The Impact of Food Drying on Microorganisms

Food drying is a centuries-old preservation technique employed to reduce moisture content, thereby inhibiting the growth of microorganisms such as bacteria, mold, and yeasts. The primary objective of this process is to achieve a moisture level that prevents microbial proliferation and extends the shelf life of food products.

Microorganisms require a certain level of water activity (aw) to thrive. Water activity is a critical factor in controlling microbial growth, defined as the ratio of the vapor pressure of water in a food to the vapor pressure of pure water at the same temperature (aw = p/p0). A lower water activity means reduced microbial activity, as it restricts the availability of free water necessary for metabolic processes.

The relationship between water activity and microbial growth is fundamental in food safety. Most molds are inhibited at aw levels below 0.9, making dried foods less susceptible to mold growth compared to fresh items with higher water content. This principle also holds for yeasts and bacteria, with mold being more likely to grow in dried foods due to their higher tolerance for low water activity.

Moreover, the stability of dried foods is sustained through proper packaging techniques that maintain the lowered water activity achieved during drying. Packaging serves as a barrier against moisture absorption, preventing rehydration and subsequent microbial growth.

Water activity values play a pivotal role in predicting food stability concerning microbial, enzymatic, chemical, and physical changes. Foods with added soluble components like sugars or salts exhibit reduced water activity, contributing to their microbial stability. Syrups and salted, partially dried foods are examples of such stable products, though vigilance is still required to prevent yeast or mold growth under certain conditions.

Fresh foods, such as meats, vegetables, and fruits, typically possess water activity levels (0.97-0.99) ideal for microbial growth. Drying these foods significantly reduces their water activity, inhibiting microbial activity and enhancing their shelf life.

Advancements in food drying technologies continue to refine this preservation method. Modern methods like freeze-drying and vacuum drying further reduce water activity, ensuring food safety and quality. These innovations underscore the importance of understanding and controlling water activity to mitigate microbial risks in food products.

In conclusion, food drying effectively inhibits microbial growth by lowering water activity to levels unsuitable for most microorganisms. This preservation technique remains indispensable in ensuring food safety, extending shelf life, and preserving nutritional quality. Ongoing research into drying technologies and water activity management continues to enhance the efficiency and effectiveness of this age-old method of food preservation.
The Impact of Food Drying on Microorganisms

Application of microorganisms in food

Microorganisms have been used for preparing food products like bread, yoghurt or curd, alcoholic beverages, cheese for a long time without knowing their involvement in fermentation.

For example, bacteria that cause milk to become sour are used in making cottage cheese, buttermilk, and yogurt. Vinegar and sauerkraut also are produced by the action of bacteria on ethyl alcohol and cabbage, respectively.

In Europe and the Middle East yoghurt is made from milk thickened by heating or the addition of dried milk solids and fermented with Lactobacillus bulgaricus and Streptococcus thermophilus. It has custard like consistency and is often flavored with ground fruit or nuts.

Acidophilus milk is made with Lactobacillus acidophilus. Butter is made from pasteurized cream, to which a lactic acid starter has been added.

Sour-cream butter is made by churning cream that has been ripened by bacterial action with a resultant production of lactic acid. The fat globules are more easily coalesced during the churning, and they take up certain desirable flavors that were produced by the bacteria during the fermentation process.

Kefir is a fizzy beverage made from fermented mares' milk using 'grains' that contain yeasts, streptococci, lactobacilli, and micrococci. The grains are strained off after their action is complete and added to the next batch of milk. Koumiss, also made from mares' milk, is a greyish-white liquid with uncurdled casein, often bottled and allowed to carbonate by the action of the microorganisms.

A culture of yeast is allowed to grow in a dough made of flour and water. As it grows the yeast produces carbon dioxide, which causes the dough to rise or become light. In addition, flavors are produced in the bread by the yeast or certain types of bacteria.

Molds are grown as feed and food and are employed to produce ingredients such as enzymes like amylase used in making bread or citric acid used in soft drinks.

Molds are major contributors in the ripening of many oriental foods. A species of Bothrytiscinerea, is used in rotting of grape for production of wine. Lactic fermentations using molds results in a unique Finnish fermented milk called viili.

Lactic acid bacteria are a heterogenous family of mainly low G+C Gram positive anaerobic, non sporulating and acid tolerant bacteria. They can ferment various nutrients through a homo or heterofermentative route into primarily lactic acid, but also into by products such as acetic acid. Formic acid, ethanol and carbon dioxide. They contribute to rapid acidification of food products, but also to flavous, texture and nutrition.

Since these lactic acid bacteria are tolerant of salt, and since salt exerts a curing action, it is employed in almost all fermented foods. Cucumbers are fermented in brines strong enough to discourage most spoilage micro-organisms, the salt and acid being soaked out before the product is used as food.
Application of microorganisms in food

Protection against food spoilage

Microorganisms can grow and multiply in all sorts of environments, which cause food spoilage problems. They change flavors and textures, and may produce toxic materials.

The microorganisms themselves may cause human disease. Although foods can be sterilized (as by heat processing)and contained in such a way as to prevent contamination by microbes during storage, it still is often necessary in some cases to forego sterilization, thus making it necessary to take other steps to prevent microbial degradation of the food.

Foods can be protected against microbial attack for long periods (months to years) by holding them at temperatures below freezing. They can be preserved for shorter periods by several days by holding them in ice or in a refrigerator at temperatures in the range 32 – 46 F (0 – 7.8 C).

Foods can also be preserved by altering them to make them incapable of supporting microbial growth. Drying is an example of this type of preservation. Food must be preserved against color and texture changes.

Quite often it is either impossible or undesirable to employ conventional preservation methods, and a large variety of food additives is available for use, alone or in combination with other additives or with mild forms of concentrations 0f 0.1% or less.

Sometimes, radiation has been experimented upon as a means of preventing microbial spoilage of food. Electric energy, radiowave, etc are known to kill microorganisms. Ionizing radiations such as alpha, beta, X-rays and ultraviolet rays have also been tried for the purpose.

Sodium diacetate and sodium or calcium propionate are used in breads to prevent mold growth and the development of bacteria that may produce a slimy material known as rope. Sorbic acid and its salts may be used in bakery products, cheeses, syrups, and pie fillings to prevent mold growth.

Sulfur dioxide is used to prevent browning in certain dried fruits and to prevent wild yeast growth in wines used to make vinegar. Benzoic acid and sodium benzoate may be used to inhibit mold and bacterial growth in some fruit juices, oleomargarines, pickles, and condiments. It also be noted that benzoic acid is a natural component of cranberries.

Mould spoilage of bread is generally prevented by the addition of food grade preservatives such as propionic, sorbic and acetic acids and their salts.

Aeration plays a major role in food spoilage. By creating anaerobic conditions, the action of spoilage organism may be reduced, Complete evacuation of air or replacement or air with CO2 or nitrogen may help in preventing the action of microorganisms. But some of the anaerobic organisms, if present, may become more virulent and cause damage to the food products.
Protection against food spoilage

Food preservation technique using High Osmotic Pressure

By principle of osmosis, jams, jellies and pickles are preserved. The application of the principle of osmotic pressure can be used as an effective antimicrobial agent.
Bacteria reach osmotic equilibrium by two means:

1. In hypertonic environments the volume of the protoplasts will shrink, and
2. In hypotonic environments the rigid wall will resist increase in protoplasts volume at a limiting volume of water; equilibriums results from turgor against the wall.

Hypertonicity and process of salting are commonly used for inhibiting microbial growth in food preservation as an addition of sugar raises osmotic pressure of the food thus making it impossible for most microorganisms to grow.

The rigid walls present in bacteria cells enable most bacteria to tolerate even extremely dilute environments. Osmotic equilibrium is achieved by development of turgor pressure against the wall. The wall of gram-positive micrococci can withstand 22 atm of pressure.

Water is withdrawn from microbial cells when they are replaced in solutions containing large amounts of dissolved substances such as sugar or salt.

Although the walls of gram-negative rods have lower tensile strength, the wall is sufficiently strong to retain the turgor pressure if the cell is suspended in water.

The most commonly used osmotic agents are sucrose and sodium chloride. Other osmotic agents such as lactose, maltodextrin, ethanol, glucose, glycerine and corn syrups have been used.

Like dehydration, high osmotic pressure may inhibit microbial growth but it cannot be relied upon to kill microorganisms, yeasts and mould are relatively resistant to high osmotic pressure.
Food preservation technique using High Osmotic Pressure

What is genetically modified food?

Genetically modified foods or GM food is based on genetically modified organisms (GMOs), which can be defined as organisms in which the genetic material DNA has been altered in a way that does not occur naturally.

The development of recombinant DNA techniques in the 1970s enabled scientists to ‘cut and paste’ DNA sequences at precise locations and then transfer selected DNA segments into the genomes of other organisms.

Experts in agriculture science generally agree that there is no best way to protect all crops from all pest and diseases.

Each approach, for example, the use of synthetic pesticides, crop and integrated pests management systems, and biotechnology-based solutions has limitations.

The possibility to alter the DNA of an organism is used to make plants adopt new traits, such as increased agricultural productivity, increased resistance to disease and pests, or improved quality and nutritional and food processing characteristics, which can contribute directly to enhancing human health and development.
What is genetically modified food?

Microorganisms in milk

As drawn from cow’s udder , milk seldom , if ever, is free from microorganisms; bacterial, molds, and yeasts are usually present in small numbers.

Possible sources of contamination during production, storage and processing including:
*Inflammation of udder
*Animal itself: skin of teats and udder
*Condition at the milking area
*The milking operator
*Utensils and equipment used during processing
*The air and environment

Contamination occurs when microorganisms enter into the milk. The control of microbial activity in milk and milk products, especially the control of bacterial and bacterial growth, is the most important function in the handling and manufacturer of dairy products.

Raw milk, when improperly handled, may undergo any of several adverse changes. It may become sour due to the growth of bacteria that produce lactic acid.

Some bacteria, especially coliforms, ferment milk and produce gas. Clostridium butyricum, Candida pseudotropicalis and Torulopsis sphaerica are some of the organisms which produce gas in milk while fermenting the lactose.

Raw milk may also be subject to peptonisation (digestion of casein), the formation of rope (viscous polymer of sugars), and sweet curding, when bacterial growth is not controlled.

Enterobacter aerogenes, Micrococcus spp. Streptococcus spp. etc cause ‘ropy milk’. They cause the milk to become viscous or stringy, and produce a gummy substance in the medium.

Dairy herds are tested for tuberculosis and tested for and vaccination for brucellosis, and most milk is pasteurized to prevent the transmission of a variety of food –borne disease. However, some pathogens may be survive the pasteurization process, possibly because they may be protected by fat in which they may become encapsulated.

Research shows that 70-90% of raw milk samples tested contained psychotropic bacteria capable of producing proteinases that were active after heating at 149 °C for 10s.

Grams positive psychotropic bacteria also can produce lipases in milk which is associated the development of rancid flavor in UHT milk.

Microorganisms in milk

Food drying effects on microorganisms

The main purpose of drying foods is to lower their moisture content to a particular level that will exclude the growth of microorganisms (bacteria, mould and yeasts).

Significant growth of microorganisms in a food product can be prevented by lowering is water content by an appropriate drying process.

The lower the water activity of a food, the less probable that microorganisms will grow. Water activity affects the growth and multiplication of microorganisms. When aw <0.9, growth of most molds is inhibited.

Generally, mold at lower water activities than yeasts and yeasts will grow at lower water activities than bacteria. For this reason, mold are opt to grow in dried foods than are yeasts or bacteria. In dried foods, the moisture content is lowered to the point which microorganisms will not grow and it is kept that way through packaging, which includes moisture.

Water activity is a vital parameter for food monitoring. Water activity (aw) is defined as the ratio of the water vapor pressure of a food (p) to that of pure water (p0) at the same temperature: aw = p/p0.

Water activity values are used extensively to predict the stability of food stuff with respect to microbial growth and enzymatic, chemical and physical changes.

Water activity also can be lowered by soluble components, such as sugar or salt. Thus, certain syrup and salted, partially dried foods (e.g. foods) are relatively stable as far as at the growth of microorganisms is concerned , although there may be conditions in which they become subject to the growth of yeasts or molds.

Most fresh foods such as fresh meat, vegetables and fruit have aw values that are close to the optimum growth level of most microorganisms (0.97-0.99).
Food drying effects on microorganisms