Alveograph
Chopin Alveograph
The Chopin Alveograph is a dough-testing instrument that inflates a thin sheet of dough into a bubble by means of air pressure. The theoretical justification for this method of analysis is its simulation of the inflation of bubbles in a dough with carbon dioxide produced by yeast during fermentation (Hlynka, 1955).
Marcel Chopin originally developed the Alveograph in France in 1920 as an empirical instrument to measure flour quality (Hoseney, 1986). He was interested in developing a dough testing instrument to use in place of baking tests to characterize the baking qualities of French wheat (Faridi, 1987). In 1927 Chopin wrote:
“it appears that the sole mechanical test on dough which corresponds exactly to the deformation which it undergoes in forming a loaf is one which consists in stretching a test sample from a compact state into a thin membrane until it finally becomes weakened to the point of rupture. This test must be made upon the dough itself and not upon the gluten contained in the dough. Furthermore, it is possible to measure the tensile strength of the membrane which is being stretched.” (Chopin, 1927).
This formed the motivation for the development of the alveograph. Chopin’s model (which he called a “extensimeter”) was designed to measure the plasticity of wheat flour doughs. This was accomplished through the measurement of (1) the ‘tenacity’ of the dough (its resistance to extension) which was estimated by the pressure required to force a uniform cylinder of dough to a take a definite form (a bubble) in a fixed period of time, and (2) the ability of the dough to be stretched into a thin membrane (Faridi, 1987).
The original design has been modified several times over the years to its current form. The instrument consists of three main components: (1) a mixer, (2) the bubble blowing apparatus (the alveograph), and (3) the recording manometer (Pyler, 1986). The most current versions include a computer component for analyzing the various measurements taken from the alveogram. Even with the several modifications that have taken place over the last 80 years the principles of operation remain the same: a dough is mixed for a set period of time to a standardized moisture content, sheeted into a flat disk and allowed to rest. After the resting period the dough is secured into the instrument and air pressure is used to inflate a bubble with the instrument measuring the pressure required to inflate the bubble (Hoseney, 1986).
Evaluation of the rheological behavior of a dough sample by blowing it into a bubble until it ruptures allows for a unique analysis approach. During the inflation of the bubble the dough piece is being extended in two directions: along a parallel and along a meridian of the bubble. This type of deformation is called bi-axial extension and in dough rheological testing is unique to the alveograph (Launay, 1987). Bi-axial extension has distinct advantages over uni- axial stretching. Physically, bi-axial extension simulates the type of deformation that takes place during fermentation and oven rise. The other difference is the rate of extension. The rate at which a bubble expands in an alveograph test changes with the volume of the bubble (Faridi, 1986).
Perhaps owing to its European origin the alveograph was originally used principally for the evaluation of European wheats which were generally weaker (lower protein content) than were varieties found in the United States (Walker, 1996). Today the alveograph is used on all types of wheat flour including very strong bread flours, however, it is still principally used in Continental Europe, French speaking Africa, Latin America and some parts of Asia (Sugden, 1998).
The alveograph testing procedure begins with mixing a dough from the wheat sample. 250g of flour are placed in the mixer and the mixer started. The appropriate amount (based on the initial moisture content of the flour sample) of 2.5% sodium chloride solution is added to the flour over a 20-second time period. After 1 minute of mixing the mixer is stopped and the side scrapped for 1 minute. The mixer is restarted and allowed to mix for six additional minutes. After mixing for 8 minutes in total the mixer is reversed and the dough is extruded out the front gate. Pieces are cut off and sheeted on the sheeter assembly and cut into round discs and placed in the resting cabinet for 20 minutes. After the rest a disc is placed in the alveograph and the inflation process is activated by switching on the airflow. The bubble is allowed to inflate and burst. The recording/analyzing computer performs the analysis automatically (MacRitchie, 2000).
The procedure used above results in an alveogram. A typical example is
shown in figure 1. The common parameters obtained from an alveogram are
the maximum over pressure (P), the swelling index (G), the average abscissa at
rupture (L), the configuration ratio (P/L) and the deformation energy (W).
The interpretation of the alveogram has been researched and debated almost from the time the alveograph was developed. The following ‘standard’ interpretation has been adapted from ‘Interpretation of the Alveogram’ (in Faridi, 1986).
The overpressure (designated as “P” in figure 1) is measured as the maximum height of the alveogram multiplied by a factor of 1.1. The P value has received much research attention and its use as an indicator of flour quality has been debated widely. Typically, P is regarded as a measure of dough tenacity as related to the maximum pressure attained in the inflation process.
The “L” value (see bottom of figure 1) is the average length, in millimeters, of the curves from the point where the dough bubble starts to inflate to the point where the bubble bursts. Very little debate has occurred over the L value, and is widely regarded as a measure of dough extensibility.
The configuration ratio, P/L, is an approximate indication of the shape of the alveogram combining the dough tenacity measurement (P) and the dough extensibility (L).
G, the swelling index, is the square root of the volume of air needed to rupture the dough bubble. Several studies have related the G value to various dough characteristics, however, it is primarily taken as a measure of dough extensibility.
And finally, the W value represents the work or energy needed to inflate the dough until it ruptures and is derived from the area under the curve. The W value is widely used as a measure of flour strength, and is often used as the principle indicator of the processing behavior of the flour.
From these five indexes the processing characteristics of a flour can be determined and its suitability for a specific end use can be evaluated. As was stated above the alveograph was initially developed for use in the evaluation of European wheats that tended to be lower in protein and thus “weaker” than the hard wheats grown in the United States. The strongest of the US wheats are typically hard spring wheats grown in the northern growing regions of the US. Initial research indicated that the alveograph was not suitable for evaluation of this wheat class (Khattak, 1974). A study was conducted by Chen and D’Appolonia (1985) to examine the influence of increased water absorption resulting from increasing levels of starch damage on alveograph values. Their research resulted in a modified method for use in evaluating hard spring wheats with the alveograph thus extending its use to a wider range of wheat flours.
On the other side of the flour strength spectrum is the application of alveography in the evaluation of soft wheat flours. In the introduction to their research Rasper, et. al. (1986) indicated that concern had been expressed about performing a stretchability test on doughs of constant water content without allowing for hydration capacity of the tested flours. This was the same question raised above by Chen and D’Appolonia for hard spring wheats. The objective of the Rasper study was to evaluate the suitability of the alveograph in quality assessment of soft white wheat flours. Their research also resulted in a modified method and determined that the Chopin Alveograph could be used successfully in the evaluation of soft wheat flours and was more sensitive to variations in quality than the widely used cookie spread test procedure.
With these two research projects and the established research prior to these studies the alveograph has been shown to be an effective and valuable tool in wheat flour quality evaluation and a good indicator of end use potential for the entire range of wheat flours.
