but allow the starchand gluten to pass through. The fiber is collected, rinsed to reclaimstarch, and then used for animal feed. The gluten is then removedby centrifuge, and the remaining starch is washed several timesuntil reaching 99.5 % purity. At this point, the process will changedepending on the desired final products.Starch is basically a polymer ofglucose sugar units. The longchain starch molecule itself isvery versatile and is used as athickening agent in foods, asa raw material for ethanol andplastics production, and asa coating agent in the paperand textile industries. TheStarchEndospermStarchandGlutenHull andFiberGermproperties of starch depend on the extent and nature of the bondsin the molecule and can be changed by various preparation methodsthat involve acids, bases, sodium hypochlorite, and enzymes. Thesemethods yield products such as modified starch, unmodified starch,dextrins, cyclodextrins, and starch derivatives. These products aregenerally dried to a powder as a final product.The starch molecule is also used as the raw material for makingall kinds of sugar syrups. For these products, the starch slurry iscombined with acid and enzymes called amylases that break thestarch molecules into dextrose (glucose) and other sugars. Thekind of enzyme, reaction time, and process conditions are varieddepending upon the product requirements for different kinds ofsugars.The product syrups are identified by a Dextrose Equivalent (DE) valuethat represents how far along this reaction (starch conversion) hasgone. The product syrups are neutralized and then ionexchanged toremove salts. Dextrose is the simplest sugar, but not the sweetest.Converting dextrose to fructose doubles its sweetness and is veryeconomical. This step is called isomerization and uses the isomeraseenzyme at a specific pH value. The direct product of isomerizationis 42 % fructose syrup, but later fractionation processes are used toproduce 90 % fructose or even crystalline fructose. Various streamsare blended to produce the commercial 55 % fructose product. Endproducts are clarified and filtered before delivery. High fructose cornsyrup (HFCS) is the primary sweetening agent used in soft drinks andother foods and beverages.
Food & BeverageThe MeasurementspH measurements are made throughout the milling process tooptimize the product yield. The bulk of the water used for rinsingand washing usually needs pH adjustment to prevent altering thenominally acidic pH present during the various steps. Rinse watersare generally recycled upstream, but must be eventually treated(and neutralized) before disposal.Specific pH applications include the following:1.The pH in the steeping tank is used to control the additionof the sulfur dioxide (or other acid) that begins to release theFigure 1 - Corn Wet Milling Process - Part 12.3.starch from the corn. Too much acid may release the starchprematurely and causes corrosion of stainless steel processlines. Not adding enough will not prepare the corn kernelsadequately.Starch modification is used to lower the viscosity of the starchproduct. This process must be conducted under controlled pHto meet product specifications.Dextrin roasters also use pH control to catalyze the process.Dextrins have lower viscosity and dissolve in water (moststarches are insoluble), making them ideal for use in adhesives.Clean,ShelledCornWaterWater3000BushelSteepTankGermSeparator(Cyclone or Tank)GermGrindingMillWaterScreensHulls & FiberMillStarchCentrifugalSeparatorGlutenStarchWasherSO 2GasSteepWaterSteamCorn OilExtractionStarch Products(See Figure 2)Corn OilIndicates pH MeasurementAnimal FeedMultiple EffectEvaporators4.5.The enzymes used in starch conversion are expensive andfunction best at well defined pH levels (typically in the 3–5range depending on the enzyme used). pH levels higher thannormal will not utilize the enzymes efficiently, while operationat lower pH may allow the reaction to proceed too far andcause plugging of downstream filters. High pH values cause theproduct to set into a viscous gel that is difficult to process.pH is also measured during the filtration of the corn syrup. pH iscontrolled to prevent crystallization of sugar in the syrup. Solidcrystals will be filtered out and will lower yield from this step.6.Corn syrup end users require that the pH of the product theyuse have specific pH values, normally in the range of 3.5 to5.5. Final quality control of pH is used to ensure that soft drinkbottlers, wineries, and breweries can confidently use the cornsyrup without affecting their final products (Figures 1 and 2).Several other pH measurements may be useful throughout theprocess.Page 2
Food & BeverageFigure 2 - Corn Wet Milling Process - Part 2Acid &EnzymesStarchWasherStarchConversion42 DE SyrupFilters, IonExchangeIsomerizer42 % FructoseAcid orOxidizerModificationTankFractionation BlendingClarifiers Filters55 % FructoseHigh FructoseCorn Syrup(HFCS)StarchDrierStarchDrierModified Starch ( Diverse Uses)Acid orCausticDextrinRoasterDextrins ( Used in Adhesives)Indicates pH MeasurementUnmodified Starch( Used in Thickening Agent)ProductspH measurement in corn syrup and starch is strongly affected by thehigh concentration of non-conductive sugars and starch present.On-line pH sensors must continue to measure pH accurately withoutallowing sugars to contaminate the inside of the sensor. The TUpHmodel 396R sensor has proven reliable in these kinds of applications.TUpH sensors are designed with high area /low permeabilityjunctions that resist coating and reference contamination.Ideal instruments to use with the Model 396R sensor includethe 56 pH analyzer and the 5081-P Two-Wire transmitter. Theseinstruments support complete on-line diagnostics of the glass andreference portions of the sensor and notify operators of potentialproblems with the pH measurement. Both instruments are HARTcompatible and AMS aware.For more information on these products, ask for product data sheets71-TUpH, 71-56, and 71-5081.Although corn products typically do have enough conductivity thatpH can be measured accurately with an on-line pH sensor, someproduct corn syrups are purified by ion exchange and have very lowconductivities. Such products can be difficult to measure with higharea junction sensors. Use the Model 385+ for sugar solutions withconductivity below 75 μS/cm.Page 3
Food & BeverageInstrumentation5081-P pH/ORP 396R RetractableSmart Two-Wire TransmitterpH/ORP TUpH Sensor• Hand-held infrared remote control link• Rugged titanium andactivates all the transmitter functions.NEMA 4X (IP65) weatherproof,••••corrosionresistant enclosure.Comprehensive pH glass and referencediagnostics.Non-volatile EEPROM memory holds data in event of powerfailure.FOUNDATION Fieldbus protocol or 4–20 mA with superimposedHART56 pH Analyzer• Process measurements and on-screendata trend graphs easily viewed onfull-color screen.• User help screens show detailedinstructions and troubleshooting inmultiple languages.• Data Logger and Event Logger –Download process data and alarm conditions with time anddate stamps via USB 2.0 data port• PID and time proportional control capabilities. Includessynchronized interval timers and four special applicationfunctions.••polypropylene construction toprovide maximum chemicalresistance.Longer sensor life and reduced maintenance.Retractable version for greater insertion depths.385+ pH/ORP Sensor• Advanced on-line sensor diagnosticsfor use with the 56 pH/ORP Analyzer or5081 pH/ORP Transmitter.• Retractable version allows safe removaland replacement under pressure withoutprocess shutdown.• Long-life, triple junction referenceelectrode provides longer service life in applications wherepoisoning ions are present.• Disposable Tefzel and titanium design provides maximumchemical resistance and economic advantage where minimaltroubleshooting and maintenance downtime are of primeimportance.www.RosemountAnalytical.comwww.analyticexpert.comwww.youtube.com/user/RosemountAnalyticalwww.twitter.com/RAIhomewww.facebook.com/EmersonRosemountAnalytical©2013 Emerson Process Management. All rights reserved.Emerson Process ManagementRosemount Analytical Inc.2400 Barranca ParkwayIrvine, CA 92606 USAT (949) 747 8500F (949) 474 email@example.comThe Emerson logo is a trademark and service mark of Emerson Electric Co. Rosemount Analytical is a markof one of the Emerson Process Management family of companies. All other marks are the property of theirrespective owners.The contents of this publication are presented for information purposes only, and while effort has beenmade to ensure their accuracy, they are not to be construed as warranties or guarantees, express or implied,regarding the products or services described herein or their use or applicability. All sales are governed by ourterms and conditions, which are available on request. We reserve the right to modify or improve the designsor specifications of our products at any time without notice.LIQ-ADS-0084-FB-Corn-Wet-Milling