The estimation of the sensory shelf-life of a perishable food product is far more complicated than merely estimating the shelf-life of a product in terms of food safety. A product could be biologically perfectly safe to eat, i.e. containing fewer pathogenic micro-organisms than a legally predetermined limit, but it may be unacceptable to the consumer in sensory terms. For example, a ready to eat packaged salad could be biologically safe, but may have visible signs of enzymatic browning, and therefore is deemed unfit for sale. Factors such as microbial load or the extent of lipid oxidation in a food can be measured, but these biological/chemical/instrumental results must, 'correlate closely with results from sensory evaluation.' Individual food producers most often determine the sensory shelf-life cut-off point of their products in terms of acceptable taste, smell, appearance, and texture based on the opinion of the consumer by using panels of judges in taste trials. Food producers are interested in shelf-life estimation, and more so extension, as it greatly reduces the level of waste, and therefore loss, to the producer. There are two main experimental methods of determining the sensory cut-off point of a food product: basic storage design, and reversed storage design. The former involves simply testing the same large batch of food, stored at normal conditions, after different intervals to determine at what stage the product has become unacceptable in sensory terms. The latter involves evaluating many different samples at once, which have been stored under normal conditions for different periods of time. Methodologies for sensory shelf-life estimation can be further divided more specifically into a) quality-based methods under the headings: difference from control test, intensity of sensory attributes, and quality rating methods; b) acceptability limit methodology, c) cut-off point methodology, and d) survival analysis. A 2012 study in Uruguay concluded that, 'the most popular approach for estimating the sensory shelf-life of food products continues to be the evaluation of product quality throughout storage with a trained assessor panel.'
Currently researched & applied methods for the shelf-life extension of perishable foods
Until recent decades, most consumers were happy as long as their extended shelf-life food products were safe to eat, i.e. in a microbial sense. Old fashioned methods of shelf-life extension often involved severe, high temperature cooking of foods such as vegetables, meats, and some fruits to kill the bacteria and spores, then drying or canning it, usually in brine with a high salt content or sugary syrup. Apart from adding salt, sugar, and possibly nitrates or other unhealthy preservatives to the foods, these methods significantly affected the vitamin content of the foods, especially vitamin C and the B vitamins, in fruits and vegetables. Many modern consumers (and food producers) look for foods that have had their shelf-life extended by healthier methods, with minimal processing, for example, prepared salad leaves and chopped vegetables/potatoes ready to cook, or raw marinated steak strips for stir frying, instead of pre-cooked, tinned, or heavily processed foods. This obviously calls for a different approach to shelf-life extension. Modifying the atmosphere within packaging, and the permeability of the packaging material itself to control water vapour, oxygen, carbon dioxide, and ethylene levels has, in recent decades, been the main area of research in terms of extending shelf-life, and is still a huge area of study and development; with some of the resulting packaging proving to be more expensive than food itself. However, other methods of shelf-life extension have been investigated in recent years, which, when combined with the extremely effective modified atmospheric conditions and packaging that already exist, could result in significant shelf life extension for many perishable products.
One recent development has come, in part, from DIT itself-the use of plasma to extend shelf-life. Dr. P.J. Cullen is working with the European-wide Safebag project, which aims to develop a, 'novel continuous in-pack decontamination system for fresh-cut produce.' Plasma has been known to scientists for a century, and used as a decontaminant in the medical packaging industry for a decade, but its applications in food preservation have never been used on a large scale. Plasma is an energetic ionised gas, but unlike most forms of energy it remains cool, and so does not destroy delicate fresh produce. Plasma is considered a fourth state of matter, where the atoms of a gas are bombarded with energy, causing loss of electrons, which results in the nuclei becoming charged/ionised, transforming the electrons and nuclei into a plasma state. Currently gases such as hydrogen, oxygen, nitrogen, and argon are used to create plasma inside the sealed bag of perishable food. Other gases and their mixtures are also being studied. The reactive plasma particles extend shelf-life by removing all organic material from surfaces. When applied to fresh cut fruits and vegetables inside a sealed bag, the carbon based organic material on the surface, such as bacteria and moulds, is destroyed. The use of plasma is an affordable and much faster alternative to chlorine washing of fresh produce, which is of concern to many consumers, and is banned in Switzerland and Germany. Chlorine can leave residue, and foods can become re-contaminated even more quickly on washed surfaces. Safebag estimate their new technology will double the current shelf-life of vegetables and fruits, which will be of huge benefit to food producers, and consumers.
Marine bioactive compounds have also been studied recently for their potential in shelf-life extension. Chitosan is a polysaccharide derived (using sodium hydroxide) from chitin, found in the shells of shrimp, lobster and crabs. It already has several commercial uses as a biopesticide, as a self-fixing polyurethane coating, and to aid the delivery of drugs via the epidermis. A study in Thailand recently found that incorporating chitosan and green tea extract into plastic food film significantly extended the shelf-life of pork sausages by delaying chemical and microbiological deterioration, and improving the sensory qualities of the meat. A South Korean study in 2010 incorporated chitosan lactate in low density polyethylene food film, and found that it maintained the red colour of sliced raw meat significantly, and inhibited the growth of Listeria monocytogenes, Escherichia coli and Salmonella enteritidis. A 2011 study in Italy incorporated chitosan and whey protein into an edible film for ricotta cheese and found that it significantly reduced the growth of psychrotrophic and mesophilic micro-organisms, and lactic acid bacteria. An Irish study this year found that feeding pigs for twenty one days pre-slaughter with seaweed extracts containing fucoidan and laminarin, extracted from Laminaria digitata, significantly reduced lipid oxidation of the meat end-product in 75% of cases. Another recent Irish study found that extracts from the seaweed, Himanthalia elongata had significant antioxidant and antimicrobial properties when used as a food additive, thus potentially extending the shelf-life of perishable foods.
Other recent areas of research include rosemary extracts, which have been found in several studies to inhibit both protein and lipid oxidation in meat, which, for example, can cause a 'soapy' taste in frankfurters. Rosemary has also been tested in combination with cocoa and olive oil polyphenol extracts, chitosan, and whey protein. In all cases, antioxidant properties were exhibited, and in the case of the rosemary/cocoa/olive oil polyphenols, inhibition of Listeria monocytogenes. This is significant since L. Monocytogenes can survive on food at temperatures as low as 0.5°C. Vitamin E has been much studied in recent years for the effects its α-tocopherol component has on meat in particular. When included in the diet of red meat producing animals it has been shown to have a stabilising effect on myoglobin, maintaining the red colour, and improve freshness of flavour. Other phytochemical extracts with antioxidant properties such as pycnogenol from pine bark, procyanidins, citroflavan-3-ol, and resveratrol from grape skins have exhibited significant effects on dairy, meat, and poultry products.
Physics is another area of study for shelf-life extension, for example, using pulsed light, high-intensity pulsed electric fields, and ultrasound for their antimicrobial effect. There are many other topical methods of food shelf-life extension, too numerous for this discussion.
