Microbial spoilage and preservation of cereal and cereal products
Cereal products are widely consumed as staple foods around the world. They are derived from cereal grains such as wheat, rice, maize, barley, oats, rye, sorghum, and millet. Cereal products include flour, bread, pasta, noodles, breakfast cereals, cakes, pastries, biscuits, crackers, and snacks.
Cereal products can be contaminated by various sources during their production, processing, storage, and distribution. Contamination can affect the quality and safety of cereal products and cause spoilage or foodborne illnesses. Some of the common sources of contamination are:
- Air and dust: Airborne microorganisms such as bacteria, molds, and yeasts can settle on cereal grains and products and cause spoilage or infection. Dust can also carry insect eggs, rodent hairs, fecal matter, and other foreign materials that can contaminate cereal products.
- Soil: Soil contains a diverse range of microorganisms and organic matter that can adhere to cereal grains during harvesting or handling. Soil can also introduce contaminants such as pesticides, fertilizers, heavy metals, and toxins that can affect the quality and safety of cereal products.
- Water: Water is used for irrigation, washing, soaking, cooking, and cooling of cereal grains and products. Water can be a source of contamination if it is contaminated with microorganisms, chemicals, or pollutants. Water can also affect the moisture content and water activity of cereal products and influence their susceptibility to microbial growth and spoilage.
- Insects: Insects such as beetles, weevils, moths, mites, flies, and cockroaches can infest cereal grains and products and cause physical damage, loss of weight and quality, and contamination with their excreta, saliva, and body parts. Insects can also transmit microorganisms and toxins that can cause spoilage or food poisoning.
- Rodents: Rodents such as rats and mice can feed on cereal grains and products and cause damage, loss of quality and quantity, and contamination with their urine, feces, hair, and saliva. Rodents can also carry pathogens such as Salmonella, Listeria, E. coli, and Leptospira that can cause foodborne diseases.
- Birds: Birds such as pigeons, sparrows, crows, and starlings can feed on cereal grains and products and cause damage, loss of quality and quantity, and contamination with their droppings, feathers, and parasites. Birds can also carry pathogens such as Campylobacter, Salmonella, and E. coli that can cause foodborne diseases.
- Animals: Animals such as dogs, cats, cows, pigs, and horses can feed on cereal grains and products and cause damage, loss of quality and quantity, and contamination with their saliva, fur, and feces. Animals can also carry pathogens such as Salmonella, E. coli, and Brucella that can cause foodborne diseases.
- Humans: Humans can contaminate cereal grains and products through improper handling, processing, storage, and distribution. Humans can introduce microorganisms such as Staphylococcus aureus, Bacillus cereus, Clostridium perfringens, and Shigella that can cause food poisoning. Humans can also introduce contaminants such as allergens, foreign objects, and adulterants that can affect the quality and safety of cereal products.
- Environmental conditions: Environmental conditions such as drought, rainfall, temperature, and sunlight can affect the growth and development of cereal crops and influence their susceptibility to microbial contamination and spoilage. Environmental conditions can also affect the storage stability and shelf life of cereal products by influencing their moisture content, water activity, pH, and nutrient availability.
These are some of the main sources of contamination that can affect cereal products. Contamination can have negative impacts on the quality, safety, and shelf life of cereal products and pose health risks to consumers. Therefore, it is important to prevent or minimize contamination by adopting good agricultural practices (GAP), good manufacturing practices (GMP), good hygiene practices (GHP), and good storage practices (GSP).
Cereals are grains that are harvested from grasses such as wheat, rice, corn, oats, barley, and rye. They are rich in carbohydrates, proteins, fats, fibers, vitamins, and minerals. However, cereals also have a high water activity (aw) of around 0.85 to 0.95 when they are fresh. Water activity is a measure of how much water is available for microbial growth. The higher the aw, the more likely the food is to spoil.
Microbial spoilage is one of the main causes of cereal deterioration. Microorganisms such as bacteria, yeasts, and molds can grow on cereals and produce enzymes, toxins, acids, alcohols, gases, and off-flavors that affect the quality and safety of the food. Microbial spoilage can occur at any stage of cereal production, from harvesting to processing to storage.
Some of the factors that influence microbial spoilage of cereals are:
- Contamination sources: Cereals can be contaminated by microorganisms from various sources such as air, dust, soil, water, insects, rodents, birds, animals, humans, environmental conditions (such as drought, rainfall, temperature, and sunlight), harvesting and processing equipment, and storage containers.
- Moisture content: Moisture content is the amount of water in a food product. It affects the aw and the shelf life of cereals. Cereals with a high moisture content (>14%) are more prone to microbial spoilage than those with a low moisture content (<12%). Moisture content can increase due to improper drying, humid storage conditions, or water absorption from the environment.
- Temperature: Temperature affects the growth rate and metabolism of microorganisms. High temperatures (>30°C) can accelerate microbial spoilage of cereals by increasing the aw and enzyme activity. Low temperatures (<10°C) can slow down microbial spoilage by inhibiting microbial growth and enzyme activity. However, some microorganisms can still grow at low temperatures and cause spoilage.
- Oxygen: Oxygen is essential for the growth of aerobic microorganisms such as molds and some bacteria. Oxygen can also promote oxidation reactions that cause rancidity and discoloration of cereals. Oxygen can enter cereals through cracks, holes, or leaks in the packaging or storage containers.
- pH: pH is a measure of how acidic or alkaline a food product is. It affects the growth and survival of microorganisms. Most microorganisms prefer a neutral pH (6.5 to 7.5) for optimal growth. However, some microorganisms can tolerate acidic (pH < 4.5) or alkaline (pH > 8.5) conditions and cause spoilage.
- Insects: Insects such as beetles, weevils, moths, and mites can infest cereals and cause physical damage, nutrient loss, odor production, and microbial contamination. Insects can also carry microorganisms from one cereal product to another or from the environment to the cereals.
- Mycotoxins: Mycotoxins are toxic secondary metabolites produced by molds that grow on cereals under favorable conditions. Some of the common mycotoxins found in cereals are aflatoxins (produced by Aspergillus spp.), ochratoxins (produced by Penicillium spp.), deoxynivalenol (produced by Fusarium spp.), and zearalenone (produced by Fusarium spp.). Mycotoxins can cause acute or chronic health problems in humans and animals such as liver damage, kidney damage, cancer, immune suppression, reproductive disorders, and neurological disorders.
To prevent or reduce microbial spoilage of cereals, various methods such as drying, debranning (removing bran layers), chlorination (using chlorine or hypochlorite), irradiation (using ionizing radiation), ozone treatment (using ozone gas), microwave treatment (using electromagnetic waves), pulsed ultraviolet light treatment (using short bursts of UV light), cold plasma treatment (using ionized gas), organic acid treatment (using acetic acid or lactic acid), or chemical preservatives (such as potassium bromate or sodium benzoate) can be used.
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 mycotoxins and the molds that produce them are:
- Aflatoxins: These are produced by Aspergillus flavus and Aspergillus parasiticus. They are carcinogenic and can cause liver damage, immune suppression, and growth retardation in humans and animals. Aflatoxins can contaminate corn, peanuts, cottonseed, and tree nuts.
- Ochratoxins: These are produced by Aspergillus ochraceus and Penicillium verrucosum. They are nephrotoxic and can cause kidney damage, immunotoxicity, and carcinogenicity in humans and animals. Ochratoxins can contaminate wheat, barley, oats, rye, coffee, cocoa, and dried fruits.
- Trichothecenes: These are produced by Fusarium spp. such as Fusarium graminearum, Fusarium culmorum, Fusarium sporotrichioides, and Fusarium poae. They are cytotoxic and can cause vomiting, diarrhea, hemorrhage, skin irritation, and immunosuppression in humans and animals. Trichothecenes can contaminate wheat, barley, oats, rye, corn, rice, and sorghum.
- Zearalenone: This is produced by Fusarium spp. such as Fusarium graminearum and Fusarium culmorum. It is estrogenic and can cause reproductive disorders, infertility, and feminization in humans and animals. Zearalenone can contaminate wheat, barley, oats, rye, corn, rice, and sorghum.
- Fumonisins: These are produced by Fusarium spp. such as Fusarium verticillioides and Fusarium proliferatum. They are neurotoxic and can cause neural tube defects, liver cancer, esophageal cancer, and leukoencephalomalacia in humans and animals. Fumonisins can contaminate corn and sorghum.
- Patulin: This is produced by Penicillium spp. such as Penicillium expansum and Penicillium griseofulvum. It is genotoxic and can cause DNA damage, oxidative stress, inflammation, and apoptosis in humans and animals. Patulin can contaminate apples, pears, grapes, berries, and their products.
Mycotoxins can pose a serious threat to food safety and human health. Therefore, it is important to prevent or reduce their occurrence in cereal crops by adopting good agricultural practices (GAP), good manufacturing practices (GMP), good storage practices (GSP), and good hygiene practices (GHP). Additionally, various physical (e.g., sorting, cleaning), chemical (e.g., acidification), biological (e.g., fermentation), or thermal (e.g., heating) methods can be used to detoxify or degrade mycotoxins in contaminated cereals.
Microorganisms can cause various defects in cereals, affecting their quality, safety, and shelf-life. Some of the common defects caused by microorganisms are:
Moldy grains: Mold growth on cereal grains can result in discoloration, odor, flavor changes, and loss of nutritional value. Moldy grains can also produce mycotoxins, which are harmful substances that can cause acute or chronic toxicity, immunosuppression, and carcinogenicity in humans and animals. Some of the major mycotoxins associated with cereals are aflatoxins, ochratoxins, fumonisins, deoxynivalenol (DON), zearalenone (ZEA), and T-2 toxin. The molds that produce these mycotoxins belong to the genera Aspergillus, Penicillium, and Fusarium. The occurrence and severity of moldy grains depend on several factors, such as environmental conditions, harvesting and storage practices, insect damage, and genetic resistance of the cereal varieties.
Heating or hot spots: Heating or hot spots are localized areas of high temperature and moisture within a cereal mass that result from microbial respiration and metabolism. Heating can cause deterioration of grain quality, such as loss of germination, discoloration, odor, flavor changes, and nutrient degradation. Heating can also increase the risk of mold growth and mycotoxin production. The microorganisms that cause heating are mainly aerobic bacteria and fungi that utilize the available carbohydrates and oxygen in the cereal. Some of the common bacteria involved in heating are Bacillus, Pseudomonas, Enterobacter, Klebsiella, Erwinia, and Micrococcus. Some of the common fungi involved in heating are Aspergillus, Penicillium, Fusarium, Alternaria, Cladosporium, and Rhizopus.
Ropy or sticky grains: Ropy or sticky grains are characterized by a slimy or viscous texture that makes the grains adhere to each other or to the equipment. Ropy or sticky grains can cause problems in milling, processing, and handling of cereals. Ropy or sticky grains are caused by bacterial exopolysaccharides (EPS) that are produced by some bacteria as a protective mechanism against environmental stress. EPS can also alter the physicochemical properties of cereals, such as water absorption, swelling power, gelatinization temperature, and pasting viscosity. Some of the bacteria that produce EPS and cause ropy or sticky grains are Leuconostoc mesenteroides, Lactobacillus plantarum, Lactobacillus brevis, Lactobacillus fermentum, Weissella confusa, and Weissella cibaria.
Souring or acidification: Souring or acidification is a defect that involves a decrease in pH and an increase in acidity of cereals due to microbial fermentation. Souring or acidification can affect the taste, aroma, texture, and nutritional value of cereals. Souring or acidification can also facilitate the growth of spoilage and pathogenic microorganisms that can tolerate low pH conditions. Some of the microorganisms that cause souring or acidification are lactic acid bacteria (LAB), such as Lactobacillus spp., Pediococcus spp., Leuconostoc spp., Streptococcus spp., and Enterococcus spp. LAB can produce lactic acid and other organic acids from the fermentation of sugars in cereals. LAB can also produce antimicrobial substances, such as bacteriocins and hydrogen peroxide, that can inhibit the growth of other microorganisms.
Discoloration: Discoloration is a defect that involves a change in color of cereals due to microbial pigments or enzymes. Discoloration can affect the appearance and consumer acceptance of cereals. Discoloration can also indicate the presence of spoilage or pathogenic microorganisms that can pose health risks. Some of the microorganisms that cause discoloration are:
- Black yeasts: Black yeasts are a group of fungi that produce melanin pigments that give a black or dark brown color to cereals. Black yeasts can grow on cereals with low water activity and high sugar content. Some examples of black yeasts are Aureobasidium pullulans, Cladosporium herbarum, Exophiala jeanselmei,
- Pink yeasts: Pink yeasts are a group of fungi that produce carotenoid pigments that give a pink or orange color to cereals. Pink yeasts can grow on cereals with high water activity and low sugar content. Some examples of pink yeasts are Rhodotorula mucilaginosa, Rhodotorula glutinis, Sporobolomyces roseus, Sporobolomyces salmonicolor.
- Red yeasts: Red yeasts are a group of fungi that produce red or purple pigments that give a red or purple color to cereals. Red yeasts can grow on cereals with high water activity and low sugar content. Some examples of red yeasts are Zygosaccharomyces bailii, Zygosaccharomyces rouxii, Candida krusei, Candida tropicalis.
- Blue-green molds: Blue-green molds are a group of fungi that produce blue or green pigments that give a blue or green color to cereals. Blue-green molds can grow on cereals with high water activity and low sugar content. Some examples of blue-green molds are Penicillium roqueforti, Penicillium expansum, Penicillium camemberti, Penicillium glaucum.
- Bacterial blight: Bacterial blight is a disease that affects rice plants and causes brown spots on the grains. Bacterial blight is caused by the bacterium Xanthomonas oryzae pv. oryzae, which produces a yellow pigment called xanthomonadin that gives a yellow color to the grains.
These are some of the common microorganisms and the defects they cause in cereals. To prevent or reduce these defects, proper hygiene, sanitation, and preservation methods should be applied throughout the cereal production chain.
Cereals are prone to microbial spoilage due to their high nutrient content and exposure to various contamination sources. Therefore, various methods have been developed to preserve cereals and extend their shelf-life. Some of the methods are:
- Pesticides: These are chemicals used to prevent and control the occurrence of pests that cause harm to crops, such as fungi, insects, and weeds. Pesticides provide crop protection from the damaging influences of pests, resulting in higher yields and better quality of cereals. However, pesticides also pose some risks to human health and the environment, so they should be used judiciously and according to the regulations.
- Drying: This is a process of reducing the moisture content of cereals to a level that prevents microbial growth and insect infestation. Drying can be done by natural methods (such as sun drying or wind drying) or artificial methods (such as oven drying or microwave drying). Drying helps to create unfavorable conditions for mold growth and the proliferation of insects, as well as preserving the nutritional and sensory qualities of cereals.
- Debranning: This is a process of removing the bran layers of cereals, which are rich in fiber, minerals, and antioxidants, but also contain most of the microbial load. Debranning improves 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 is reduced up to 87%.
- Chlorine and hypochlorite: These are chlorine-based compounds that are widely used in cereal processing for microbial control. Chlorine gas or sodium hypochlorite solution can be applied to cereals or flours to reduce the bacterial and fungal counts. Chlorine and hypochlorite also have bleaching effects on cereals, which may improve their appearance but also affect their nutritional value.
- Irradiation: This is a process of exposing cereals or flours to a certain amount of ionizing radiation, such as gamma rays or electron beams. Irradiation has been successfully used for the control of microorganisms on cereals and flours, as well as for insect disinfestation and sprout inhibition. Irradiation does not significantly alter the chemical composition or sensory properties of cereals, but it may induce some changes in their physical properties.
- Ozone: This is a triatomic oxygen molecule (O3) that has strong oxidizing and disinfecting properties. Ozone can be generated by electrical discharge or ultraviolet radiation and applied to cereals or flours as a gas or in aqueous solution. The use of ozone as a fungicide for decontamination of cereal grains has been investigated in several studies, showing promising results against molds such as Aspergillus and Penicillium.
- Microwave (MW) treatment: This is a process of applying electromagnetic waves with frequencies within 300 MHz to 300 GHz to cereals or flours. Microwave energy can heat up the water molecules in cereals or flours, causing thermal inactivation of microorganisms. Microwave treatment can also cause non-thermal effects, such as membrane disruption or DNA damage, on microorganisms. Microwave treatment has been shown to be effective for the reduction of microbial load and insect infestation on cereals.
- Pulsed ultraviolet (UV) light treatment: This is a non-thermal technology that uses short pulses of intense UV light to decontaminate foods and food contact surfaces. Pulsed UV light can damage the DNA and proteins of microorganisms, leading to their death or inactivation. Pulsed UV light is considered to be more efficient in microbial inactivation than continuous 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: This is a partially ionized gas that consists of UV photons, neutral or excited atoms and molecules, negative and positive ions, free radicals, and free electrons. Cold plasma can be generated at atmospheric or low pressure and applied to cereals or flours as a gas or plasma jet. Cold plasma has multiple mechanisms of action against microorganisms, such as oxidation, etching, sputtering, UV irradiation, electric field effects, and shock waves. Cold plasma has shown potential for the inactivation of pathogenic fungi (such as Aspergillus spp. and Penicillium spp.) and bacteria (such as Geobacillus stearothermophilus and Bacillus amyloliquefaciens) on cereals.
- Organic acids: These are natural or synthetic compounds that have acidic properties and can act as food additives and preservatives. Organic acids can inhibit microbial growth by lowering the pH of cereals or flours, disrupting the cell membrane of microorganisms, or interfering with their metabolic pathways. Some examples of organic acids used for grain preservation are acetic, citric, lactic, or propionic acids. Adding organic acids or a combination of organic acids and NaCl to tempering water has been found to reduce microbial contamination in cereals. It was reported that the combination of lactic acid (5.0%) and NaCl (52%) was the most effective against aerobic plate count and Enterobacteriaceae.
Cereal products are derived from cereal grains such as wheat, rice, corn, oats, barley, rye, and millet. They include flour, bread, pasta, noodles, breakfast cereals, cakes, pastries, cookies, crackers, and snacks. Cereal products are widely consumed as staple foods and provide significant amounts of carbohydrates, proteins, dietary fiber, vitamins, minerals, and phytochemicals to the human diet.
However, cereal products are also susceptible to microbial spoilage during their production, processing, storage, distribution, and consumption. Microbial spoilage can affect the quality, safety, and shelf-life of cereal products and cause economic losses and health risks. Microbial spoilage can be caused by bacteria, yeasts, molds, or their metabolites such as toxins and enzymes. The type and extent of microbial spoilage depend on various factors such as the type of cereal product, the moisture content, the water activity (aw), the pH, the temperature, the oxygen availability, the packaging material and method, the hygiene practices, and the presence of preservatives.
The main types of microbial spoilage of cereal products are:
Mold spoilage: Mold is the most common and visible form of spoilage in cereal products. Mold can grow on the surface or inside the product and produce fuzzy or powdery growths of various colors such as green, black, white, yellow, or pink. Mold can also produce mycotoxins that are harmful to human and animal health. Some of the common molds that spoil cereal products are Aspergillus spp., Penicillium spp., Fusarium spp., Alternaria spp., Cladosporium spp., Rhizopus spp., and Mucor spp.
Bacterial spoilage: Bacteria can cause spoilage in cereal products by producing acids, gases, odors, flavors, slime, or pigments. Bacterial spoilage is usually less visible than mold spoilage but can be more dangerous due to the potential production of toxins or pathogens. Some of the common bacteria that spoil cereal products are Bacillus spp., Clostridium spp., Lactobacillus spp., Enterobacteriaceae spp., Pseudomonas spp., Acetobacter spp., and Staphylococcus spp.
Yeast spoilage: Yeasts can cause spoilage in cereal products by producing alcohols, acids, carbon dioxide, flavors, or foam. Yeast spoilage is usually less common than mold or bacterial spoilage but can affect the texture and taste of the product. Some of the common yeasts that spoil cereal products are Saccharomyces spp., Candida spp., Pichia spp., Zygosaccharomyces spp., Debaryomyces spp., and Kluyveromyces spp.
In order to prevent or control microbial spoilage of cereal products, various methods can be used such as drying, refrigeration or freezing, heat treatment or pasteurization, irradiation, packaging with modified atmosphere or vacuum, adding preservatives or antioxidants, or using natural antimicrobials such as spices or herbs.
In this article, we will discuss the microbial spoilage and preservation of some of the major cereal products such as flour, bread, pasta, and noodles.
Flour is a powder made by grinding cereal grains, seeds, or roots. It is one of the most important ingredients in bakery products, as well as pasta and noodles. Flour can be made from wheat, rye, barley, corn, rice, oats, millet, sorghum, quinoa, buckwheat, and many other plants.
Flour has a low moisture content of less than 13%, which prevents the growth of most microorganisms. However, flour can still be contaminated by mold spores, bacteria, and insects during harvesting, processing, storage, and handling. The contamination sources of flour are similar to those of cereal grains, such as air and dust, soil and water, insects and rodents, birds and animals, humans and equipment.
The most common type of microbial spoilage in flour is mold growth. Mold can produce visible mycelium or colonies on the surface or inside the flour. Mold can also produce mycotoxins, which are harmful substances that can cause acute or chronic effects on human and animal health. Some of the molds that can contaminate flour are Aspergillus, Penicillium, Fusarium, Alternaria, Cladosporium, Mucor, Rhizopus, and Eurotium.
Bacteria can also cause spoilage of flour, but they are less common than mold. Bacteria can cause changes in the color, odor, flavor, texture, and pH of flour. Some of the bacteria that can contaminate flour are Bacillus, Lactobacillus, Acetobacter, Pseudomonas, Micrococcus, and Enterobacter. One of the most serious bacterial spoilage conditions in flour is known as "rope", which is caused by the growth of Bacillus species. Rope causes the dough to become sticky and slimy with a foul smell.
Yeasts are another type of microorganism that can spoil flour. Yeasts are usually present in low numbers in flour and do not cause significant problems. However, some yeasts can cause surface spoilage of flour by producing gas bubbles and alcohol. Some of the yeasts that can contaminate flour are Saccharomyces, Candida, Pichia, Cryptococcus, Sporobolomyces, Rhodotorula, and Trichosporon.
The spoilage of flour can affect the quality and safety of bakery products made from it. Therefore, it is important to prevent or control the microbial contamination of flour by using proper methods of harvesting, processing, storage, and handling.
Flour is a dry product that has a low water activity (aw) and is usually safe from microbial spoilage. However, flour can still be contaminated by mold, bacteria, and insects during production, storage, or transportation. Therefore, proper preservation methods are needed to extend the shelf life and quality of flour.
Some of the methods for preserving flour are:
- Storing in an air-tight container. This prevents exposure to moisture, oxygen, light, and pests that can cause rancidity or spoilage of flour. Flour can be stored in an air-tight container for 6–10 months at room temperature .
- Refrigerating or freezing. This reduces the temperature and slows down the chemical reactions and microbial growth that can affect the flour. Flour can be refrigerated for up to 1 year or frozen for up to 2 years . However, it should be brought to room temperature before using to avoid lumps.
- Using oxygen absorbers. These are small packets that contain iron powder that reacts with oxygen and removes it from the container. This creates an anaerobic environment that inhibits mold and insect growth. Flour can be stored with oxygen absorbers for a long shelf life.
- Using chemical additives. These are substances that are added to flour to prevent or delay microbial spoilage or insect infestation. Some examples of chemical additives are potassium bromate, benzoyl peroxide, calcium propionate, sodium benzoate, sodium dehydroacetate, sodium polyacrylate, phosphate salts, BHA, and BHT. However, some of these additives may have adverse effects on human health or the environment and should be used with caution.
- Using non-thermal technologies. These are physical processes that use energy sources other than heat to inactivate microorganisms or insects in flour. Some examples of non-thermal technologies are irradiation, ozone, microwave, pulsed ultraviolet light, and cold plasma. These technologies can reduce the microbial load and extend the shelf life of flour without affecting its nutritional or sensory properties.
These are some of the methods for preserving flour and preventing its spoilage. By following these methods, you can ensure that your flour stays fresh and safe for your baking needs.
Bakery products are foods made from dough or batter that are baked in an oven or fried in oil. They include bread, cakes, pastries, pies, cookies, biscuits, muffins, doughnuts, croissants, and many others. Bakery products are popular worldwide and have a long history of consumption and production.
Bakery products are susceptible to microbial growth due to their high nutrient content and water activity. The main nutrients in bakery products are carbohydrates, proteins, lipids, vitamins, and minerals. These nutrients provide a source of energy and growth factors for microorganisms. The water activity (aw) of bakery products is the measure of the availability of water for microbial growth. The aw of bakery products ranges from 0.75 to 0.98, depending on the type and formulation of the product. The higher the aw, the more favorable the conditions for microbial growth.
The most common microorganisms that cause spoilage of bakery products are molds, yeasts, and bacteria. Molds are filamentous fungi that produce spores and can grow on the surface or inside the bakery products. They cause discoloration, off-odors, off-flavors, and texture changes in bakery products. Some molds can also produce mycotoxins, which are harmful substances that can cause acute or chronic effects on human health. Yeasts are single-celled fungi that can ferment sugars and produce carbon dioxide and ethanol. They can cause gas production, souring, and alcohol formation in bakery products. Bacteria are single-celled prokaryotes that can multiply rapidly under favorable conditions. They can cause spoilage of bakery products by producing acids, gases, enzymes, pigments, and odors.
The main factors that influence the microbial growth and spoilage of bakery products are:
- The type and quality of raw materials: The raw materials used for making bakery products include flour, water, sugar, salt, yeast, eggs, milk, butter, oil, fruits, nuts, spices, and additives. The quality and hygiene of these raw materials affect the microbial load and contamination of the final product. For example, flour can be contaminated with mold spores or bacteria during milling or storage; water can be contaminated with microorganisms from the source or pipes; eggs and milk can be contaminated with Salmonella or Listeria; fruits and nuts can be contaminated with molds or insects; spices can be contaminated with aflatoxins or ochratoxins; additives can be contaminated with preservatives or antioxidants.
- The processing conditions: The processing conditions include mixing, kneading, shaping, proofing, baking or frying, cooling, slicing, packaging, and storage. The processing conditions affect the microbial growth and spoilage of bakery products by changing the physical and chemical properties of the product. For example, mixing and kneading can introduce air and distribute microorganisms throughout the dough; proofing can allow yeast fermentation and gas production; baking or frying can kill most microorganisms by heat but also create a crust that can protect them from drying; cooling can reduce the temperature but also increase the aw; slicing can expose the product to air and contamination; packaging can create a barrier or a modified atmosphere that can inhibit or promote microbial growth; storage can affect the temperature and humidity that can influence microbial growth.
- The environmental factors: The environmental factors include temperature, humidity, oxygen availability, pH, light exposure, and sanitation. The environmental factors affect the microbial growth and spoilage of bakery products by creating favorable or unfavorable conditions for microorganisms. For example, temperature affects the rate of microbial growth and enzyme activity; humidity affects the aw and moisture loss or gain of the product; oxygen availability affects the type of microorganisms that can grow (aerobic or anaerobic); pH affects the acidity or alkalinity of the product and the activity of acid-tolerant or acid-producing microorganisms; light exposure affects the color and flavor of the product and the activity of light-sensitive microorganisms; sanitation affects the cleanliness and hygiene of the equipment, surfaces, and personnel involved in the production and handling of bakery products.
To prevent or reduce microbial spoilage of bakery products, several methods can be used, such as reformulation, freezing, and the use of preservatives. These methods will be discussed in detail in the following points.
Bread is a staple food that is made from flour, water, yeast, and salt. It undergoes a fermentation process by yeast that produces carbon dioxide and ethanol, which leaven the dough and give it a characteristic flavor and texture. Bread can be spoiled by various microorganisms, such as molds, bacteria, and yeasts, that cause different defects on the product.
Mold spoilage is the most common and visible type of spoilage in bread. Molds are filamentous fungi that grow on the surface or inside the bread, producing spores that can spread to other products. Molds can cause discoloration, off-odors, off-flavors, and texture changes in bread. Some molds can also produce mycotoxins, which are harmful substances that can cause acute or chronic health effects in humans and animals.
The most common genera of molds involved in bread spoilage are Penicillium, Aspergillus, Cladosporium, Fusarium, Monilia, Endomyces, Rhizopus, and Mucor. Some examples of mold defects observed on bread are:
- Blue mold: caused by Penicillium species, such as P. roqueforti and P. expansum. It appears as blue-green spots or patches on the bread surface or crumb.
- Black mold: caused by Aspergillus species, such as A. niger and A. flavus. It appears as black or dark brown spots or patches on the bread surface or crumb.
- Red mold: caused by Monilia species, such as M. sitophila and M. fructigena. It appears as pink or red spots or patches on the bread surface or crumb.
- White mold: caused by Endomyces species, such as E. albicans and E. magnusii. It appears as white or grayish spots or patches on the bread surface or crumb.
- Soft rot: caused by Rhizopus species, such as R. stolonifer and R. arrhizus. It causes softening and liquefaction of the bread crumb, with a foul odor and a cottony growth of white mycelium.
- Dry rot: caused by Mucor species, such as M. mucedo and M. racemosus. It causes hardening and shrinking of the bread crumb, with a musty odor and a grayish-brown growth of coarse mycelium.
Bacterial spoilage is less common than mold spoilage in bread, but it can cause serious defects that affect the quality and safety of the product. Bacteria can grow in the dough or in the baked bread, producing acids, gases, enzymes, toxins, or pigments that alter the appearance, flavor, texture, or aroma of the bread.
The most common bacterial spoilage condition in bread is known as rope. Rope is caused by spore-forming bacteria of the genus Bacillus, such as B. subtilis, B. licheniformis, B. mesentericus, and B. cereus. Rope causes sticky and slimy streaks in the bread crumb, with a sour odor and taste. Rope can also cause swelling and bursting of the bread loaf due to gas production.
Other bacterial defects observed on bread are:
- Souring: caused by lactic acid bacteria (LAB), such as Lactobacillus and Leuconostoc species. Souring causes a decrease in pH and an increase in acidity in the bread crumb, with a sour flavor and aroma.
- Pink discoloration: caused by Serratia marcescens, a gram-negative rod-shaped bacterium that produces a red pigment called prodigiosin. Pink discoloration appears as pink spots or streaks on the bread crumb or crust.
- Green discoloration: caused by Pseudomonas fluorescens, a gram-negative rod-shaped bacterium that produces a green pigment called pyoverdin. Green discoloration appears as green spots or streaks on the bread crumb or crust.
Yeast spoilage is the least common of all types of microbial spoilage in bread, but it can cause some defects that affect the sensory quality of the product. Yeasts are single-celled fungi that can grow aerobically or anaerobically in the dough or in the baked bread, producing carbon dioxide, ethanol, acetic acid, lactic acid, or other metabolites that influence the flavor, aroma, or texture of the bread.
The most common yeasts involved in bread spoilage are Saccharomyces, Debaryomyces, Kluyveromyces, Pichia, Candida, and Zygosaccharomyces. Some examples of yeast defects observed on bread are:
- Alcohol fermentation: caused by Saccharomyces species, such as S. cerevisiae and S. exiguus. Alcohol fermentation causes a rise in ethanol content and a decrease in pH in the bread crumb, with an alcoholic odor and taste.
- Acetic acid fermentation: caused by Acetobacter species, such as A. aceti and A. pasteurianus. Acetic acid fermentation causes a rise in acetic acid content and a decrease in pH in the bread crumb, with a vinegar-like odor and taste.
- Surface spoilage: caused by various yeasts that grow on the surface of the bread, producing pigments, acids, or gases that cause discoloration, souring, or swelling of the bread crust. Surface spoilage can be observed as red, green, blue, or black spots or patches on the bread crust.
Bakery products are prone to spoilage by mold, yeast, and bacteria due to their high moisture content, neutral pH, and rich nutrient composition. Therefore, various methods have been used to extend their shelf-life and maintain their quality. Some of these methods are:
- Reformulation: This involves reducing the available water in bakery products by dehydration, evaporation, freeze-drying, or adding osmotically active ingredients such as sugar and salt. This lowers the water activity and inhibits microbial growth.
- Freezing: This is a common method for preserving bakery products, especially those with cream fillings or toppings. Freezing prevents microbial growth and slows down staling by reducing the rate of starch retrogradation.
- Chemical preservatives: These are substances that inhibit microbial growth by altering their metabolism, membrane permeability, or enzyme activity. Some examples of chemical preservatives used in bakery products are acetic acid, sorbic acid, propionic acid, and their salts. These preservatives are effective against mold and yeast, but may have adverse effects on sensory and nutritional properties of bakery products.
- Bio-preservatives: These are natural or biotechnological products that have antimicrobial activity against spoilage microorganisms and pathogens. The most important bio-preservatives for bakery products are lactic acid bacteria (LAB) of sourdough. LAB produce organic acids, hydrogen peroxide, diacetyl, and bacteriocins that inhibit mold, yeast, and rope bacteria. LAB also improve the flavor, texture, and nutritional value of bakery products by producing aroma compounds, enzymes, and vitamins. Sourdough fermentation also delays starch retrogradation and extends the shelf-life of bread.
- Packaging: This is a physical barrier that protects bakery products from environmental factors such as oxygen, moisture, light, and microbial contamination. Packaging materials should be impermeable, inert, flexible, and resistant to mechanical damage. Packaging can also be modified by adding oxygen scavengers, carbon dioxide emitters, or antimicrobial agents to enhance the preservation of bakery products.
Pasta and noodles are produced by mixing milled wheat flour and water to form an unleavened dough, followed by kneading and shaping. The dough is then cut into various shapes and sizes, such as spaghetti, macaroni, lasagna, vermicelli, etc. for pasta, and thin strips or sheets for noodles. Pasta and noodles are usually dried before packaging and distribution, but they can also be sold fresh or frozen.
Pasta and noodles have a low moisture content (around 10-12%) and a low water activity (around 0.5-0.6), which makes them resistant to microbial spoilage. However, they can still be contaminated by microorganisms during the manufacturing process, such as from the raw materials, the equipment, the environment, or the handlers. Some of the microorganisms that can cause spoilage of pasta and noodles are molds (such as Aspergillus, Penicillium, Fusarium, etc.), bacteria (such as Enterobacter, Bacillus, Clostridium, etc.), and yeasts (such as Saccharomyces, Candida, etc.). These microorganisms can produce off-flavors, off-odors, discoloration, gas formation, or toxin production in pasta and noodles.
To prevent or reduce microbial spoilage of pasta and noodles, several methods can be used. Some of these methods are:
- Hygienic practices: This involves maintaining good sanitation and hygiene throughout the production process, from the selection of raw materials to the packaging and storage of the final product. This includes washing and disinfecting the equipment and utensils, cleaning and sanitizing the premises, wearing protective clothing and gloves, avoiding cross-contamination, and following good manufacturing practices (GMPs).
- Drying: This is the most common method for preserving pasta and noodles. Drying reduces the moisture content and water activity of the product, making it unfavorable for microbial growth. Drying can be done by hot air, microwave, infrared, or vacuum methods. The optimal drying conditions depend on the type and shape of the product, but generally aim to achieve a moisture content of less than 13% and a water activity of less than 0.65.
- Refrigeration or freezing: This is another method for preserving pasta and noodles that are not dried. Refrigeration or freezing lowers the temperature of the product, slowing down the metabolic activity of microorganisms. Refrigerated pasta and noodles can have a shelf-life of up to several weeks, while frozen pasta and noodles can have a shelf-life of up to several months. However, refrigeration or freezing can also affect the quality of the product, such as causing texture changes or freezer burn.
- Packaging: This is a method for protecting pasta and noodles from external contamination and moisture loss during storage and distribution. Packaging can be done by using various materials, such as paper, plastic, metal, or glass. The packaging material should be impermeable to air, moisture, light, and microorganisms. The packaging should also be sealed properly to prevent leakage or tampering.
- Chemical additives: This is a method for inhibiting microbial growth in pasta and noodles by adding certain substances to the product or the packaging material. Some of the chemical additives that are used for this purpose are potassium bromate, benzoyl peroxide, calcium propionate, sodium benzoate, sodium dehydroacetate, sodium polyacrylate, phosphate salts BHA (butylated hydroxyanisole), BHT (butylated hydroxytoluene), etc. These additives can act as bleaching agents, preservatives, antioxidants, or stabilizers. However, they should be used in accordance with the regulations and guidelines of the food safety authorities.
These are some of the methods for preserving pasta and noodles from microbial spoilage. By applying these methods properly, pasta and noodles can have a longer shelf-life and better quality for consumers.
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