Analytical protocols for the determination of total dietary fibre (TDF) in food are very easy to discuss theoretically, but rather more challenging to perform practically. It is not unusual for the procedure to become one of the most frustrating routine tests in the food laboratory, and the result can easily be misunderstood by the food business client. Often that is because of the expectations placed upon both the test and the obtained results. In respect of fibre testing, nothing is simple.
The first thing to consider is the question “what is total dietary fibre?” The answer is surprisingly easy. There is a relatively recent Codex definition describing exactly what should be considered as dietary fibre1. This has been widely adopted. There are also standard methods of analysis for the determination of dietary fibre in food published by the AOAC. So there should be no problem.
Of course, there are always problems. The first being that the two standard, if not classic methods for determining TDF (AOAC 985.29 and 991.43) do not detect some of that which is defined as TDF by Codex. This is actually for the very good reasons that not only were some of these components once not considered true dietary fibre, but they are also relatively insignificant in many food products. This means that for a sizeable majority of food samples then the classic AOAC methods are still perfectly fit for purpose; which certainly came as something of a relief to the testing community.
Well, perhaps “relief” is rather a strong word to use. Although 985.29 and 991.43 are the classic methods, they are challenging to perform, and come laden with an appreciable uncertainty of measurement. Both methods are based on the assumption that if you remove everything that isn’t dietary fibre from a sample of food, then anything that you still have at the end can only be dietary fibre. The following basic steps are required.
- Chemically remove sugar and fat from a food material
- Use enzymes to digest starches and proteins under controlled conditions of temperature and pH
- Precipitate out anything that has not been digested. Filter and collect the residue. This should only contain dietary fibre, residual protein and inorganic salts.
- Dry and weigh the residue, and then analytically determine the protein content and the ash content.
- Subtract the protein and ash from the total residue, and whatever remains is TDF.
Now, as I am sure can be appreciated, these 5 instructions turn into a very complicated procedure when enacted in the laboratory under the necessarily controlled conditions. It becomes even more complicated when a large number of tests have to be performed simultaneously, as would be the case in a high-throughput contract testing facility.
However, let us return, for a moment, to the idea that these classic methods do not detect some of that which is defined as TDF. In order to achieve complete testing for all fibre molecules it is necessary to use an alternative approach, e.g. AOAC 2009.01. This method requires the analyst to insert some HPLC analysis into the above 5-point procedure. This further complicates and elaborates the situation. Nonetheless, it must be recognised that such a method may give a more complete and hence accurate measure of the total dietary fibre content of a food. In fact, there are occasions when these recent Codex-compliant methods are absolutely necessary. If a food product has been fortified with small molecule soluble fibres then it is essential that AOAC 2009.01 (or an equivalent method) should be applied. In order to do that, it is vital that the food business discusses this requirement with its servicing laboratory before samples are submitted. This will allow the laboratory to fully appreciate the scope of the analytical demands, and then to fully advise and act accordingly.
One might then ask why any of the classic approaches to TDF analysis are still in use? Why would anyone wish to test using a method that does not detect components within the TDF definition? The answer is of course very simple and revolves around money. The classic methods can be performed very effectively and very efficiently, and in large volumes. It is not easy, but it can be done. The more recent Codex-compliant methods take appreciably more time and effort, and present appreciably greater technical challenges and capital outlay. The price charged by a laboratory for one of these tests will probably be something of the order of five times that charged for the classic analysis. The bottom line is that the vast majority of foods tested for TDF do not really need a Codex-compliant method and there is little desire to spend food testing budgets unnecessarily.
Nevertheless, surely it is preferable to report better analytical data? Well, indeed it is; but now we must consider another element of TDF test that has already been mentioned – the uncertainty of measurement (UOM). Evaluation of UOM can be decidedly tricky. Different people may have different views on the best way to do it, and there will be a number of alternative approaches used by a range of laboratories. There is not necessarily “one right way” to do it, although there will be many wrong ways. However, I am certainly quite comfortable with the idea that the best estimate for UOM of TDF testing on a range of food samples in any laboratory is likely to be between 20% and 30% regardless of the method applied. I am also quite comfortable with the idea that this uncertainly will increase at low levels – e.g. anything below 3g/100g TDF.
What does this mean to the food business operator? The most obvious scenario is as follows. Two identical samples are submitted to a laboratory for TDF testing. The obtained results for TDF are 2g/100g for one sample and 4g/100g for the other. Upon seeing this the laboratory manager will probably congratulate the lab analysts on a job well done. However, the client may be concerned that one result is 100% higher than the other and be rather less impressed. Unfortunately, in the world of food testing that may just be about as good as it gets for many samples. It also demonstrates why classic methods are perfectly fit for purpose. For most individual samples, it is unlikely that that any individual test using a Codex-compliant method would give significantly different results.
Having said all of that, it is absolutely necessary to state that testing for TDF is definitely not some sort of chemistry-based random number generator. There are a series of key elements to getting the procedures to work properly. If they are implemented and followed correctly then analytical performance is significantly improved. Even then, certain matrices, such as meat or cheese, can be exhaustingly difficult to test effectively. However, if performed well then analysis can be repeatable and reproducible, and even professionally satisfying to those involved. If analysis is performed without due attention to the key elements then it can be a total nightmare.
The availability of specialised equipment for TDF testing is generally restricted to enzyme digestion and filtration stages. The options range from automated through to relatively manual approaches. There are advantages and disadvantages associated with all alternatives, and much of these pros and cons will be to do with laboratory capacity and sample throughput as much as any technical benefit. Analysis for the determination of total dietary fibre is a very “hands-on” procedure, and in such cases the simplest approach may often turn out to be optimal.
- Jones, J. M. (2014). CODEX-aligned dietary fiber definitions help to bridge the “fiber gap.” Nutrition Journal, 13, 34. http://doi.org/10.1186/1475-2891-13-34
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