INTRODUCTION TO PLANT BREEDING

AGRONOMY 815 / COURSE NOTES

P. STEPHEN BAENZIGER, 338 Keim Hall, 472-1538

DEPARTMENT OF AGRONOMY / UNIVERSITY OF NEBRASKA

THE MIXOGRAPH

Introduction

Wheat protein content can demand a premium price in the marketplace. Protein measurement is routinely done at elevators and throughout the grain marketing system on all received grain. NIR instruments have provided inexpensive, rapid determinations of wheat moisture and protein contents. Protein content is important to the milling and baking industry because it can predict baking performance.

However, protein content can be a flawed predictor. Drought conditions can stress grain and produce high protein wheats. Yet flours milled from this wheat bake poor quality breads. Protein quality is the difference. High protein content alone is not enough. The proteins must have functional properties for water uptake (absorption) and mixing strength to insure proper gluten protein development.

Gluten proteins are not formed unless water is present and mixing occurs. During the bread making process gluten proteins are responsible for the following:

• Gas retention during fermentation which results in good loaf volume and loaf softness.
  
  
• Bread crumb strength so that peanut butter can be successfully spread without tearing the bread slice.
  
  
• Bread crumb texture that is soft to the touch, yet does not stick to the roof of your mouth.

Therefore, an early generation prediction test for gluten quality is imperative for a successful wheat breeding program.

Analyzing for Protein Quality

In the University of Nebraska/USDA-ARS wheat breeding program the Mixograph is used to predict protein quality. This instrument has been manufactured by National Manufacturing Co., Lincoln, Nebraska, since 1939. During that year the University of Nebraska started using the Mixograph to analyze wheat flour samples. Currently, the Mixograph is used throughout the world.

The Mixograph uses a combination of rotating and fixed pins to mix flour and water. This mixing action approximates commercial dough mixing. During this process water is distributed throughout the dough and gluten proteins are formed.

The Mixograph curve provides information about protein quality (Figure 1):

• The amount of water absorption affects the placement of the curve on the paper and the peak time. Higher water absorption levels indicate higher quality wheat flour proteins.
  
  
• The curve peak is the highest point obtained on the Mixogram. At this point the dough has optimum development. A well-defined peak indicates proteins that are suitable for bread making.
  
  
• The time required to produce the curve peak is an indicator of gluten protein strength. Longer peak times of well-defined curves indicate stronger gluten proteins.
  
  
• After the peak occurs, the curve descends. The width of the Mixogram curve and the angle of descent indicate the tolerance of the dough to over mixing. Well-defined curves with wide bands and low angles of descent indicate strong tolerance to over mixing and superior protein quality.

The Mixograph can be used to evaluate wheat protein quality in a breeding program. Experimental lines with high protein content may differ markedly in dough mixing properties (Figure 2). Lines that have erratic mixing curves can be discarded (Figure 3).

The Nebraska Wheat Quality Lab uses the Mixograph extensively to evaluate early generation wheat lines. In a typical year 5,000 analyses will be done. Mixogram curves will be evaluated for peak times and tolerance scores (Figure 4). Wheat breeders can then make decisions about discarding lines with no baking potential.

Conclusion

The Mixograph has a long history of predicting wheat flour protein quality in Nebraska and worldwide. The results of this test are excellent indicators of the baking potential of experimental lines. Wheat breeders can then focus on lines that have the greatest promise for success in the marketplace.

Figure 1

MIXOGRAM CURVE

Figure 2

SIMILAR FLOUR PROTEIN CONTENTS
DIFFERENT MIXOGRAM CURVES

Figure 3

ERRATIC MIXOGRAM CURVES

Figure 4

MIXOGRAM CURVE TOLERANCE SCORES