MPI (Milling Performance Index)
"Milling in laboratories is more than just the simple transformation of grain into flour." By saying that, we wish to emphasize the fact that there is a great wealth of information to use during this operation. Moreover, one must use the right equipment.
In 2006, the French industry began working together on the problems involved with the milling of wheat. This "milling quality consortium"* combined the skills of different contributors from the cereals industry. It consisted of 3 sections:
- The first was very scientific, and aimed at understanding the reasons for the behavior of milled wheats. Several texts have been published on the subject.
- The second section involved creating a reference mill that was usable across the industry. This pilot is currently operational on the Enilia-ENSMIC site.
- The aim of the third section was to develop a laboratory mill capable of analyzing and thus better understanding the behavior of milled wheats. The result was the LabMill and its patented milling diagram.
Figure 1: Milling Diagram for the LabMill
Figure 1 shows this milling diagram. It is a simplified industrial mill which retains the major principles of the industrial mill--in particular, the use of grooved cylinders to grind and smooth cylinders to reduce the size of the middlings (soon, article "Milling"). It was also designed to obtain a yield that is as close as can be to industrial milling output, all while producing flour representative of rheological and baking quality.
Such progressive milling allows one to measure exactly how the wheat is milled and split up. In the end, there are no less than 17 different products that are obtained in the course of milling. A calculation sheet, provided with the LabMill, allows one to concentrate all this data.
It is, nonetheless, clear that a simplified version of this information can make for interesting reading. This is what led to the calculation of the Milling Performance Index (MPI).
The MPI consists of 3 digits (or indices) that describe the behavior of wheat.
- The resistance index is calculated using the formula (W=Weight):
At this point, we concentrate on what is happening at G1 (Grinding 1, Figure 1). We calculate the sum of the small particles (fine middlings and flour), which we then compare to the quantity of large particles (coarse middlings and waste matter). The idea is that the greater this relationship, the more the wheat tends to produce fine particles as early as the first grinding (soft wheat). This information is interesting because it effectively describes how the wheat will split under the impact of the ribbed cylinders.
Based on this relationship, a score of 1 to 7 is given. 7 indicates a wheat that is easy to crush.
- The dissociation index is calculated with the formula :
For this index, we look at the products obtained after 2 passes through the grinding cylinders (G1 and G2, Figure 1). In this step, we compare the mass of fine middlings to the mass of coarse middlings. If the amount of coarse middlings is high, it means the wheat resists dissociation and will tend to produce coarse rather than fine middlings. On a scale of 1 to 7, it would be graded 1. On the other hand, if the quantity of fine middlings is greater, the wheat would receive a score of 7.
- The reduction index is calculated with this formula :
This index compares the quantity of flour produced by reduction to the quantity of fine middlings that feed the said reduction. The aim is to know the efficiency of producing flour from fine middlings. The more flour obtained through reduction, the stronger the relationship and the less the middlings "resist" reduction. Here, too, the scale is from 1 to 7.
In the end, the MPI provides a simple interpretation of the efficiency of extracting flour from a given wheat.
Beyond the extraction rate, which is an important piece of data for millers, it is also important to have wheats which behave as consistently as possible from batch to batch. The reason for this is that this directly impacts the milling yield even when mill settings remain constant. Trials realized on more than 150 different wheats from around the world have clearly demonstrated that the same extraction rate can be obtained from wheats with very different milling behaviors. Some produce more flour early in milling, and others produce more later on.
This demonstration, however, also shows that selecting wheats based on the MPI enables one to noticeably improve the consistency of milling. Such that one can see the use of combining the two types of data and selecting wheats by choosing the one with the higher extraction rate for a given MPI and have a higher extraction rate with the same mill settings. One further bit of interesting information: the MPI is subject to the law of mixtures (a flour consisting of a 50%/50% mix of Flour A and Flour B has an MPI equal to the sum of 50% of the MPI of Flour A and 50% of the MPI of Flour B) and thus can act as an interesting means of optimizing flours or wheats.
Laboratory milling is an indispensible step in the cereals laboratory process. One cannot but recommend skipping the simple judgment of extraction rate in favour of the real analysis of information-rich data. The MPI helps you do this.
* Milling Quality Consortium - Consortium Valeur Meunière (AFSA, Arvalis-institut du végétal, ANMF, Danone Vitapole, INRA, IRTAC, Ulice, CHOPIN Technologies).