Chapter3 Deterioration factors and their control
Contents -Previous -Next
A summary of overall deterioration reactionsin fruits and vegetables is presented below.
Enzymes which are endogenous to plant tissuescan have undesirable or desirable consequences. Examples involving endogenousenzymes include a) the post-harvest senescence and spoilage of fruit andvegetables; b) oxidation of phenolic substances in plant tissues by phenolase(leading to browning); c) sugar - starch conversion in plant tissues by amylases;d) post-harvest demethylation of pectic substances in plant tissues (leadingto softening of plant tissues during ripening, and firming of plant tissuesduring processing).
The major factors useful in controlling enzymeactivity are: temperature, water activity, pH, chemicals which can inhibitenzyme action, alteration of substrates, alteration of products andpre-processing control.
3.2.1 Sensory quality
The two major chemical changes which occur duringthe processing and storage of foods and lead to a deterioration in sensoryquality are lipid oxidation and non-enzymatic browning. Chemical reactionsare also responsible for changes in the colour and flavour of foods duringprocessing and storage.
3.2.1.1 Lipid oxidation rate and course of reactionis influenced by light, local oxygen concentration, high temperature, thepresence of catalysts (generally transition metals such as iron and copper)and water activity. Control of these factors can significantly reduce theextent of lipid oxidation in foods.
3.2.1.2 Non-enzymic browning is one of the majorcauses of deterioration which occurs during storage of dried and concentratedfoods. The non-enzymic browning, or Maillard reaction, can be divided intothree stages: a) early Maillard reactions which are chemically well-definedsteps without browning; b) advanced Maillard reactions which lead to theformation of volatile or soluble substances; and c) final Maillard reactionsleading to insoluble brown polymers.
3.2.1.3 Colour changes
Chlorophylls. Almost any type of food processingor storage causes some deterioration of the chlorophyll pigments.Phenophytinisation (with consequent formation of a dull olivebrown phenophytin)is the major change; this reaction is accelerated by heat and is acidcatalysed.
Other reactions are also possible. For example,dehydrated products such as green peas and beans packed in clear glass containersundergo photo-oxidation and loss of desirable colour.
Anthocyanins. These are a group of more than150 reddish water-soluble pigments that are very widespread in the plantkingdom. The rate of anthocyanin destruction is pH dependent, being greaterat higher pH values. Of interest from a packaging point of view is the abilityof some anthocyanins to form complexes with metals such as Al, Fe, Cu andSn.
These complexes generally result in a changein the colour of the pigment (for example, red sour cherries react with tinto form a purple complex) and are therefore undesirable. Since metal packagingmaterials such as cans could be sources of these metals, they are usuallycoated with special organic linings to avoid these undesirablereactions.
Carotenoids. The carotenoids are a group ofmainly lipid soluble compounds responsible for many of the yellow and redcolours of plant and animal products. The main cause of carotenoid degradationin foods is oxidation. The mechanism of oxidation in processed foods is complexand depends on many factors. The pigments may auto-oxidise by reaction withatmospheric oxygen at rates dependent on light, heat and the presence ofpro- and antioxidants.
3.2.1.4 Flavour changes
In fruit and vegetables, enzymically generatedcompounds derived from long-chain fatty acids play an extremely importantrole in the formation of characteristic flavours. In addition, these typesof reactions can lead to significant off-flavours. Enzyme-induced oxidativebreakdown of unsaturated fatty acids occurs extensively in plant tissuesand this yield characteristic aromas associated with some ripening fruitsand disrupted tissues.
The permeability of packaging materials is ofimportance in retaining desirable volatile components within packages, orin permitting undesirable components to permeate through the package fromthe ambient atmosphere.
3.2.2 Nutritional quality
The four major factors which affect nutrientdegradation and can be controlled to varying extents by packaging are light,oxygen concentration, temperature and water activity. However, because ofthe diverse nature of the various nutrients as well as the chemical heterogeneitywithin each class of compounds and the complex interactions of the abovevariables, generalizations about nutrient degradation in foods will inevitablybe broad ones.
Vitamins. Ascorbic acid is the most sensitivevitamin in foods, its stability varying markedly as a function of environmentalconditions such as pH and the concentration of trace metal ions and oxygen.The nature of the packaging material can significantly affect the stabilityof ascorbic acid in foods. The effectiveness of the material as a barrierto moisture and oxygen as well as the chemical nature of the surface exposedto the food are important factors.
For example, problems of ascorbic acid instabilityin aseptically packaged fruit juices have been encountered because of oxygenpermeability of the package and the oxygen dependence of the ascorbic aciddegradation reaction.
Also, because of the preferential oxidationof metallic tin, citrus juices packaged in cans with a tin contact surfaceexhibit greater stability of ascorbic acid than those in enamelled cans orglass containers. The aerobic and anaerobic degradation reactions of ascorbicacid in reduced-moisture foods have been shown to be highly sensitive towater activity, the reaction rate increasing in an exponential fashion overthe water activity range of 0.1-0.8.
One major undesirable physical change in foodpowders is the absorption of moisture as a consequence of an inadequate barrierprovided by the package; this results in caking. It can occur either as aresult of a poor selection of packaging material in the first place, or failureof the package integrity during storage. In general, moisture absorptionis associated with increased cohesiveness.
Anti-caking agents are very fine powders ofan inert chemical substance that are added to powders with much larger particlesize in order to inhibit caking and improve flowability. Studies in onionpowders showed that at ambient temperature, caking does not occur at wateractivities of less than about 0.4.
At higher activities, however, (aw > 0.45)the observed time to caking is inversely proportional to water activity,and at these levels anti-caking agents are completely ineffective. It appearsthat while they reduce inter-particle attraction and interfere with thecontinuity of liquid bridges, they are unable to cover moisture sorptionsites.
3.4.1 Microbiological
Micro-organisms can make both desirable andundesirable changes to the quality of foods depending on whether or not theyare introduced as an essential part of the food preservation process or ariseunintentionally and subsequently grow to produce food spoilage.
The two major groups of micro-organisms foundin foods are bacteria and fungi, the latter consisting of yeasts and moulds.Bacteria are generally the fastest growing, so that in conditions favourableto both, bacteria will usually outgrow fungi.
Foods are frequently classified on the basisof their stability as non-perishable, semi-perishable and perishable. Forexample, hermetically sealed and heat processed (e.g. canned) foods are generallyregarded as non-perishable. However, they may become perishable under certaincirc*mstances when an opportunity for recontamination is afforded followingprocessing.
Such an opportunity may arise if the can seamsare faulty, or if there is excessive corrosion resulting in internal gasformation and eventual bursting of the can. Spoilage may also take placewhen the canned food is stored at unusually high temperatures: thermophilicspore-forming bacteria may multiply, causing undesirable changes such asflat sour spoilage.
Low moisture content foods such as dried fruitand vegetables are classified as semi-perishable. Frozen foods, though basicallyperishable, may be classified as semi-perishable provided that they are properlystored at freezer temperatures.
The majority of foods (e.g. meat and fish, milk,eggs and most fresh fruits and vegetables) are classified as perishable unlessthey have been processed in some way. Often, the only form of processingwhich such foods receive is to be packaged and kept under controlled temperatureconditions.
The species of micro-organisms which cause thespoilage of particular foods are influenced by two factors: a) the natureof the foods and b) their surroundings. These factors are referred to asintrinsic and extrinsic parameters.
The intrinsic parameters are an inherent partof the food: pH, aw, nutrient content, antimicrobial constituentsand biological structures. The extrinsic parameters of foods are those propertiesof the storage environment that affect both the foods and their microorganisms.The growth rate of the micro-organisms responsible for spoilage primarilydepends on these extrinsic parameters: temperature, relative humidity andgas compositions of the surrounding atmosphere.
The protection of packaged food from contaminationor attack by micro-organisms depends on the mechanical integrity of the package(e.g. the absence of breaks and seal imperfections), and on the resistanceof the package to penetration by micro-organisms.
Metal cans which are retorted after fillingcan leak during cooling, admitting any microorganisms which may be presentin the cooling water, even when the double seam is of a high quality. Thisfact is widely known in the canning industry and is the reason for the mandatorychlorination of cannery cooling water.
Extensive studies on a variety of plastic filmsand metal foils have shown that microorganisms (including mounds, yeastsand bacteria) cannot penetrate these materials in the absence ofpinholes.
In practice, however, thin sheets of packagingmaterials such as aluminium and plastic do contain pinholes. There are severalsafeguards against the passage of micro-organisms through pinholes infilms:
- because of surface tension effects, micro-organisms cannot pass through very small pinholes unless the micro-organisms are suspended in solutions containing wetting agents and the pressure outside the package is greater than that within;
- materials of packaging are generally used in thicknesses such that pinholes are very infrequent and small;
- for applications in which package integrity is essential (such as sterilisation of food in pouches), adequate test methods are available to assure freedom from bacterial recontamination.
3.4.2 Macrobiological
3.4.2.1 Insect Pests
Warm humid environments promote insect growth,although most insects will not breed if the temperature exceeds about 35C° or falls below 10 C°. Also many insects cannot reproducesatisfactorily unless the moisture content of their food is greater thanabout 11%.
The main categories of foods subject to pestattack are cereal grains and products derived from cereal grains, other seedsused as food (especially legumes), dairy products such as cheese and milkpowders, dried fruits, dried and smoked meats and nuts.
As well as their possible health significance,the presence of insects and insect excrete in packaged foods may render productsunsaleable, causing considerable economic loss, as well as reduction innutritional quality, production of off-flavours and acceleration of decayprocesses due to creation of higher temperatures and moisture levels.
Early stages of infestation are often difficultto detect; however, infestation can generally be spotted not only by thepresence of the insects themselves but also by the products of their activitiessuch as webbing, clumped-together food particles and holes in packagingmaterials.
Unless plastic films are laminated with foilor paper, insects are able to penetrate most of them quite easily, the rateof penetration usually being directly related to film thickness. In general,thicker films are more resistant than thinner films, and oriented films tendto be more effective than cast films. The looseness of the film has alsobeen reported to be an important factor, loose films being more easily penetratedthan tightly fitted films.
Generally, the penetration varies dependingon the basic resin from which the film is made, on the combination of materials,on the package structure, and of the species and stage of insects involved.The relative resistance to insect penetration of some flexible packagingmaterials is as follows:
- excellent resistance: polycarbonate; poly-ethylene-terephthalate;
- good resistance: cellulose acetate; polyamide; polyethylene (0.254 mm); polypropylene (biaxially oriented); poly-vinyl-chloride (unplasticised);
- fair resistance: acrylonitrile; poly-tetra-fluoro-ethylene; polyethylene (0.123 mm);
- poor resistance: regenerated cellulose; corrugated paper board; kraft paper; polyethylene (0.0254 - 0.100 mm); paper/foil/polyethylene laminate pouch; poly-vinylchloride (plasticised).
Some simple methods for obtaining insect resistanceof packaging materials are as following:
- select a film and a film thickness that are inherently resistant to insect penetration;
- use shrink film over-wraps to provide an additional barrier;
- seal carton flaps completely.
3.4.2.2 Rodents
Rats and mice carry disease-producing organismson their feet and/or in their intestinal tracts and are known to harboursalmonella of serotypes frequently associated with food-borne infectionsin humans. In addition to the public health consequences of rodent populationsin close proximity to humans, these animals also compete intensively withhumans for food.
Rats and mice gnaw to reach sources of foodand drink and to keep their teeth short. Their incisor teeth are so strongthat rats have been known to gnaw through lead pipes and unhardened concrete,as well as sacks, wood and flexible packaging materials.
Proper sanitation in food processing and storageareas is the most effective weapon in the fight against rodents, since allpackaging materials apart from metal and glass containers can be attackedby rats and mice.
Summary
Major causes of food deterioration include thefollowing:
- growth and activities of micro-organisms, principally bacteria, yeasts and moulds;
- activities of natural food enzymes;
- insects, parasites and rodents;
- temperature, both heat and cold;
- moisture and dryness;
- air and in particular oxygen;
- light;
- time.
Extrinsic factors controlling the rate of foodDETERIORATION reactions are mainly:
- Effect of temperature;
- Effect of water activity (aw);
- Effect of gas atmosphere;
- Effect of light.
Contents -Previous -Next
