Bran Index: a new flour quality parameter

by Chrystal Shannon
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Digital imaging of wheat flour has been shown to improve bread quality and milling performance

he Warburton Group is an independent, family-owned U.K. bakery company with a reputation for the highest quality standards for its products. The company operates 10 major bakeries in England and Scotland, and maintains its high standards through strict quality control at all stages of their process — ingredient supply, production, packaging and marketing. Wheat is purchased directly in Canada and the United Kingdom, and Warburton bakeries are supplied with flour from a number of leading U.K. flour mills.

Recently, Warburton participated in a research project that provided evidence for a new standard in wheat flour quality utilizing Digital Image Analysis (DIA) techniques. The research took place over a 20-month period with two objectives: to determine the influence of bran levels in flour and to establish optimum quality control levels to ensure consistent performance in high-quality breadmaking.

The research project addressed the common problem concerning the consistency of wheat flour. Although the flours are produced to high specifications, there are variations that are not detected by standard flour-testing specifications. Flours often meet the same specification but vary in performance in the bakery, giving concerns over loaf texture and volume.

The very high standards demanded by Warburton put pressure on the millers to provide consistent quality. However, the current test methods do not provide adequate controls at this level.

The SPX Bran Index measurement proved to be a simple and quick method to provide effective quality control at the mill and bakery. Use of this measurement parameter also provides valuable process control information for the miller.


The main focus of the research was on the effects and levels of bran in the flour, a subject that has been of interest to researchers for many years. The KJM Color Grade has been traditionally used in the U.K. to measure the bran effects and has worked well in defining general grading of flours. However, where low levels of bran are concerned and different bran fractions are involved, sensitive discrimination is not possible or is concealed by the effect of the background flour "color."

The decision was made to work with new digital imaging techniques that are now commercially available and have been refined in recent years for more practical quality control lab applications. Initially, the two systems used were the Branscan 2000 and the Maztech SPX Speck Expert. Both systems provided useful data, and researchers were encouraged by the bran differences detected with these instruments that were not indicated by KJM Color Grade measurements.

The early data showed relatively large inconsistencies in flours from five independent milling companies, and attempts were made to narrow the range. Bran Index levels were recorded over the first nine months of the project (see Figure 1 at right). Initial high bran levels and inconsistencies were found in most flours supplied.

Cooperation with the independent flour millers achieved a dramatic improvement in consistency and enabled Warburton to begin defining acceptable bran levels. The suppliers were restricted to those who could produce flour within these limits.


The two instrument types used in the research provided similar results but had different techniques. There was a general agreement in grading of flours, but there was not a good direct correlation between individual readings.

Some questions were raised concerning reproducibility, and it was felt that to achieve a higher level of discrimination and a practical method, the repeatability and variation in instruments had to be optimized. There were also practical considerations about the operation, sample handling and ease of use.

It was decided that the project would proceed using the Maztech SPX system. This offered many practical advantages to the operator and provided the reproducibility required at this high quality level.

The SPX Speck Expert is manufactured by Maztech MicroVision Ltd., Ottawa, Ontario, Canada. Maztech products are distributed by Perten Instruments and Calibre Control International.

The SPX was introduced in 1997 as a method for speck counting in semolina. In 1998, the SPX was adapted for quantifying bran in flour. Using the Bran Index, all bran layers were determined to be darker than starchy endosperm and thus measured directly using grey levels and particle size. Bran Index is independent of mineral content but related to overall flour brightness.

While the SPX system was effective, the ultimate need for both suppliers and purchasers to determine bran at such low levels meant that there was a need to have a stable calibration and performance between instruments. Maztech upgraded the instrument calibration routines and reproducibility between instruments by introducing a new method of normalization. This utilizes a specially designed reference disk (see below), which can be presented to the instrument at routine intervals. The instrument automatically normalizes and maintains consistent measurements with other instruments.

The improvement was clearly demonstrated by results from an independent flour ring test. A number of production flours were included and tested on a range of instruments in the U.K. and Canada. Results were plotted and reproducibility was achieved (see Figure 2 on page 49). Another ring test was scheduled last year using a wider range of flours.


The project continued over a number of months. A large amount of data were needed to determine the required specification for Bran Index. Because all flours supplied to Warburton must be of a high quality, it was necessary to identify those deliveries that had a different analysis and that performed differently in the plant. It was not practical to create samples artificially; therefore, all deliveries were monitored and compared with routine bread quality reports.

These reports have been based on sensory evaluations for many years and have been recently supplemented by new bread imaging techniques, which have proved to correlate well with the sensory methods and have become a reliable means of detecting the variations in flour quality.

Results were collected over 20 months and the data analyzed (see Figure 3 on page 50). Warburton was able to determine an optimal range of Bran Index for the flours.

The Bran Index gives an assessment of the bran level in the flour, including measurements well below the normal visible range. This measurement takes into account the size and color of bran (grey level) and provides a single figure indicating the level of contamination.

The calibration used in the instrument had been developed over a long period of cooperation between Maztech and U.K. milling companies. The measurement is particularly suited to white flours and optimizes the system for breadmaking flour.

Although the instrument also provides a simultaneous measurement of speck count (number of bran specks), the measurement is considered secondary for this work. At this level, only large variations would be significant and would probably be detected easily by other quality control test methods.

The data (see Figure 4 on page 50) are for deliveries from one particular supplier over the period of the trial. There are a number of interesting factors represented by this chart:

n The initial work is shown clearly where the deliveries varied considerably.

n As the project continued, a satisfactory level of Bran Index was determined and achieved for the most part by the miller.

n Problems associated with mill start-up after the end-of-the-year plant shutdown can be clearly identified.

n A point where new crop wheat is introduced to the mill is shown by the inconsistency of data.

n Once the SPX was introduced into the mill, flour supplies achieved the highest level of consistency.


Bran Index is now being used routinely by Warburton as a very important quality parameter. As a result of this research, it is expected that a number of organizations will develop new applications for DIA in flour milling. This may include measurements on bran fractions, milling streams/machine flours, extraction rates and process monitoring.

The objective of this project was to determine a method of identifying flours that did not perform well in baking, even though they met existing quality control requirements. This was achieved, and the resulting consistency of product had benefits for both baker and miller.

The Bran Index method is now adopted as a quality control parameter, and it is considered that use of this technology may be expanded into other flour milling and baking areas. More work is being done to identify these applications and benefits, but there is a lack of understanding about the function of bran and its components under process conditions. It is hoped that DIA will contribute to this understanding in the future, as more sophisticated instruments and software become available.

The research project was necessarily quite long in order to detect enough samples that did not perform adequately in the plant. The specification for flour quality (Bran Index) was therefore refined based on a large amount of data from the final bread quality. This had the added benefit that the instrument and the technique were fully evaluated and found to be stable over the long research period.

In testing flours to meet the bakers’ specification, millers were able to gain further benefits by monitoring process conditions. Comparing adjustments to the mill with changes in Bran Index, the miller was able to maximize machine efficiency and extraction rates, and to respond quickly when problems occurred.

It is interesting to note the increasing use of DIA techniques. In this work, bread imaging systems are becoming the reference for the new flour imaging methods. Similar technologies are being employed at different parts of the process to improve final quality.

A great deal of cooperation took place between Warburton and its suppliers, enabling specific events and causes of problems to be identified. Suppliers are now using these new guidelines for routine quality control, and Warburton said it is pleased with the improved consistency of the flour and the final bread quality.