The following points highlight the three main types of plant products that are derived from plants to fulfil human food requirements. The types are:- 1. Cereals 2. Fruits and Fruit Products 3. Vegetables and Vegetable Products.
Plant Product Type # 1. Cereals:
The cereals, which all belong to the Gramineae or the grass family, are one of the most important sources of carbohydrates in the human diet. Some of the more important cereal crops are listed in Table 5.9. Wheat, rice and maize are by far the most important cereal crops on the basis of world-wide tonnage and well over three hundred million tons of each are produced annually.
However, each cereal species is adapted to grow in a particular range of climatic conditions although plant breeding programmes have extended the ranges of several of them.
The common wheat is the major cereal of temperate parts of the world, and is grown extensively in both northern and southern hemispheres, whereas the durum wheat is grown extensively in the Mediterranean region and in the warmer, drier parts of Asia, North and South America. Rye can be grown in colder parts of the world and is an important crop in central Europe and Russia.
Maize, which originated from the New World, is now grown extensively throughout the tropics and subtropical regions of both northern and southern hemispheres. Similarly, although rice was originally of Asian origin, it is also grown in many parts of the world.
Although sorghum and some of the millets are not very important in terms of world tonnage, they are especially well adapted to growing in warm, dry climates and may be locally the most important cereals in such regions as those bordering the southern edge of the Sahara desert.
The microbiology of cereals, during growth, harvest and storage is dominated by the moulds and it is convenient to consider two groups of fungi. The field fungi are well adapted to the sometimes rapidly changing conditions on the surfaces of senescing plant material in the field.
Although they require relatively high water activities for optimum growth, genera such as Cladosporium, Alternaria and Epicoccum are able to survive the rapid changes that can occur from the desiccation of a hot sunny day to the cool damp conditions of the night.
The genus Fusarium includes species which have both pathogenic and saprophytic activities. Thus F. culmorum and F. graminearum can cause both stem rot and head blight of wheat and barley in the field and these field infections may lead to more extensive post harvest spoilage of these commodities if they are stored at too high a water activity.
By contrast the so called storage fungi seem to be well adapted to the more constant conditions of cereals in storage, and generally grow at lower water activities (Table 5.10). The most important genera of the storage fungi are Penicillium and Aspergillus, although species of Fusarium may also be involved in spoilage when grain is stored under moist conditions.
Water activity and temperature are the most important environmental factors influencing the mould spoilage of cereals, and the possible production of mycotoxins, and Table 5.11 shows the relationship between water content and water activity for barley, oats and sorghum the same information as an isotherm for wheat.
Although xerophilic moulds such as Eurotium spp. and Aspergillus restrictus may grow very slowly at the lower limit of their water activity range .(0.71 corresponding to about 14% water content in wheat at 25 °C) once they start growing and metabolizing they will produce water of respiration and the local water activity will steadily rise allowing more rapid growth.
Indeed it could increase sufficiently to allow mesophilic mould spores to germinate and grow; the process being, in a sense, autocatalytic. There is a sequence of observable consequences of the process of mould growth on cereals starting with a decrease in germ inability of the grain.
This is followed by discolouration, the production of mould metabolites including mycotoxins, demonstrable increase in temperature (self-heating), the production of musty odours, caking and a rapid increase in water activity leading finally to the complete decay with the growth of a wide range of micro-organisms.
Preservation of High-Moisture Cereals:
Although not directly relevant to human foods, the availability of high-moisture cereals, such as barley, provides a highly nutritious winter feed for cattle. Long-term storage of such material can be achieved by a lactic acid fermentation comparable to the making of silage, or by the careful addition of fatty acids such as propionic acid.
If this process is not carried out carefully then it may be possible to have sufficient, propionic acid to inhibit the normal spoilage moulds associated with cereals in a temperate climate, but not enough to inhibit Aspergillus flavus.
It has been shown that, even though partially inhibited in its growth, this mould can produce aflatoxin B1 at enhanced levels under these conditions. If such material is fed to dairy cattle there is the possibility of aflatoxin M1 being secreted in the milk and it then becomes a problem in human foods and not just a problem of animal feeds.
Pulses, Nuts and Oilseeds:
The pulses are members of the huge legume family of plants, the Fabaceae also known as the Papilionaceae and Leguminosae, which form a major source of vegetable proteins and include such important crops as peas, beans, soya, groundnuts and lentils.
Although many species of peas and beans are familiar to us as fresh vegetables, millions of tons of the mature seeds of soya beans and groundnuts are harvested for longer term storage every year and may be susceptible to mould spoilage if not stored under appropriate conditions.
Several of the leguminous seeds, such as groundnuts and soya beans, are also valuable sources of vegetable oils but there are plants from many other diverse families which are now used to provide food quality vegetable oils, rapeseed from the crucifers, sunflower seed from the daisy family, oil palm and olives to mention just a few.
Edible nuts may also come from a botanically wide range of tree species and many of them are rich in oil and give similar microbiological problems as oilseeds.
Seeds rich in oil, such as groundnuts, have a much lower water content at a particular water activity than cereals, thus groundnuts with a 7.2% water content have a water activity of about 0.65-0.7 at 25 °C.
Apart from the problem of mycotoxin formation in moulded oilseeds, several mould species have strong lipolytic activity leading to the contamination of the extracted oils with free fatty acids which may in turn undergo oxidation to form products contributing to rancidity.
The most important lipolytic moulds are species of Aspergillus, such as A. niger and A. tamarii, Penicillium and Paecilomyces, while at higher water activities species of Rhizopus may also be important. Figure 5.8 shows the influence of moisture- content, and damage on the formation of free fatty acids (FFA) in groundnuts stored for 4 months.
It can be seen that in wholesome nuts there is a steady increase in FFA formation with an increase in moisture content and this is due to the plants own lipolytic enzymes.
However, damaged groundnuts show a more rapid rise at low moisture contents, presumably due to increased contact of enzymes and substrate as a result of damage, but they also show an especially rapid rise in FFA at moisture contents greater than 7.2% corresponding to the active growth of lipolytic moulds.
If cereals, pulses, oilseeds and tree nuts are harvested with as little damage as possible and dried to an appropriate water content it should be possible to store them for considerable periods of time so long as they are not exposed to excessive temperature abuse during storage.
The problems which may arise when large storage facilities, such as silos, are not carefully designed to avoid temperature differentials arising within the stored commodity.
The migration of water in these circumstances can result in the germination of fungal spores and the growth of mycelium creating a localized region of active fungal activity releasing further water of respiration into the region. In this way, despite the commodity initially going into store at what was judged to be a safe water content, it may nevertheless go mouldy over a period of time.
It should be noted that, although a commodity may be dry enough to avoid direct microbiological spoilage, it may not be secure against the ravages of pests such as insects and rodents and their activity may lead to secondary invasion and mould spoilage.
Plant Product Type # 2. Fruits and Fruit Products:
Despite the high water activity of most fruits, the low pH leads to their spoilage being dominated by fungi, both yeasts and moulds but especially the latter. The degree of specificity shown by many species of moulds, active in the spoilage of harvested fruits in the market place or the domestic fruit bowl, reflects their possible role as pathogens or endophytes of the plant before harvest.
Thus Penicillium italicum and P. digitatum show considerable specificity for citrus fruits, being the blue mould and green mould respectively of oranges, lemons and other citrus fruits.
Penicillium expansum causes a soft rot of apples and, although the rot itself is typically soft and pale brown, the emergence of a ring of tightly packed conidiophores bearing enormous numbers of blue conidiospores, has led to this species being referred to as the blue mould of apples.
This particular species has a special significance because of its ability to produce the mycotoxin patulin which has been detected as a contaminant in unfermented apple juices but not in cider.
Other common diseases of apples and pears include the black spot or scab, caused by the ascomycete Venturia inaequalis (anamorph Spilocaea pomi = Fusicladium dendriticum), and a brown rot caused by another ascomycete, Monilinia-fructigena (= Sclerotinia fructigena, anamorph Monilia fructigena). Apple scab spoils the appearance of fruit, and would certainly reduce its commercial value, but does not cause extensive rotting of the tissue.
The brown rot, however, can lead to extensive damage of fruit both on the tree and in storage. The typical brown rot is usually associated with rings of brown powdery pustules of the imperfect, or anamorph, stage, however fruit which is infected, but apparently healthy when it goes into store, can be reduced to a shiny black mummified structure in which much of the fruit tissue has been replaced by fungal material and the whole apple has become a functional sclerotium, or overwintering resting body, of the fungus.
Although rarely seen in the United Kingdom, it is this structure which may germinate in the spring to produce the stalked apothecia of the perfect, or teleomorph, stage.
An especially widespread mould on both fruits and vegetables is the grey mould Botrytis cinerea, which is the imperfect stage of another ascomycete, Botryotinia fuckeliana (= Sclerotinia fuckeliana). Its role in the spoilage of strawberries.
Infection of grapes on the vine by this same mould can lead to drying out of the grape and an increase in sugar concentration and wines made from such contaminated fruit are -considered to be very special. Under these circumstances the fungus has been referred to as La Pourriture Noble – the noble rot!
To avoid excessive mould spoilage of harvested fruit during storage and transport it is necessary to harvest at the right stage of maturity and avoid damage and bruising. Mouldy fruit should be removed and destroyed and good hygiene of containers and packaging equipment is essential to prevent a build-up of mould propagules.
The development of international trade in many fruit species has led to the use of some biocides (Figure 5.9) to prevent mould spoilage. Benomyl has proved useful where it can be applied to the surface of fruits, such as citrus and bananas, in which the skins would normally be discarded (this, of course, is not the case for citrus used for marmalade and other preserves).
In some parts of the world moulds like Penicillium digitatum have developed increased resistance to benzoyl. Biphenyl is quite an effective protectant when incorporated into the wrapping tissues of fruit such as oranges when they are individually wrapped. Captan has been used as a spray for strawberries in the field to control Botrytis but its use must be stopped well before harvest.
Reduced temperature and increased carbon dioxide concentration may also be useful in controlling mould spoilage during storage and transport but many fruits are themselves sensitive to low temperatures and enhanced CO2 levels and appropriate conditions need to be established for each commodity.
Canned fruits are normally given a relatively low heat treatment because of their low pH and although most mould propagules would be killed the ascospores of some members of the Eurotiales are sufficiently heat resistant to survive.
Species of Byssochlamys are the best known but the increasing use of more exotic fruits is providing cases where spoilage of canned fruits have been due to such organisms as Neosartorya fischeri (anamorph Aspergillus fischerianus) and Talaromyces flavus var. macrosporus (anamorph Penicillium sp.)
Plant Product Type # 3. Vegetables and Vegetable Products:
The higher pH values of the tissues of many vegetables makes them more susceptible to bacterial invasion than fruits although there are also a number of important spoilage fungi of stored vegetables.
The bacteria involved are usually pectinolytic species of the Gram-negative genera Erwinia, Pseudomonas and Xanthomonas, although pectinolytic strains of Clostridium can also be important in the spoilage of potatoes under some circumstances, and the non-sporing Gram-positive organism Corynebacterium sepedonicum causes a ring rot of potatoes.
Table 5.12 lists a range of micro-organisms which may cause spoilage of fresh vegetables.
The role of plant pathogens in subsequent spoilage post-harvest may be complex, thus Phytophthora infestans causes a severe field disease of the potato plant, frequently causing death of the plant, but it may also remain dormant within the tubers and either cause a rot of the tubers during storage, or a new cycle of disease in the next season’s crop.
However, the most frequent agents of spoilage are not the plant pathogens themselves but opportunistic micro-organisms which gain access to plant tissue through wounds, cracks, insect damage or even the lesions caused by the plant pathogens.
All freshly harvested vegetables have a natural surface flora, including low numbers of pectinolytic bacteria, and it is becoming increasingly evident that healthy tissue of the intact plant may also contain very low numbers of viable microorganisms (endophytic).
The onset and rate of spoilage will depend on the interactions between the physiological changes occurring in the tissues after harvest and changes in microbial activity. Harvesting itself will produce physiological stress, principally as a result of water loss and wilting, and cut surfaces may release nutrients for microbial growth. This stress may also allow growth of the otherwise quiescent endophytic flora.
The most frequently observed form of spoilage is a softening of the tissue due to the pectinolytic activity of micro-organisms. Pectin, the methyl ester of α-1, 4-poly-D- galacturonic acid, and other pectic substances are major components of the middle lamella between the cells making up plant tissue and once it is broken down the tissue loses its integrity and individual plant cells are more easily invaded and killed.
Pectic substances may be quite complex and include un-esterified pectic acid as well as having side chains of L-rhamnose, L-arabinose, D-galactose, D-glucose and D- xylose. Several distinct enzymes are involved in the degradation of pectin and their role is illustrated in Figure 5.10.
The prevention of spoilage during storage and transport of vegetables must involve a range of measures. The control of the relative humidity and the composition of the atmosphere in which vegetables are stored is important but there is a limit to the reduction of relative humidity because at values below 90-95%, loss of water from vegetable tissues will lead to wilting.
It is essential to avoid the presence of free water on the surfaces of vegetables and temperature control may be just as important to prevent condensation. The presence of a film of water on the surface will allow access of motile bacteria such as Erwinia and pseudomonads to cracks, wounds and natural openings such as stomata.
A combination of constant low temperature, controlled relative humidity, and a gas phase with reduced oxygen (ca. 2-3%) and enhanced CO2 (ca. 2-5%) has made it possible to store the large hard cabbages used in coleslaw production for many months making the continuous production of this commodity virtually independent of the seasons.
Vegetables should not normally be a cause of public health concern but the transmission of pathogenic bacteria such as Salmonella and Shigella is possible by direct contamination from faeces of birds and animals, the use of manure or sewage sludge as fertilizer, or the use of contaminated waters for irrigation.
Celery, watercress, lettuce, endive, cabbage and beansprouts have all been associated with Salmonella infections, including typhoid and paratyphoid fevers, and an outbreak of shigellosis has been traced to commercial shredded lettuce.
Not all pathogens are necessarily transmitted to vegetables by direct or indirect faecal contamination. Organisms such as Clostridium botulinum have a natural reservoir in the soil and any products contaminated with soil can be assumed to be contaminated with spores of this organism, possibly in very low numbers.
This would not normally present a problem unless processing or storage conditions were sufficiently selective to allow subsequent spore germination, growth and production of toxin. In the past, this has been seen mainly as a problem associated with under processed canned vegetables, but now it must be taken into consideration in the context of sealed, vacuum or modified-atmosphere packs of prepared salads.
Those salads containing partly cooked ingredients, where spores may have been activated and potential competitors reduced in numbers, could pose particular problems. In 1987 a case of botulism caused by Clostridium botulinum type A was associated with a pre-packed rice and vegetable salad eaten as part of an airline meal.
Similar risks may occur in foil-wrapped or vacuum packed cooked potatoes or film-wrapped mushrooms and in all these cases adequate refrigeration appears to be the most effective safety factor.
Another group of pathogens naturally associated with the environment includes the psychrotrophic species Listeria monocytogenes which is commonly associated with plant material, soil, animals, sewage and a wide range of other environmental sources.
Raw celery, tomatoes and lettuce were implicated on epidemiological grounds as a possible cause of listeriosis which occurred in several hospitals in Boston, USA in 1979, although direct microbiological evidence was missing.
An outbreak of listeriosis in Canada in 1981 was associated with coleslaw. Strains of L. monocytogenes can certainly grow on shredded cabbage and salad vegetables such as lettuce at temperatures as low as 5 °C and modified-atmospheres seem to have no effect on this organism.
In the UK, routine surveillance of foods by the Public Health Laboratories revealed that, out of 567 samples of processed vegetables and salads examined, 87 (15%) were found to contain Listeria spp. while 72 (13%) contained L. monocytogenes specifically.
Two other psychrotrophic organisms which are readily isolated from the environment are Yersinia enterocolitica and Aeromonas hydrophilic. Both may be expected to be associated with vegetables and could grow to levels capable of causing illness if care is not taken during the growth, harvesting, storage and treatment of these commodities.