Purchasing a loaf of bread is a near-everyday experience for many consumers. Choice of brand depends primarily on what is important to the consumer in terms of taste and texture. One additional consideration is how fresh the bread remains for an extended period of time after it is bought. Aging of bread is referred to as “staling.” The average person thinks of it as hardening of the bread with a firmer and less-desirable texture.
Bakeries, especially large ones, conduct texture and staling tests on daily production batches to ensure that performance criteria for freshness and life expectancy are satisfied. The instrument used for testing is called a texture analyzer, which works by pushing a probe into the food item being evaluated. Rate of penetration by the probe is specified in the test method. A load cell inside the instrument measures the resistance to penetration and records the force in scientific units of grams, or Newtons. Choice of load cell force range and resolution is typically indicated in the method. When testing sliced breads in the U.S., 4,500-gram load cell with resolution of 0.5 grams is generally sufficient. Higher capacity load cells are available from manufacturers of texture analyzers if needed.
Figure 1 shows a cylindrical probe with 36-millimeter (mm) diameter positioned above two bread slices. It is called TA-AACC36 and comes from a specification created by the American Association of Cereal Chemists. This is the preferred choice when evaluating sliced bread for firmness and springiness. It is a relatively inexpensive item and attaches to any texture analyzer with standard M3 threaded coupling.
Texture Profile Analysis
The standard method for characterizing bread is a two-cycle test called Texture Profile Analysis (TPA). The probe pushes down into two bread slices stacked on top of one another at 1 mm/second to a depth of 4 mm. The instrument begins to record the measured force after a trigger load of 5 grams is detected. When the probe reaches 4 mm, it reverses direction and returns to its starting position. While this takes place, the bread will spring back to some extent. The probe then commences its second penetration cycle. The point of contact may take place slightly later than the first cycle because the bread does not fully recover to its original position. The probe pushes down again to a distance of 4 mm and records the measured force as before. The peak force measured during the second cycle may be lower due to internal structural damage during the first compression cycle.
Preparation of samples for the staling test involves placement of bread slices on a tray. Removal from the original packaging allows exposure to room humidity for a defined time interval to accelerate the staling process. Four-hour increments are a typical choice. The above TPA test is conducted on fresh slices taken out of the packaging while those on the tray remain untouched for four hours before testing.
Figure 2 shows graphical data from the TPA test on fresh bread slices (Sample A) versus those that have been left on the tray to stale (Sample B). The y-axis is registered in units of grams force while the x-axis is simply the timeline in seconds. Sample A exhibits a peak load of 184.5 grams on the first cycle when the probe has compressed the bread slices to a depth of 4 mm. The second cycle has a slightly lower peak load of 179 grams. Sample B by comparison has higher peak loads of 371.5 grams and 361.5 grams on cycles 1 and 2, respectively.
Sample A is softer as indicated by the significantly lower peak force values compared to Sample B. The internal structure of the bread has changed during the four-hour staling process to become more firm and rigid. The consumer will obviously notice the higher resistance to biting and chewing the bread slices that constitute Sample B.
Plotting the data using force versus distance for the same tests produces the graph in Figure 3, making it easy to perform mathematical calculations that quantify the amount of work done to compress the bread slices. The area under each curve is the equivalent work value for Sample A and Sample B respectively during the first compression cycle. Unit of measurement for work done is millijoules. Sample A has a value of 4.54 while Sample B is 9.75. This calculation confirms that the consumer will easily sense the difference between fresh and stale bread slices.
The final parameter used to evaluate the samples is springiness. This is technically defined as the ratio of spring-back distance compared to the maximum deformation. Both samples recovered almost completely after each cycle, therefore, the spring-back distance is close to the 4 mm compression distance. Springiness in both cases is relatively close to 1.
Springiness index is the ratio of springiness to the actual deformation after the completion of cycle 1. Since each slice recovered substantially to its original thickness, the actual deformation of each slice was relatively small compared to the thickness of the slice. Therefore, the springiness index will be a numerical value much greater than 1. Sample A is 5.08 and Sample B is 5.34. Comparatively speaking, fresh and stale slices were fairly similar.
The obvious advantage of TPA is the ability to numerically quantify behavior of the bread slices using deformation tests that simulate biting and chewing. (See the compiled measurement data in Table 1.) Comparing test data to standards for freshness and staling provides a meaningful yardstick to ensure that each batch will meet customer expectations.
McGregor is director of high-end lab instrument sales at AMETEK Brookfield, Instrumentation & Specialty Controls Division. Reach him at email@example.com. Chiang is sales manager, texture, for the company. Reach him at firstname.lastname@example.org.