Consumers like to see what they are buying, which is why so many products come in clear plastic wrappers. However, research by food scientists in the Department of Food Science and Technology at Virginia Tech in the US has provided significant evidence that visible wavelengths of light cause taste and odour changes to food.
Materials research for protecting food from light damage focuses on UV light in the 200 to 400nm range: the range that can damage skin.
“These are the same wavelengths that cause nutritional and sensory damage in food,” said Susan Duncan, professor of food science and technology.
Light is a common initiator of these oxidation reactions. Many foods and beverages contain photosensitisers that absorb specific wavelengths of light, both UV and visible, and initiate free radical oxidation reactions in food. Natural pigments found in food that commonly act as photochemical initiators are flavonoids, riboflavin (vitaminB2), chlorophyll, heme, and vitaminK.
Photooxidation is especially prevalent in foods and beverages packaged in translucent or transparent packaging materials and displayed under high intensity fluorescent light. These oxidation reactions ultimately lead to the production of off-odour and flavour compounds and destroy vitamins and other nutrients.
When sensitisers such as riboflavin become excited, they undergo one of several transformations:
1. The excited electron re-enters the original bonding orbital, giving off energy in the form of light or heat;
2. The molecule abstracts a hydrogen from the substrate, or;
3. The compound transfers its energy to another molecule through sensitisation.
Theoretically, once these transformations occur, the sensitiser goes back to its ground state and is available to be excited again. In reality, however, some sensitiser molecules are destroyed during these reactions and are no longer available for further free radical initiation.
Photosensitiser initiated photo-oxidation can proceed by one of two mechanisms depending on the conditions that are present at the time of the reaction. The excited triplet state of the sensitiser can react with biomolecules, such as lipids or proteins, to form free radicals either by the abstraction of an electron or a hydrogen (typeIreaction) or by reacting directly with molecular oxygen to form singlet oxygen (typeIIreaction).
UV wavelengths are not the only ones that cause damage, but they are important from the perspective of the food processors, who want beverages to look appealing. Packaging has moved away from paperboard to polymers such as polyethylene, so the consumer can see the product. “Then they started to have colour and flavour problems,” added Duncan.
Adding UV absorbers to the packaging helped and still allowed the consumer to see the product. However, this did not totally resolve the problem.
“The only way to completely protect the product is to use a totally opaque container. But generally, consumers like to see a product, particularly milk, to make sure it isn’t curdled, or juice to make sure there is no sedimentation. But we also want a product to have a long shelf life.”
The Virginia Tech researchers have tested a number of new materials that are not currently being used for food packaging. One material was a translucent sleeve over wrap with an iridescent shimmer that reflects wavelengths. “We found evidence of improvement, but still not as good as opaque,” Duncan said.
But she believes that material scientists can develop better materials, once they become attuned to the challenges of food packaging.
“We want to find manufacturers to work with us to develop packaging products that will work with milk and the visible wavelengths. Food scientists and material scientists working together is what is on the horizon and why we took our food research to the polymer section of the American Chemical Society (ACS) meeting.”
Duncan and her co-workers from Virginia Tech and Eastman Chemical presented their paper “Packaging solutions for sensory degradation in foods and beverages due to photooxidation” at the recent 234th ACS national meeting in Boston, Massachusetts.
Getting at gastroenteritis
Meanwhile, scientists at the UK’s Institute of Food Research and their collaborators at Dutch veterinary pharmaceutical company Intervet have deciphered and published the complete genome sequence of a strain of Campylobacter jejuni.
The food-poisoning bacterium C. jejuni is one of the major causes of gastroenteritis in humans, causing diarrhoea, stomach cramps and in rare cases a nervous condition called Guillain-Barré syndrome (Fig.1).
Humans are commonly infected by eating undercooked poultry meat, which is contaminated during processing of the chickens. Surprisingly, the Campylobacter bacterium is commonly carried in the gut of birds without causing disease in the birds.
Like many bacteria, C. jejuni is able to avoid our body’s defences by altering the nature and content of its surface. These alterations are achieved by having regions of the bacterial chromosome that are able to make small random variations, resulting in different surface structures.
Genomic variability has been a problem for researchers investigating C. jejuni, since it potentially also causes differences between laboratories and even between experiments. In a project funded by the UK Biotechnology and Biological Sciences Research Council (BBSRC) and Intervet, the Campylobacter group at IFR has determined and analysed the complete genome sequence of Campylobacter jejuni strain 81116 (also known as NCTC11828). This strain was selected because of its previously reported genomic stability over time.
The genome sequence reported by IFR and Intervet is 1,628,114 bases in length and notable for having fewer of the variable regions than the previously reported C.jejuni sequences. Strain 81116 is widely studied as it is amenable to genetic alterations, and grows well in poultry allowing this important natural reservoir to be studied. Thus the reported sequence will provide useful information for Campylobacter researchers worldwide, and is predicted to be a valuable resource for the research community.
