When Testing for Off-Flavors, Chromatography and the Human Nose Are a Winning Combination

Quality control testing in the food sector is vitally important to make sure that what we eat is safe, meaning that it is free of contaminating allergens, bacteria, fungi and parasites.

But other quality factors are also important from a consumer point of view, such as flavor.

Any off-flavors can cause significant economic damage to the food industry if their cause cannot be determined and remedied.

But detecting off-flavors is not as simple as just running a sample through some scientific apparatus. For this, food scientists must rely on our most sensitive, well-trained piece of equipment: the human nose.

What causes off-flavors in food?

“Off-flavor” is a general term used in food chemistry to describe an unpleasant taste or odor in a food. Some off-flavors are perfectly natural and develop over time. For example,as fruit begins to mature to over-ripeness and then rancidity, it can develop an unpleasant bitterness or a vinegar-like taste and smell. But other off-flavors are the result of specific processes that occur in the food industry.

"The use of recycled materials is a source for many off-flavors that may migrate into our food,” said Barbara Siegmund, PhD, an analytical chemist and leader of the Food Chemistry and Human Sensorics group at TU Graz, in a keynote talk delivered at this year’s 11^th International Symposium on Recent Advances in Food Analysis (RAFA) conference.

“Other potential reasons or causes for the formation of off-flavors could be chemical reactions, could be oxidative processes, could be reactions induced by thermal treatments or degradation. It could be off-flavor compounds in packaging material that might migrate into the food and cause off-flavor.”

“And there are many other reasons, like genetic factors and so on,” Siegmund continued. “And there are microorganisms. Microorganisms produce primary and secondary metabolites, and in most cases, it is the secondary metabolites and not the primary metabolites that cause off-flavors.”

The challenge of detecting unpleasant tastes

Unsurprisingly, given the wide variety of potential sources for off-flavors, many different compound classes could be of interest when it comes to discussing and identifying off-flavors. But that is not the only significant challenge.

“What we need to keep in mind when we talk about flavor and off-flavor is the term odor threshold,” Siegmund said. “The odor threshold represents the minimum concentration of a compound that needs to be present in order to evoke a sensory impression in us humans.”

Crucially, the odor threshold of many potential food contaminants is incredibly low — in some cases, this can be down to the low nanograms per kilogram/liter level or picograms per kilogram/liter level.

“We’re talking about ultra-trace concentrations,” Siegmund told the RAFA audience. “And this is where the human nose comes into play.”

The nose knows

For these off-flavor-causing compounds, the human nose is far more sensitive than the typical gas chromatography (GC) apparatus, Siegmund explained. Crucially, where a GC will output a chromatogram that requires further interpretation — or in some cases, a chromatogram so complex that it defies interpretation — a sensory panel made up of trained human taste testers can quickly deliver actionable feedback.

“We are not asking hedonistic questions; it’s no use to ask if you like the sample or not. It’s a clear analytical task with high-trained panelists, and we aim for as much information as we can can get” Siegmund said. “If we have a panelist who tells us ‘this is stinky,’ well, this is correct, but it is not enough.”

“What we like most is when the panelist can tell us ‘this is a pungent, bell pepper-like smell, and it could be IBMP isobutyl methoxypyrazine [IBMP] or IPMP [isopropyl methoxypyrazine],’ because then we can perform targeted analysis.”

These types of human sensory evaluation panels are not just an essential element in improving the efficiency of off-flavor determination, they can also be used in conjunction with more traditional analytical chemistry approaches to generate more detailed insights.

One example of this is gas chromatography-olfactometry (GC-O), an analytical technique that uses gas chromatography to separate volatile compounds from a sample extract and funnel them to an “olfactory detection port” — a small outlet where a waiting human assessor can sniff them and record their evaluations. In some cases, GC-O can also be combined with more traditional gas chromatography-mass spectrometry (GC-MS) to improve its analytical power.

“Why is this important?” Siegmund asked the RAFA audience. “Because we get information on the retention behavior of the compounds; we get information on other qualities of the compounds. And with this, we can go into more targeted analyses because we can focus on specific regions in chromatograms for identification.”

The ideal sensory panelist

Given the important role that sensory evaluation can play in identifying off-flavors, you might be thinking to yourself, would I make a good sensory panelist?

As already mentioned, a good panelist can give very specific and detailed descriptions of what they are tasting. They often have a particularly high sensitivity to common off-flavor compounds, and should be very familiar with the chemicals’ odors. In the best-case scenario, Siegmund said, a panelist should be able to correlate the odors and flavors they experience to certain chemical compound families.

“And do not forget, they need to have very high levels of motivation if we’re going to keep bringing them tainted or off-flavor samples again and again,” Siegmund added.

Another critical attribute is the absence of any anosmia, the partial or total absence of the sense of smell.

“Anosmia is a sensory distortion — some of you may have suffered from temporary anosmia during the coronavirus pandemic if you lost your sense of smell,” Siegmund explained.

Ansomia can also be genetic, or the result of other infections or nasal injuries. People with general anosmia do not tend to participate in sensory evaluation. However, cases of “specific anosmia” are a different matter, and surprisingly common.

“A specific anosmia is where a person cannot smell a specific smell, usually due to genetic variation in the expression of our olfactory receptors,” Siegmund said. “Basically all of us show a type of specific anosmia. When we look at the frequency of specific anosmia, that ranges between 5-50% for certain compounds, so we have to keep that in mind.”

“We really have to be aware of flavor, because off-flavor in food commodities causes enormous damage and enormous food waste.”

“Sensory evaluation and GC are key techniques to identify off-flavor-causing compounds,” Siegmund continued. “You have to be very, very strict with your panelists, you have to train them well, but also take care of them. And if they’re good, try to keep them.”