Important note regarding use and 'misuse' of TPA testing
Over the past few years TPA testing has caused much concern. In general, TPA is a very popular method of testing, and this is considered so as it provides very quick calculation of parameters which are 'believed to correlate with sensory analysis'. The user is almost encouraged to not think about the analysis and in doing so takes down the results as true. One should always consider the parameters before testing and only bear in mind those parameters which are actually applicable to the product. After all, how can the springiness parameters possibly be repeatable for a product such as chocolate; likewise, unless of very special formulation, bread could never be considered to be adhesive, and therefore these results should not be reported.
Another area of abuse is the degree of compression. Often when presumably limited by force capacity, we find that results are shown for compression to, for example, 30%. When considering that the analysis of the results were developed to correlate with sensory analysis, one must clearly bear in mind that compression in the mouth is never 'small deformation' (i.e. a gently squeeze) and that the probe/fixture used should always assume the principle of compression (i.e. only flat probes larger than the sample should be used for testing).
Attention should be drawn to a letter published in the Journal of Texture Studies which confirms the above guidelines:
Letter to the Editor: Issues Pertaining to the Texture Profile Analysis
As a founder of the Texture Profile Analysis (TPA) in the 60's, I am very gratified that the method presently enjoys considerable use and popularity. I am especially pleased that a number of researchers have built on it, as I have hoped they would. Some of this work constitutes indisputable improvements (as e.g., using only the down, i.e. left-hand side, portion of the force-deformation curves to calculate cohesiveness, or a sounder and easier method of quantifying springiness). Other researchers have just expanded the number of quantified parameters by adding new ones (e.g. hardness 2) without demonstrating their usefulness.
However, I am deeply perturbed by what I would call a 'misuse' of the method, and a poor understanding of the meaning of the parameters and the manner in which the method should be executed. As examples of 'misuse' I would cite an article published not so long ago in the Journal of Texture Studies in which a penetrating needle (rather than a compressing plate) was used, and a recent publication in the Journal of Sensory Studies in which 'chewiness' of Lifesavers of candy was calculated and correlated with the sensory evaluation. Some eyeballing of the sample during compression and some common sense would reveal that: a) hard candy is not chewed but sucked in the mouth, and b) when compressed to 70% (as was done in that work) the Lifesaver is crushed/shattered and, thus, the parameter of springiness is meaningless. Penetration causes a totally different structural damage than compression; penetrating the sample twice in the same spot leads to meaningless data and the test should not be called TPA.
A very prevalent misunderstanding of the meaning of the parameters deals with the calculation of both chewiness and gumminess for the same sample. The original description of TPA parameters (J. Food Sci. 23, 390-396, 1963) defined chewiness as applicable to solids and gumminess as applicable to semisolids. Prof. Bourne and I had clarified this issue in our Letters-to-the-Editor first in the Journal of Food Science (vol. 60, p. viii. 1995) and then in the Journal of Texture Studies (vol. 27, pp. vi-vii, 1996). As evidenced by references cited in subsequent publications employing the TPA some researchers have read these letters. A number have not.
It seems to me that time is ripe to have a 'round table' discussion (either face-to-face or on the Internet) of how TPA should be executed. It could lead to the much needed development of some guidelines and, ultimately, a standardisation of the method.
Alina Surmacka Szczesniak, formerly Principal Scientist, General Foods Technical Center, and founding editor of Journal of Texture Studies.
Using TPA Testing in Exponent
A Project called TPA.prj is supplied with Exponent. This project pulls together all the components required to perform a basic TPA (Texture Profile Analysis) test.
The project can be found in the Exponent Samples directory. You will probably need to modify the Texture Analyser Settings to suit your particular product.
To copy the project using version 220.127.116.11 (or higher):
Locate the TPA project in the Project panel > Sample Projects. Right click on TPA.PRJ and select Copy from the pop-up menu. Click on the ...button to the right of the Destination folder box. Locate a suitable folder to copy the project to. You can create new folders if required with the Make New Folder button. Click on OK.
Change the destination name to something more personal e.g. 'Butter TPA.prj'. Click on OK. When the prompt asks if you want the project added to your project list we recommend that you select YES.
Select the Project List and expand the Personal Project shortcuts. Double click on the TPA entry and follow the prompts.
All of the files in the samples folder are write protected to avoid accidents. If you need to change them, we recommend that you make some copies or follow the procedure above.
TPA Project Files
The TPA Project consists of certain files. These files all play a role in the final analysis.
TPA.PRJ: this is the project file that contains all of the references to the other files so that Exponent knows how to do a TPA test.
TPA.SET: this file contains all the Texture Analyser settings required to run a TPA compatible test. The settings for speed and distance will probably need changing to suit your sample. The actual values required are normally found on a try it and see basis. The settings are edited in the T.A. Settings window.
TPA.PRF: this file contains the details of the axis types and values, etc. You can see these settings in the Graph Preferences window.
TPA32.MAC: this is the macro that issues all of the commands to find and measure the required data from a graph. There may be a need for changes to the TPA macro due to the particular properties of your sample and the forces derived.
If the sample has no Fracture peak, then a pop-up box will appear stating Peak Not Found. Click on OK to continue. The three force peaks are found in REVERSE order (see graph above) so that a missing fracture peak will still produce the correct values for other results. If the force peaks appear towards the end of the graph, failure to notice this is most likely to cause all TPA values to be incorrectly calculated. In this case, try selecting a larger Force Threshold in your Graph Preferences.
After the peaks of the curve are detected the macro then instructs the cursor to search forwards and drop 6 anchors, in order, onto the curve (as shown above). If the message, e.g. 'Unable to place cursor at specified position' is displayed, then it is likely that the Distance Threshold is too high. Again failure to notice this is most likely to cause most of the TPA values to be incorrectly calculated. In this case try selecting a lower Distance Threshold in your Graph Preferences.
If the sample has no adhesiveness properties at all, the force might not return to zero between the two compression strokes. If this is the case, then the anchors will not be positioned correctly. To correct this problem, you might need to adjust the Go To Force value in the macro from 0.0 to something greater than the Texture Analyser trigger force. The Adhesiveness result will then show a positive value, which should be ignored.
TPA.RSL: this is the results file that has formulae programmed in to calculate the TPA data from the measurements. Unless this particular results spreadsheet is open when the TPA Macro is run, the TPA parameters will not be calculated in the spreadsheet columns as expected.