Microscopic and Macroscopic Identification of Moulds in foods
Like bacteria, moulds play an important role in foods. They can usually be seen by the naked eye (unlike bacteria). They are multi-cellular, consist of many cells connected with together.
Moulds are saprophytes and break down complex organic materials into simpler substance. This can encourage the decay of rotting matter. In this way moulds contribute to food spoilage, i.e. the visible decay of foods that occurs after ripening or damage but which is itself does not normally lead to food poisoning.
Basically most foods can be affected by moulds. Their presence will be apparent when they start to produce blue/green ‘fur’ or orange spots on foods.
Sometimes mould growth is useful, for example they can help ripening process in some certain cheeses. Examples of product in which moulds are used are Brie and Camembert which have a white mould on the outside; Danish Blue and Stilton have blue moulds through them.
A few moulds can cause infection in man for example farmer’s lung, athlete’s foot, etc.
Typically a mould constitutes long, branched, threadlike filaments of cells called hyphae that form a mycelium, a tangled mass or tissue like aggregation.
In some fungi, protoplasm streams through hyphae, uninterrupted by cross walls. These hyphae are called coenocytic. The hyphae of other fungi have cross walls called septa with either a single pore or multiple pores that permit protoplasmic streaming. These hyphae are termed septate.
Moulds reproduce by the production of spores which are finally blown away to settle on some suitable foodstuff and develop into new mycelium.
Mould spores can be formed either by asexual or sexual methods.
Asexual reproduction occurs in several ways:
1. A parent cell can divide into two daughter cells by central constriction and formation of a new cell wall.
2. Somatic vegetative cells may bud to produce new organisms. This is very common in the yeasts.
3. The most common method of asexual reproduction is the production of spores. Asexual spore formation occurs in an individual fungus through mitosis and subsequent cell division.
The following are the most common moulds present in foods:
a- Rhizopus Species
Rhizopus nigricans is commonly known as the bread mould because it frequently occurs in this food.[Copyright note: http://www.labreports.info] They may also attack vegetables because of their sugar content and low PH and cause Rhizopus soft rot. This makes the vegetables soft and mushy. Among those affected are beans, carrots, potatoes, cabbages, sprouts, cauliflower, turnips, cucumbers and tomatoes.
In addition, Rhizopus species are believed to attack fruits causing Rhizopus rot. It infects grapes, blackberries, strawberries, oranges, lemons, peaches, plums, pears.
b- Aspergillus Species
Aspergillus often appears as a green mould when present on foods such as oilseeds, edible nuts and cereals, but the aflatoxins which may be produced are not visible to the naked eye. Aflatoxins may even be present when no outward indication of mould can be observed.
Aspergillus species are widely distributed and cause numerous kinds of spoilage.
Aspergillus niger cause black mould rot and it affects grapes, cherries, peaches, apricots, plums and prunes. They also attack fresh meat and bacon which because of its low Aw content, is ideal for attack by moulds. They also attack butter and pickles.
Furthermore, they are able to decay tobacco, cigars, nuts and bread and grow on clothing in humid weather.
c- Penicillium Species
Penicillium species are capable of attacking fruits causing blue mould rot which attacks most fruits. They also attack beef causing green patches, bacon, butter and pickles. Spores of Penicillum roqueforti and Penicillium camemberti are used in the manufature of Camembert and Roquefort cheese. The enzymes of these moulds break down the fats and proteins in the cheese causing it to have its own strange flavour.
Penicillium notatum and Penicillium chrysogenum produce penicillin which has been developed for medical use).
d) Mucor species
Mucor colonies are able to grow fast at 25-30°C and can immediately cover the surface of the agar. Typically it resembles cotton candy with a fluffy figure and approximately several cm high. Its Preliminary colour (front section) is white and then it gradually turns darker to a grayish brown. From the back , it is permanently white. [752]
