Throwing new light on sun taste
Photo-oxidation is a major problem in the storage of a number of different kinds of food. Dairy products are particularly vulnerable, as they contain molecules that are sensitive to light. "Sun taste" in milk is a case in point.
"Understanding and measuring light-induced oxidation in dairy products" was carried out between 2006 and 2008 and was financed by the Norwegian Research Council. Jens Petter Wold from Nofima Mat (previously Matforsk) was the project manager. Other participants in the project included the University of Copenhagen and the Norwegian Radium Hospital.
A recently completed research project has revealed new information on how and why photo-oxidation occurs. This will help the dairy industry protect their products against this deterioration in quality. "Sun taste" occurs in milk and other dairy products when they are exposed to light over time, for instance in the sun or in refrigerated displays in the shops.
Not only riboflavin
Understanding the mechanisms behind photo-oxidation will also help us gain a better understanding of how these effects can be reduced. Dairy products contain riboflavin, which is highly sensitive to light and may start photo-oxidation. It has been an accepted fact that riboflavin is responsible for photo-oxidation in dairy products. Historically, however, a few studies have indicated that photo-oxidation must have other additional causes. Just prior to this project, researchers at Nofima Mat discovered that there are other light sensitive substances in dairy products, namely chlorophyll residue as well as various types of porphyrins. This was discovered using fluorescence spectroscopy measurements made directly on products such as butter, cheese, sour cream and milk. Figure 1 shows how a single fluorescent spectrum contains quantitative information on several different light sensitive molecules (photosensitising agents). When the product is exposed to light, several substances will be degraded. This degradation can be measured directly using this method. This means that it’s now possible to measure the initiation of the oxidation processes, and also to see which sensitising agents that actually contribute to the initiation.
Chlorophyll and porphyrins are highly efficient, and in our project we have found that riboflavin probably plays a modest role in photo-oxidation. As part of our project, we characterized the various light sensitive substances in two different ways. The first was to compare the fluorescence peaks in for instance butter with fluorescence spectra from obvious candidates, ie. various porphyrins and chlorines. This was then combined with carefully designed light exposure experiments and fluorescence spectroscopy. In this way, we have established the existence of active photosensitising agents and also partly determined their properties. In addition to riboflavin we have found protoporphyrin, hematoporphyrin, a derivate of chlorophyll a as well as two as yet undefined porphyrins. By using chemometrics, we can follow the concentration for each of these light sensitive substances as a function of for example light colour, duration of exposure to light and oxygen level in packaging. This gives us entirely new knowledge of the photo-oxidation processes in dairy products.
By using fluorescence spectroscopy we have also been able to quantify the amount of protoporphyrins and hematoporphyrins in butter. The concentrations are extremely low, between 0 and 0.02 ppm. This low concentration may be the reason why these substances in dairy products have not received much attention earlier. Apparently, increasing fat levels in products are matched by higher porphyrin levels – high levels in butter oil and butter, somewhat lower in cheese and even less in cream and milk. What’s conspicuous is that although these concentrations are low, they are sufficient to initiate and cause marked photo-oxidation.
All light destroys
Various light sensitive substances absorb light at different frequencies. The traditional belief is that visible light in the 400-500 nm range (violet and blue) has been the most harmful, as this is the area where riboflavin absorbs light. The results from our project show that light from the entire visible spectre initiates photo-oxidation. We have seen that blue and violet light has the greatest effect on white cheese as porphyrins and chlorophyll absorb a lot of light in this area, followed by red light, but even yellow and green light initiate photo-oxidation. The situation is somewhat different for butter, where for instance negative effect of green light is almost as strong as red light. As such our results are somewhat discouraging for the industry: all visible light will initiate oxidation in dairy products. The best protection is therefore to block as much light as possible. The reason why green light is less harmful to some products is a lower absorption of green light.
In addition to explaining some photoreactive mechanisms, we have shown that fluorescence measurements directly on samples of butter or cheese is an extremely efficient and sensitive method for measuring and quantifying light-induced oxidation. Both the degradation of the various sensitising agents and the creation of oxidation products can be measured, but it is the actual degradation of light sensitive substances that is the best marker for early photo-oxidation, as it is not possible to measure fluorescence from oxidation products until long after the sensory quality has deteriorated. The same is to a considerable extent valid for standard methods that are used in this area. In this project we have consistently used sensory analysis as reference tools for early/weak photo-oxidation, as this method has been proven to be more sensitive and more precise. The only method that’s as sensitive as sensory science is fluorescence.
Which substances cause photo-oxidation? It’s hard to give a precise answer. As there are six different sensitising agents in the same system, with spectra that overlap a lot, it’s hard to attribute higher importance to some of them and not others. Through designed experiments, however, we have seen that the photodegradation of various porphyrins and chlorophyll have a great effect on oxidation, and that riboflavin probably does not play an important role, in spite of the riboflavin concentrations being 10,000 times higher than several of the other substances. Various photosensitive substances will contribute with different effects depending on the product, lighting conditions and the amount of oxygen available.
It is well known that different molecules are degraded in different ways by different kinds of light, but we have observed that they are also affected in different ways by various concentrations of oxygen. We have also seen that where there are many naturally occurring light sensitive substances, these will to a certain extent compete about the absorption of incidental light. This is why some light sensitive substances that occur in very low concentrations can generate much stronger photo-oxidation than the substances that are present in substantially higher concentrations. This is a complex matter which is hard to thoroughly explain. We have started comparing photodegradation of the light sensitive substances with the creation of free radicals. This work will continue in cooperation with the University of Copenhagen.
Treatment of cancer
The photo-oxidation processes that occur in dairy products are the same processes that form the basis of so-called photodynamic cancer therapy (PDT): certain photosensitising agents are injected into the blood or applied directly onto the skin affected by skin cancer. The photosensitising agents accumulate in the cancerous tissue and are then exposed to light. This light causes photo-oxidation, which destroys the cancerous tissue. The Norwegian Radium Hospital has used this type of treatment for many years and has led pioneering research in insight into and further development of this method. In this project, our cooperation with the Norwegian Radium Hospital has been extremely important, both in order to understand some of our results and in our planning of important further steps in our work.
PDT usually uses a single photosensitising agent, and most published studies on how treatment is affected by factors such as oxygen concentration and exposure time to light has been made on systems with only one light sensitive substance. As dairy products contain at least six light sensitive substances, the systems are very complex, and detailed interpretation of the results is difficult to obtain.
Efficient analysis hinges on the use of chemometric tests in order to separate the various components, and in this area we have had a fruitful cooperation with the University of Copenhagen.