STARCH HYDROLYSIS

The cornstarch separated from the kernel by the wet milling process is generally 99% pure and contains 0.25-0.35% protein, 0.5-0.6% lipid and less than 0.1% minerals. 35% of the industrially prepared cornstarch is utilized by the food industry; the remainder of the starch is further refined or modified for use in the paper and construction industries. A significant proportion of the cornstarch derived from the wet-milled process used for food goes into the fermentation of beer. To be useful to the breweries, it first has to be converted into dextrose or d-glucose. This is an example of down-stream processing; a raw material is isolated and used as a starting material in another process.

STARCH FOUND IN CORN Cornstarch is found in granules within the kernel as a long polymer composed of two structural classes: amylose and amylopectin. The basic repeating unit for both types of starch is d-glucose molecules, connected by glycosidic bonds. The polymer chains and the formation of the intermolecular network traps water and results in gel formation and solution thickening. After the starch is completely hydrolyzed or broken down its basic component is d-glucose also called dextrose or corn sugar.

MOLECULAR STRUCTURE OF STARCH Amylose is a linear polymer of short 1,4 linked glucose chains. Typically the amylose fraction is about 25-30% of the starch molecules found in corn and has a molecular weight of about 250,000. The percentage of amylose in the starch is genetically determined. Genetic modifications producing high-amylose (50-70%) cornstarch are also found. Amylopectin comprises about 70-75% of the starch found in the corn kernel and has a molecular weight of about 50-500 million. Amylopectin is a branched polymer of the basic repeating units of 1,4 linked glucose with branches of 1,6 linked glucose. The branching occurs irregularly in the starch, approximately one per twenty-five glucose units.

AMYLASES Amylases are a class of enzymes that are capable of digesting these glycosidic linkages found in starches. Amylases can be derived from a variety of sources. Amylases are present in all living organisms, but the enzymes vary in activity, specificity and requirements from species to species and even from tissue to tissue in the same organism. Alpha-amylase (1,4 aD-Glucan-glucanohydrolase) acts upon large polymers of starch at internal bonds and cleaves them to short glucose polymers. a-amylase catalyzes the hydrolysis of internal a1-4 glucan bonds in polysaccharides containing 3 or more a1-4 linkages; it results in a mixture of maltose and glucose. Amyloglucosidase works on the shorter polymers and splits off single glucose sugars. Bacterial a-amylase is particularly suited for industrial usage since it is inexpensive and is thermally stable. Amylose AmylopectinASSAY OF THE AMOUNT OF STARCH IN CORN FRACTIONS It may be desirable to determine the amount of starch in each fraction of the corn kernel to determine the efficiency of the fractionation process and to determine the suitability of the particular batch of corn or the genetic makeup of the particular corn strain. Therefore a simple assay for the presence and amount of starch is possible. The assay that is used is based on the hydrolysis of the starch by the above enzymes. The final product of the hydrolysis, glucose, is then determined by the glucose oxidase assay.

DETERMINATION OF THE AMOUNT OF GLUCOSE PRESENT Glucose is determined by an enzymatic reaction. The enzyme glucose oxidase converts glucose to gluconic acid and hydrogen peroxide in equal molar amounts. The amount of glucose is then determined by the reaction of the peroxide with the chromophore, aminoantipyrine, to produce a color change that is detected at 505nm.

SYNOPSIS Sample is prepared and primary digestion with a-amylase Secondary digestion with amyloglugosidase results primarily in glucose Glucose is digested with Glucose oxidase to gluconic acid and hydrogen peroxide Hydrogen peroxide is changed to an active form by hydrogen peroxidase The product of the peroxidase reaction complexes with the chromophore, aminoantipyrine Color change is followed by change in absorbance at 505nm

SAFETY Read the MSDS information for all chemicals used in this exercise. Phenol and aminoantipyrine are considered harmful or hazardous materials, use gloves when using these compounds. Wear lab coat and
proper eyewear.

MATERIALS cornstarch (from commercial source or use the cornstarch isolated from Corn Wet Milling Lab) bacterial a-amylase (Sigma A-3403) amyloglucosidase (Boehringer Mann 846) glucose oxidase (Sigma G-6500) anhydrous glucose (Sigma G-5000)

Glucose oxidase-Peroxidase reagent (GOP) N a 2HP O 4-12 H 2O 11.5g KH 2P O4 2.5g Phenol 500mg 4-aminoantipyrine 75mg (Sigma A4382) Peroxidase 3500U Glucose oxidase 3500U

Dissolve in water and make up to 500mL. pH the solution to 7.0 and filter through a glass microfiber filter (Whatman GF/A) Store at 4 ° protected from light and use within one month

Amyloglucosidase (AMG) 2mg/mL Amyloglucosidase 30mg dissolve in 15mL of AMG buffer

AMG Buffer(pH 4.6) Citric Acid monohydrate 460mg Sodium citrate dihydrate 840mg

Dissolve in 100mL water check pH to 4.6PREPARATION OF SAMPLE

Weigh samples of 50-100 mg starch and place in 10mL tubes. Add 2mL of water and 0.2mL of a-amylase to each tube Mix immediately on vortex mixer Add 6 mL of water and mix by repeated inversions Immerse the tubes in a water bath 8 5° with shaking for 3 minutes make sure that there is a rapid geletinisation Keep at 8 5° with occasional shaking for 30 minutes Cool to 2 0° Dilute to a total volume of 100 mL

DIGESTION WITH AMG Place the following in screw capped tubes that have a tight fitting seal 1) Prepare blank Add 0.2 mL of a-amylase to 100 mL of purified water 2) Prepare Glucose standard make up fresh 1mg/mL glucose standard from anhydrous glucose 3) Prepare samples Take 1mL of the above prepared sample or dilution of above in purified water

Mix gently Incubate in a 6 0° water bath for 30 minutes Cool to 2 0° Add exactly 8 mL of water to each of the tubes Filter the samples through Whatman glass fiber filter (GF/A) Take a total of 1mL from the AMG digestion for analysis of total glucose

ANALYSIS OF TOTAL GLUCOSE 1) Prepare blank Take 1 mL of the digested blank ( a-amylase) 2) Prepare glucose standards From the digested 1mg/mL make dilution to cover the range 50-100ug 3) Prepare the samples Take samples from the above AMG digestion make dilutions where appropriate


### Analyte volume water GOP Blank water Blank α -amaylase Samples glucose 10 μ g glucose 20 μ g glucose 40 μ g glucose 80 μ g glucose 100 μ g glucose 160 μ g --- 1 mL. 1 mL 10 μ L 20 μL 40 μL 80 μL 100 μL 160 μL 1 mL.

---

--- 990 μ L 980 μ L 960 μL 920 μ L 900 μ L 840 μ L 5 mL. 5 mL. 5 mL. 5 mL. 5 mL. 5 mL. 5 mL. 5 mL. 5 mL. adjusted volume AMG AMG Buffer Blank α -amylase Glucose @ 1mg/mL Samples 1 mL. 1 mL. 1 mL 1 mL.

--- 1 mL --- 1 mL.

---Use tightly stoppered screw capped tubes. Place tube in a water bath 3 5° for 45 minutes, keep protected from the light. Cool to room temperature for 10 minutes in the dark. Measure the absorbance in 1-cm cells at 505 nm against the water blank. Read absorbance within 30 minutes. Subtract the a-amylase reading from the sample reading. Determine the amount of glucose in the samples by comparing them to the standard curve. Multiply the amount of glucose in the samples by 0.9 to give an indication of the amount of starch in the samples. This is an adjustment from free glucose to anhydro glucose as occurs in starch.

REFERENCES Karkalas, John, An improved enzymatic Method for the Determination of Native and Modified Starch, J Sci. Food Agric., 36, 1019, 1985. Amer. Assoc. Cereal Chem. Approved Methods, 9th ed. Method 76-12, 1995.