Microbial spoilage and preservation of cereal and cereal products

  • Cereal grains, such as wheat, corn, rye, oat, rice, etc., are important nutrients and energy sources for humans.
  • Cereal grains are the most commonly consumed food group worldwide and they are grown on about 60% of the cultivated land in the world.
  • Cereals are consumed in various forms in the food industry.
  • Cooked cereals are eaten directly after cooking (rice, maize).
  • Flours are made by grinding cereals (such as wheat, maize, rice, and rye) and products processed from them, e.g. biscuits, cookies, etc.
  • Bread is usually made from flours of wheat and rye by yeast fermentation.
  • Manufactured dried cereal products produced from wheat, maize, oats, and rice.
  • However, cereals are also used to produce dough, batter, pasta, noodles, pastries, cake, etc.
  • These products are subject to physical, chemical, and microbiological spoilage that affects the taste, aroma, leavening, appearance, and overall quality of the end consumer product.
Microbial spoilage and preservation of cereal and cereal products
Microbial spoilage and preservation of cereal and cereal products

Contamination source of cereal products

  • Air and dust
  • Soil
  • Water
  • Insects
  • Rodents
  • Birds
  • Animals
  • Humans 
  • environmental conditions (such as drought, rainfall, temperature, and sunlight)
  • harvesting and processing equipment 
  • Contaminated equipment and unsanitary handling.
  • Storage condition and storage temperature
  • Shipping containers

Spoilage of cereal

  • Cereals usually contain 70–75% carbohydrates, 8–15% protein, fat, fiber, vitamins, and minerals with near-neutral pH and hence are susceptible to microbial growth leading to spoilage.
  • Microbial growth is normally prevented due to sufficiently low water activity (i.e. below 0.70).
  • Mold is considered the primary organism for causing spoilage in cereal.
  • Bacteria can also cause spoilage of cereals, but yeasts cause few spoilage problems.
  • The most commonly associated bacterial families with cereals are Bacillaceae, Micrococcaceae, Lactobacillaceae, and Pseudomonadaceae.
  • Yeast that is found in cereal includes Candida, Cryptococcus, Pichia, Sporobolomyces, Rhodotorula, Trichosporon
  • Mold spores in cereals and flour are chiefly Aspergillus, Penicillium, Alternaria, Mucor, Cladosporium, Fusarium, Helminthosporium, Cladosporium, and Rhizopus.
  • Mycotoxins are the toxic secondary metabolites produced by mold that is found in cereal crops under favorable growth conditions. The genera of molds producing mycotoxins are Aspergillus, Penicillium, and Fusarium.
  • A high incidence of mycotoxin infections in cereals has been observed worldwide.

Some examples of mycotoxin and the mold that produce them are:

Mold Mycotoxin
Aspergillus parasiticus Aflatoxins B1, B2, G1, G2
Aspergillus flavus Aflatoxins B1, B2
Penicillium islandicum Islanditoxin, luteoskyrin
Fusarium sporotrichioides Trichothecenes: T-2 toxin

Some of the microorganisms and the defects caused by them in cereals are

Microorganisms Defects in cereal
Alternaria, Fusarium, Drechslera, Cladosporium, and Botrytis Blights and blemishes
Aspergillus fumigatus, A. penicillioides, A. ochraceus Discolored germs
A. candidus Powdery white patches
A. flavus Greenish discoloration
Eurotium Discolored germs, green eye
Fusarium Red streaking, particularly on maize
Penicillium  Blue coloration
Alternaria Darkening
Fusarium or Alternaria Pink or black tips in wheat
Claviceps purpira Ear rot (ergotism) in grain and corn

Preservation of cereal

Pesticides

  • The chemicals used to prevent and control the occurrence of pests causing harm to crops that includes including fungicides, herbicides, and insecticides.
  • The pesticides provide crop protection from the damaging influences of pests, higher yields, and better quality of cereals.

Drying

  • Grains are dried to a low moisture content until the moisture content level guaranteeing safe storage conditions( i.e. equivalent to <0.70 aw)
  • Drying helps to create unfavorable conditions for mold growth and the proliferation of insects.

Debranning

  • Debranning is a process during which the bran layers are removed.
  • This technique is found to improve the yield and degree of refinement of flour, as well as allowing the production of good quality grains.
  • After debranning, grains are found to be microbiologically purer as the total microbial contamination was reduced up to 87%.

Chlorine and hypochlorite

  • The chlorine-based methods are widely used in cereal for microbial control.
  • Sodium hypochlorite has also been used frequently.

Irradiation

  • It is a process that involves exposing cereal food to a certain amount of ionizing radiation.
  • Irradiation has been successfully used for the control of microorganisms on cereals and flours.

Ozone

  • Ozone (O3) is the triatomic oxygen formed by the addition of a free radical of oxygen to molecular oxygen.
  • The use of ozone as a fungicide for decontamination of cereal grains has been investigated in several studies.

Microwave (MW) treatment

  • Microwaves are electromagnetic waves with frequencies within 300 MHz to 300 GHz.
  • It is widely used for the inactivation of microorganisms associated with cereal grains.
  • Microwave energy can also be used for the control of stored grain insects.

Pulsed ultraviolet (UV) light treatment

  • Pulsed UV light treatment is a non-thermal technology that can be used both for decontamination of foods and food contact surfaces.
  • Pulsed UV light is considered to be more efficient in microbial inactivation than UV light, offering safer and faster decontamination.
  • The antimicrobial efficacy of this technology against microorganisms occurring on stored cereal grains has been studied.

Non-thermal (cold) plasma

  • Cold plasma can be generated at atmospheric as well as low pressure and consists of UV photons, neutral or excited atoms and molecules, negative and positive ions, free radicals, and free electrons.
  • As a non-thermal process, cold plasma does not cause damage to the food product after treatment.
  • This process has shown the inactivation of two pathogenic fungi, Aspergillus spp. and Penicillium spp. in cereals.
  • This process has also shown the inactivation of Geobacillus stearothermophilus and Bacillus amyloliquefaciens.

Organic acid

  • Organic acids are used as food additives and preservatives and can also be used for grain preservation.
  • Adding organic acids (acetic, citric, lactic, or propionic) or a combination of organic acids and NaCl added to tempering water is found to reduce microbial contamination in cereal.
  • It was reported that the combination of lactic acid (5.0%) and NaCl (52%) was the most effective against aerobic plate count and Enterobacteriaceae.

Spoilage of cereal products

Flour spoilage

  • The moisture content of the flour is less than 13% that prevents the growth of microorganisms.
  • However, the addition of water to flour tends to make it susceptible to microbial growth in flour.
  • The molds found in flours are mostly Eurotium species and Aspergillus candidus. The molds produce typical mycelium in flour.
  • The spoilage flour contains many psychrotrophs, flat sour bacteria, and thermophilic spore-forming bacteria such as Acetobacter spp, Bacillus spp, Lactic acid bacteria.
  • If acid-forming bacteria are present in flour, acid fermentation occurs followed by alcoholic fermentation by yeasts and then acetic acid by Acetobacter spp.
  • Bacillus spp is known for producing lactic acid, gas, and acetoin in flour.

Preservation of flour 

  • Flour can be preserved in an air-tight container for 6-10 months and a vacuum-packed container for 1-2 years.
  • It can also be preserved in the freezer.
  • It can also be preserved by filling it in oxygen absorbers containers for a long shelf-life of flour.

Bakery product

  • There are a wide variety of bakery products including leavened and unleavened bread, rolls, buns, croissants, muffins, cakes, doughnuts, pastries, pancakes, waffles, biscuits, cupcakes, and sweet rolls.
  • The nutrient content of bakery products includes carbohydrates, proteins, lipids, vitamins, and minerals.
  • Therefore, bakery products are susceptible to microbial growth due to their high nutrient content and also because the most common factor of these products is water.
  • The most famous bakery product that is consumed worldwide for a very long period is bread.
  • The ambient temperatures, product pH levels between 5.4 and 7.5, and water activity in the range of 0.75–0.98 promote spoilage of baked cereal foods with mold, yeast, and rope bacteria.
  • The most common source of microbial spoilage of bread is mold growth. The bacterial spoilage condition is known as ‘rope’ caused by the growth of the Bacillus species. The least common of all types of microbial spoilage in bread is that caused by certain types of yeast.
  • Typical genera of mold involved in spoilage are Penicillium, Aspergillus, Cladosporium, Fusarium, Monilia, Endomyces, Rhizopus, and Mucor.
  • Yeasts that can cause surface spoilage of bakery products include Saccharomyces, Debaryomyces, Kluyveromyces, Pichia, Candida, and Zygosaccharomyces.

Some defects observed on bread due to microbial spoilage are

Microorganisms Defect in bread
Rhizopus stolonifer (bread mold) white cottony mycelium and black dot
Penicillium expansum, Penicillium stoloniferum Green color
Aspergillus niger greenish- or purplish-brown to black color
Monilia sitophila  Pink or reddish color on the bread
Aspergillus glaucus  green and gray-green color
Aspergillus flavus Olive green color
Mucor Gray color
Endomycopsis fibuligera, Trichosporon variable Chalky defect
Bacillus subtilis, Bacillus mesentericus,  Bacillus licheniformis, Clostridium spp, Lactobacillus spp,  Leuconostoc spp Ropiness
Serratia marcescens Red or bloody bread
Saccharomyces cerevisiae   Alcoholic off-flavor
  • Pastry spoilage is similar to bread, but the pastry filling or topping is more susceptible to microbial growth.
  • Pastries include cakes and baked shells filled with cream, or sauces that contain egg or milk products that support the growth of spoilage-causing bacteria. 
  • However, due to high sugar concentration, pastries are rarely spoiled by bacteria.
  • The most common spoilage is caused by mold. They cause sugar fermentation and make them sour in taste with an alcoholic odor.

Preservation of bakery products

  • Several methods have been used to control mold growth on bakery products that include reformulation, freezing, and the use of preservatives.
  • Reformulation involves a reduction of available water in bakery products. Reduction in product aw can be achieved by dehydration, either through evaporation or freeze-drying or by high osmotically active additives e.g., sugars and salts, incorporated directly into the food.
  • Freezing has been used for the long-term preservation of bakery products particularly, cream-filled products. 
  • Cakes, cookies, shortcakes, and pancakes are commonly frozen and marketed in frozen form.
  • Preservatives are most commonly used to control mold growth in baked goods.
  • Chemical and natural permitted chemical preservatives which act as mold inhibitors in bread include acetic, sorbic, propionic acids and their salts. Natural food preservatives, such as cultured products, raisins, vinegar, are used in bakery products.

Pasta and noodle spoilage

  • Both pasta and noodle manufacture includes no cooking step, microbial contamination may occur during the mixing and drying process.
  • They are stored and distributed in dry form so spoilage is rare.
  • During the manufacturing process, it may spoil from bacterial or fungal growth.
  • For example, Enterobacter (Aerobacter) cloacae have been found in pasta and noodle that causes gas production.

Preservation of pasta and noodle

  • Pasta and noodles are produced by mixing milled wheat flour and water to form an unleavened dough, followed by kneading and shaping.
  • Pasta and noodle are packed in sealed plastic to extend their shelf-life.
  • Pasta should be dried or refrigerated promptly after manufacture.
  • Some of the chemical additives used to inhibit microbial growth are Potassium bromate, benzoyl peroxide, calcium propionate, sodium benzoate, sodium dehydroacetate, sodium polyacrylate, phosphate salts BHA, BHT.

References

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