Selecting the correct TPA testing procedure

Over the past few years Texture Profile Analysis testing has been the cause of much concern. In general, TPA is a very popular method of testing, as it provides very quick calculation of parameters which are 'believed to correlate with sensory analysis'. 

The user, however, can be almost encouraged to not think about the analysis and in so doing, accept all of the results as being true. 

The following is a set of points to consider when choosing TPA as your test procedure.

Size of Compression Probe versus Sample
When the probe is larger than the sample, the forces registered are largely due to uniaxial compression. However, when the opposite is true, the forces derive largely from puncture, a combination of compression and shear. Various papers throughout the decades of using TPA have reported the use of probes both larger and smaller than the test samples. 

Early papers on TPA report the use of puncture probes, but in 1968 Prof. Malcolm Bourne was the first to adopt true uniaxial compression to perform TPA tests. Generally speaking, most recent work done on TPA uses compression probes of the same size as or larger than the sample size, so that the forces registered in such TPA tests are largely due to uniaxial compression forces and the whole of the sample piece is tested. 

Extent of Deformation
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%. If the purpose of testing is to imitate the highly destructive process of mastication in the mouth, as in the TPA's origins, deformation values to break the sample must be reached. 

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). For low force texture analysers it may not be possible to compress to a high % deformation as one would like. 

When developing a TPA method for testing a range of samples, it is always suggested that the method (i.e. the extent of deformation) should be developed on the hardest sample. Checking that the required extent of deformation is suitable for this hardest sample will ensure that all other 'softer' samples are able to be tested using the same method.

Usually – for example, in gelled systems – compressions greater than 70-80% completely break the sample. The second compression cycle does not usually find a weakened sample with just the first internal cracks, but portions or small pieces of the initial sample.

Deformation levels between 20-50% have commonly been applied in recent works. At these levels, samples may not break (a fracturability peak does not appear in the curve) but it is still possible to obtain information such as hardness (force to a given deformation), springiness, cohesiveness, and their derivatives gumminess or chewiness.

Crosshead Speed
The user must also be consistent with the loading rate (crosshead speed) which may vary depending on the food. It has been shown that as the crosshead speed increases, the force required to achieve a particular compression also increases, since a slower rate allows greater relaxation of the gel.  

Since humans use different forces and chewing rates depending on the textural characteristics of the food evaluated, both the magnitude of the force applied and the rate at which it is applied must be taken into account when testing conditions are established on the texture analyser. The aim must be to reproduce those conditions associated with the sensory evaluation of the specific type of food. One point to note is that the post-test speed should be set to the same as the Test Speed, especially if the cohesiveness parameters are to be calculated correctly.

Time Elapsed Between Bites
The texture analyser gives the option of selecting a variable time period to elapse between bites. If a time period is selected, it should be specified in the testing conditions, since the amount of time between bites clearly determines TPA parameters such as springiness, cohesiveness, gumminess and chewiness, mainly in those systems with a highly viscous component.

Sample Preparation
For the TPA parameters to be relevant, it should be mentioned that they can be used to compare different foods only if the sample dimensions are consistent. As an example, the meaning of the term hardness may be different even for the same food type, if different percent compressions are used in replications, or if the samples differ in height or contact area. For the accurate measurement of adhesiveness, the probe must not lift up the sample on its return, so a suitable means of holding the bottom surface of the sample needs to be identified if this is likely to occur.

Relevance of Results
TPA has been found to be very useful for a large proportion of samples, but not all TPA parameters are applicable to every sample. For instance, springiness values for the testing of chocolate are not likely to be repeatable, since springiness is not an important textural characteristic of chocolate – just as adhesiveness of bread is unimportant and therefore not applicable. 

Therefore, it is recommended that before performing a TPA test you ask yourself 'What are the important textural parameters that I am interested in for this particular product?' rather than collecting all of the TPA parameters and judging the results without prior thought to their relevance.

Important Note regarding Use and 'Misuse' of TPA 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.

References 
PONS, M. & FISZMAN, S. M. (1996).  Instrumental texture profile analysis with particular reference to gelled systems.  Journal of Texture Studies, 27, 597-624.


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