Drying Method of Food Preservation with Types, Examples
Drying is one of the oldest and most common methods of food preservation. It involves removing water from food products to prevent microbial growth and enzymatic activity, thereby extending their shelf-life. Drying also reduces the weight and bulk of food products, making them easier to transport and store.
There are different methods of drying food products, but they can be broadly classified into two categories: sun drying and artificial dryer.
Sun drying is a natural and widely used method for food preservation, especially for Mediterranean herbs, spices and dried fruits. It involves exposing food products to direct sunlight or solar heat, assisted by the movement of surrounding air. Sun drying requires a dry and sunny climate, with temperatures above 98°F (37°C) and low humidity. It also requires careful protection of food products from insects, dust, rain and night moisture. Sun drying is relatively slow and depends on the weather conditions. It can take several days to dry food products completely.
Artificial dryer is a term that covers various types of dryers that use controlled heat and air circulation to dry food products. Artificial dryers can be classified into solid surface dryers, adiabatic dryers, foam-mat dryers and freeze dryers. Artificial dryers have several advantages over sun drying, such as faster drying rate, better quality control, less risk of contamination and spoilage, and year-round operation. However, artificial dryers also have some disadvantages, such as higher capital and operating costs, higher energy consumption and possible loss of nutrients and flavor due to high temperatures.
In conclusion, sun drying and artificial dryer are two different methods of drying food products with their own pros and cons. Depending on the type of food product, the desired quality, the available resources and the environmental factors, one method may be more suitable than the other for food preservation.
The main purpose of drying is to extend the shelf-life of foods by lowering water activity. Lowering the water activity will inhibit microbial growth and enzyme activity, which are the main causes of food spoilage and deterioration. Drying also helps reduce the weight and bulk of dried foods, reducing transportation and storage costs. Dried foods are also convenient to use as they can be rehydrated easily and quickly when needed.
Drying is one of the oldest and most widely used methods of food preservation. It has been practiced since ancient times by exposing foods to natural sources of heat, such as the sun, wind, or fire. Nowadays, artificial dryers are also available that can control the temperature, humidity, and air flow to achieve optimal drying conditions. Some examples of dried foods are raisins, prunes, apricots, dates, figs, meat jerky, fish, cheese, milk powder, instant coffee, tea leaves, herbs, spices, and cereals.
Drying can also enhance the flavor, color, texture, and aroma of some foods by concentrating their natural sugars, acids, and oils. For instance, dried tomatoes have a more intense and sweet flavor than fresh ones. Drying can also create new products that have different characteristics from their original forms. For example, foam-mat drying can produce porous and crispy snacks from liquid foods.
However, drying also has some drawbacks that affect the quality and safety of dried foods. Drying can cause loss of some nutrients, especially water-soluble vitamins and minerals, due to exposure to heat and oxygen. Drying can also cause changes in the physical structure of foods, such as shrinkage, cracking, hardening, and browning. These changes can affect the appearance, texture, and rehydration properties of dried foods. Moreover, drying can introduce contamination risks from dust, insects, rodents, or microorganisms if proper hygiene and sanitation practices are not followed during the drying process.
Therefore, it is important to select the appropriate drying method and conditions for each food product to achieve the desired quality and safety standards. Some factors that influence the selection of drying methods are the state and composition of the food product, the properties of the desired finished product, the heat susceptibility of the product, the temperature tolerance and its effect on the quality of the product, the capital and processing cost, and the environmental factors. These factors will be discussed in more detail in point 8.
Drying is a process of removing water from food by applying heat and/or air flow. The principle of drying is based on two concepts: mass transfer and heat transfer.
Mass transfer refers to the movement of water from the interior of the food to the surface, and then from the surface to the surrounding air. Mass transfer depends on the water content, water activity, and structure of the food, as well as the temperature, humidity, and velocity of the air.
Heat transfer refers to the transfer of thermal energy from a heat source to the food and then to the water in the food. Heat transfer depends on the type and intensity of the heat source, the thermal conductivity and specific heat of the food, and the latent heat of vaporization of water.
The rate of drying is determined by the rate-limiting step in either mass transfer or heat transfer. In some cases, both steps may be equally important. The rate of drying can be influenced by various factors, such as:
- The size, shape, and thickness of the food pieces
- The initial moisture content and water activity of the food
- The temperature and humidity of the drying air
- The velocity and direction of the air flow
- The type and design of the dryer
- The pretreatment methods applied to the food before drying
The goal of drying is to achieve a desired final moisture content and water activity that will ensure the safety, quality, and stability of the dried food. The final moisture content and water activity depend on the type and characteristics of the food, as well as the intended use and storage conditions. Generally, lower moisture content and water activity result in longer shelf-life and better preservation of nutrients, color, flavor, and texture. However, excessively low moisture content and water activity may cause undesirable changes in some foods, such as hardening, cracking, or browning.
Drying is a complex process that involves physical, chemical, and biological changes in food. Therefore, it is important to understand the principle of drying and its effects on food quality and safety. By selecting appropriate drying methods and conditions, it is possible to produce dried foods that are nutritious, delicious, and convenient for consumption.
Water is the most abundant component in most foods, and it plays a crucial role in their quality, stability, and safety. However, not all water in foods is the same. Depending on how water interacts with other food components, it can be classified into two types: bound water and free water.
Bound water refers to water that is strongly attached to other molecules in the food matrix, such as proteins, carbohydrates, or lipids. Bound water has a low mobility and a high chemical potential, meaning that it is not easily removed by drying or freezing. Bound water also does not participate in chemical reactions or microbial growth, so it does not affect the shelf-life of foods.
Free water refers to water that is loosely attached or not attached at all to other molecules in the food matrix. Free water has a high mobility and a low chemical potential, meaning that it is easily removed by drying or freezing. Free water also participates in chemical reactions and microbial growth, so it affects the shelf-life of foods.
The ratio of bound water to free water in foods depends on several factors, such as the composition, structure, temperature, and pH of the food. Generally, foods with high protein or carbohydrate content have more bound water than foods with high fat or oil content. Foods with a complex or porous structure have more free water than foods with a simple or compact structure. Foods with a low temperature or a high pH have more bound water than foods with a high temperature or a low pH.
The distinction between bound water and free water is important for the drying process because it determines how much energy is required to remove water from foods. Removing free water requires less energy than removing bound water because free water has a lower chemical potential and a higher vapor pressure than bound water. Therefore, the drying rate of foods depends on the amount and availability of free water in the food matrix.
The drying process also affects the ratio of bound water to free water in foods. As free water is removed from foods, the remaining water becomes more bound to other molecules and less available for removal. This leads to a decrease in the drying rate and an increase in the energy required to complete the drying process. Therefore, the quality and safety of dried foods depend on the amount and distribution of bound water and free water in the food matrix.
Moisture and water activity are two important factors that affect the quality and safety of dried foods. Moisture refers to the total amount of water present in a food product, while water activity measures how much of that water is available for microbial growth, enzymatic reactions, and chemical changes.
Water activity is a dimensionless value between 0 and 1 that indicates the relative humidity of the food. A higher water activity means more free water and a higher risk of spoilage. A lower water activity means less free water and a longer shelf life. Water activity can be influenced by temperature, pressure, solutes, and pH.
The relationship between moisture and water activity is not linear, but depends on the type and composition of the food. Some foods have a high moisture content but a low water activity, such as honey, jam, or salted fish. These foods are stable because most of the water is bound to solutes or other molecules and cannot support microbial growth. Other foods have a low moisture content but a high water activity, such as fresh fruits, vegetables, or meat. These foods are perishable because they have enough free water to allow spoilage.
The goal of drying is to reduce the moisture and water activity of foods to a safe and acceptable level. The optimal level depends on the type of food, the drying method, the storage conditions, and the intended use. Generally, most dried foods have a moisture content of 10% or less and a water activity of 0.6 or less. However, some foods may require higher or lower values to maintain their quality and functionality.
Moisture and water activity can be measured by various methods, such as gravimetric analysis, hygrometry, refractometry, or electrical conductivity. These methods can help determine the drying rate, the endpoint of drying, and the quality of the final product. Moisture and water activity are also useful indicators for monitoring the storage stability and shelf life of dried foods.
The phenomenon of drying can be discussed under two periods: constant rate period and falling rate period.
Constant rate period
When food in the dryer is applied with heat, water moves from the interior of the food at the same rate as it evaporates from the surface, and the surface remains wet until a certain critical moisture content is reached. This is known as the constant rate period. During this period, moisture is easy to remove. The rate of drying depends on the temperature and humidity of the drying air, the surface area and shape of the food, and the air velocity.
Falling rate period
When the moisture content of food falls below the critical moisture content level, the drying rate slowly decreases, and the moisture of food enters equilibrium with the drying air. This is known as the falling rate period. During this period, the rate of water movement from the interior to the surface decreases compared to water evaporating from the surface. During this period, moisture is hard to remove, and the drying process is much slower. The rate of drying depends on the internal structure and composition of the food, the diffusion coefficient of water in the food matrix, and the temperature gradient within the food.
The constant rate period and the falling rate period have different implications for the quality and efficiency of drying. The constant rate period is desirable because it allows for faster and more uniform drying. However, it also exposes the food to high temperatures that may cause thermal degradation of nutrients, flavors, colors, and textures. The falling rate period is less desirable because it prolongs the drying time and increases the energy consumption. However, it also reduces the thermal damage to the food quality attributes.
Therefore, an optimal drying process should aim to maximize the constant rate period and minimize the falling rate period by adjusting the drying conditions such as temperature, humidity, air velocity, and product thickness. Different types of dryers have different capabilities and limitations in achieving this goal. In general, dryers that use hot air or gas are more suitable for constant rate drying, while dryers that use solid surfaces or vacuum are more suitable for falling rate drying.
There are different types of dryers that can be used to dry food products, depending on the characteristics of the food, the desired quality of the final product, and the cost and efficiency of the drying process. Some of the common types of dryers are:
Solid surface dryer: In this type of dryer, heat is transferred to food through a solid surface, such as a metal drum or a plate, and water vapor is removed by a vacuum pump or, in some cases, by circulating air. This type of dryer is suitable for drying thin films or sheets of food products, such as milk, fruit juices, coffee extracts, etc. The advantages of this type of dryer are that it can produce high-quality products with low moisture content and minimal thermal damage. The disadvantages are that it requires high capital investment and maintenance costs, and it may not be suitable for drying foods with high viscosity or solids content.
Adiabatic dryers: In this type of dryer, heat is transferred to food by hot gas, such as air or steam. The hot gas transfers heat to the water in the food and carries out the water vapor thus produced. This type of dryer is suitable for drying foods with low to medium moisture content and high porosity, such as grains, cereals, fruits, vegetables, meat, etc. The advantages of this type of dryer are that it can handle large quantities of food with low energy consumption and operating costs. The disadvantages are that it may cause some loss of nutrients, flavor, color, and texture due to exposure to high temperatures and oxidation.
Foam-mat dryer: In this type of dryer, liquid foods are prefoamed using an edible foaming agent, such as egg white or gelatin. The stable foam is then spread on a mat or a belt and dried using hot air. This type of dryer is suitable for drying heat-sensitive, viscous, sticky, liquid food products, such as cheese whey, tomato paste, honey, etc. The advantages of this type of dryer are that it can produce porous and lightweight products with high rehydration capacity and good sensory properties. The disadvantages are that it may require additional steps for foam formation and stabilization, and it may not be effective for drying foods with high solids content or low foaming capacity.
Freeze drying: Freeze drying (Lyophilisation) is the technique of food preservation in which water is frozen and then converted directly to vapor by sublimation. This type of dryer is suitable for drying high-value foods with natural aromas or textures, such as fruits, vegetables, mushrooms, herbs, spices, coffee, meat, seafood, etc. The advantages of this type of dryer are that it can produce high-quality products with low moisture content and minimal thermal damage. The disadvantages are that it requires high capital investment and operating costs, and it may take longer time to dry compared to other methods.
There are many factors that influence the choice of a suitable drying method for a food product. Some of the main factors are:
- State and composition of the food product: The physical form, shape, size, and moisture content of the food product affect the rate and uniformity of drying. For example, liquid foods require different dryers than solid foods. The chemical composition of the food product also determines its sensitivity to heat, oxidation, and microbial spoilage. For example, foods rich in protein or fat may need lower temperatures or shorter drying times to prevent browning or rancidity.
- Properties of the desired finished product: The quality attributes of the dried food product, such as color, flavor, texture, rehydration, and shelf-life, depend on the drying method used. For example, freeze-drying preserves the natural appearance and aroma of foods better than other methods, but it is also more expensive and time-consuming. The desired moisture content and water activity of the dried food product also influence the choice of drying method. For example, some foods may need to be dried to very low moisture levels to prevent microbial growth, while others may need to retain some moisture for better texture or flavor.
- Heat susceptibility of the product: Some foods are more prone to thermal degradation or nutrient loss than others during drying. For example, foods rich in vitamins or enzymes may lose their activity or potency at high temperatures or prolonged exposure to heat. Therefore, such foods may require gentle drying methods, such as vacuum drying or freeze-drying, that minimize heat damage. Alternatively, some pretreatments, such as blanching or sulfiting, may be applied to protect the heat-sensitive components of the food product before drying.
- Temperature tolerance and its effect on the quality of the product: Some foods can tolerate higher temperatures or longer drying times without significant quality deterioration, while others may become discolored, hardened, or shriveled at high temperatures or long drying times. For example, fruits and vegetables can be dried at relatively high temperatures (50-70°C) without much loss of quality, while meats and dairy products may need lower temperatures (40-60°C) to prevent protein denaturation or fat oxidation. The temperature tolerance of the food product also depends on its initial moisture content and water activity. For example, foods with high moisture content or water activity may need higher temperatures or longer drying times to reach a safe level of water activity for storage.
- Capital and processing cost: The cost of drying a food product depends on several factors, such as the type and size of the dryer, the energy consumption and efficiency of the dryer, the labor and maintenance costs, and the packaging and storage costs. Different dryers have different initial and operating costs. For example, freeze-dryers are more expensive to purchase and operate than hot-air dryers, but they produce higher quality products with longer shelf-life. The processing cost also depends on the scale and capacity of the dryer. For example, batch dryers may be more suitable for small-scale operations or seasonal products, while continuous dryers may be more economical for large-scale operations or year-round products.
- Environmental factors: The environmental conditions, such as temperature, humidity, air velocity, and solar radiation, affect the performance and efficiency of some dryers. For example, sun drying is a simple and cheap method of drying foods, but it depends on favorable weather conditions and may expose the food products to dust, insects, birds, or rodents. Therefore, sun drying may not be feasible or safe in some regions or seasons. Similarly, some dryers may require special ventilation or emission control systems to prevent air pollution or fire hazards.
These are some of the main factors that affect the selection of drying methods for food preservation. However, there is no single best method for all foods. Each food product has its own characteristics and requirements that need to be considered before choosing a suitable drying method. Therefore, it is important to conduct experiments and tests to evaluate the effects of different drying methods on the quality and safety of the food product.
Drying is one of the oldest and most widely used methods of food preservation. It has many advantages over other methods, such as:
- It reduces the weight and volume of food, making it easier to store and transport.
- It extends the shelf-life of food by lowering the water activity, which inhibits microbial growth and enzyme activity.
- It preserves the natural flavor, color, aroma, and texture of food, as well as most of the nutrients and phytochemicals.
- It requires less energy and equipment than other methods, such as canning, freezing, or refrigerating.
Drying can be applied to a variety of foods, such as fruits, vegetables, herbs, spices, grains, legumes, nuts, seeds, meat, fish, dairy products, eggs, and even prepared dishes. Some examples of dried foods are:
- Dried fruits: such as raisins, apricots, figs, dates, prunes, cranberries, apples, bananas, etc. They can be eaten as snacks or used in baking, cooking, or making beverages.
- Dried vegetables: such as tomatoes, mushrooms, peppers, onions, garlic, carrots, celery, etc. They can be rehydrated and used in soups, stews, sauces, salads, or casseroles.
- Dried herbs and spices: such as basil, oregano, thyme, rosemary, parsley, mint, sage, bay leaves, cinnamon, cloves, ginger, turmeric, etc. They can be used to season and flavor various dishes and drinks.
- Dried grains and legumes: such as rice, wheat, corn, oats, barley, quinoa, beans, peas, lentils, chickpeas etc. They can be cooked and consumed as staple foods or used to make breads, cereals,pastas,noodles,soups,salads,hummus,falafel etc.
- Dried nuts and seeds: such as almonds,walnuts,cashews,pistachios,sunflower seeds,pumpkin seeds,chia seeds etc. They can be eaten as snacks or used to make nut butters,milks,oils,bars etc.
- Dried meat and fish: such as beef,jerky,biltong,pemmican,salami,sausages,baccon,tuna,salmon,cod,herring,sardines etc. They can be eaten as snacks or used to make sandwiches,salads,pies etc.
- Dried dairy products: such as milk,powder,butter,ghee,yogurt,powder cheese,powder etc. They can be reconstituted and used in baking ,cooking ,or making beverages .
- Dried eggs: such as whole egg powder ,egg white powder ,egg yolk powder etc. They can be reconstituted and used in baking ,cooking ,or making omelets ,scrambled eggs ,pancakes etc.
- Dried prepared dishes: such as soups,stews,chili,casseroles,rice dishes,pasta dishes,noodle dishes etc. They can be rehydrated and heated for a quick and easy meal.
As you can see,drying is a versatile and convenient method of food preservation that can help you save money,time,and space while enjoying a variety of delicious and nutritious foods.
There are many types of dryers available for food preservation, each with its own advantages and disadvantages. Some of the common dryers and the foods preserved by them are:
Solid surface dryer: This dryer uses a solid surface to transfer heat to the food and remove water vapor by a vacuum or air circulation. It is suitable for drying liquid or semi-solid foods that can form a thin film on the surface, such as milk, citrus juice, tomato paste, etc. The product is recovered in the form of fine powder or flakes. Some examples of solid surface dryers are drum dryer, vacuum shelf dryer, and continuous vacuum dryer.
Adiabatic dryer: This dryer uses hot gas to transfer heat to the food and carry out the water vapor. It is suitable for drying solid or granular foods that can withstand high temperatures and air velocities, such as grains, cereals, nuts, fruits, vegetables, meat, etc. The product is recovered in the form of dried pieces or granules. Some examples of adiabatic dryers are cabinet dryer, tunnel dryer, kiln dryer, spray dryer, and air-lift dryer.
Foam-mat dryer: This dryer uses a preformed foam of liquid food to spread on a mat and dry with hot air. It is suitable for drying heat-sensitive, viscous, sticky, or liquid foods that can be foamed with an edible foaming agent, such as eggs, cheese, yogurt, honey, etc. The product is recovered in the form of dried foam or powder. Some examples of foam-mat dryers are tray foam-mat dryer and conveyor foam-mat dryer.
Freeze dryer: This dryer uses freezing and sublimation to remove water from the food. It is suitable for drying high-value foods that have natural aromas or textures that need to be preserved, such as coffee, mushrooms, herbs, spices, strawberries, meats, seafood, cream, confectionery, enzymes, microbial cultures, etc. The product is recovered in the form of dried pieces or powder. Some examples of freeze dryers are batch freeze dryer and continuous freeze dryer.
Pretreatments are the processes applied to food products before drying to improve the quality and efficiency of the drying process. Pretreatments can help to inactivate enzymes, prevent browning, enhance color, flavor and texture, reduce microbial load, and increase shelf-life of dried foods. Some of the common pretreatments before drying are:
Blanching: Blanching is the process of exposing food products to hot water or steam for a short time. Blanching helps to inactivate naturally occurring enzymes in foods that are responsible for enzymatic browning and off-flavor development. Blanching also helps to remove surface dirt, reduce microbial load, soften tissues, and improve rehydration. Blanching time and temperature depend on the type and size of the food product. For example, fruits and vegetables are usually blanched at 80-100°C for 1-10 minutes, while meat and fish are blanched at 100-120°C for 5-15 minutes.
Sulfur dioxide treatment: Sulfur dioxide treatment is the process of exposing food products to sulfur dioxide gas or solution for a short time. Sulfur dioxide treatment helps to preserve texture, flavor, vitamin content (ascorbic acid and carotene), and color of the food. Sulfur dioxide treatment also helps to reduce enzymatic browning and acts as an antimicrobial agent. Sulfur dioxide treatment is mainly used for fruits such as apricots, peaches, apples, grapes, and raisins. The concentration and duration of sulfur dioxide treatment depend on the type and quality of the food product. For example, fruits are usually treated with 0.5-2% sulfur dioxide solution for 5-30 minutes, or with 1000-6000 ppm sulfur dioxide gas for 1-24 hours.
Salting or curing: Salting or curing is the process of adding salt or other curing agents such as nitrites or nitrates to food products before drying. Salting or curing helps to dehydrate the food product by creating an osmotic pressure that draws out water from the cells. Salting or curing also helps to preserve the food product by inhibiting microbial growth and enhancing flavor and color. Salting or curing is mainly used for meat, fish, and cheese products. The amount and type of salt or curing agent depend on the type and quality of the food product. For example, meat and fish are usually salted with 10-25% salt by weight for 1-7 days, while cheese is usually salted with 1-4% salt by weight for a few hours to a few days.
Dipping in sulfite: Dipping in sulfite is the process of immersing food products in a sulfite solution for a short time before drying. Dipping in sulfite helps to dehydrate the food product by reducing its water-holding capacity. Dipping in sulfite also helps to improve the drying characteristics and quality of the food product by preventing browning, preserving vitamins, enhancing color and flavor, and reducing microbial load. Dipping in sulfite is mainly used for fruits such as bananas, pineapples, mangoes, and papayas. The concentration and duration of dipping in sulfite depend on the type and quality of the food product. For example, fruits are usually dipped in 0.1-5% sulfite solution for 1-5 minutes.
Cooking: Cooking is the process of heating food products to a high temperature for a short time before drying. Cooking helps to destroy microorganisms and enzymes that can cause spoilage or deterioration of the food product. Cooking also helps to decrease the water-holding capacity of the food product by denaturing proteins and starches. Cooking also helps to improve the texture, flavor, color, and rehydration of the food product. Cooking is mainly used for meat, fish, eggs, milk, cereals, legumes, and vegetables. The temperature and time of cooking depend on the type and size of the food product. For example, meat and fish are usually cooked at 100-150°C for 10-30 minutes, while cereals and legumes are usually cooked at 80-100°C for 15-60 minutes.
Influence of drying on the nutritive value of food
Drying is an effective method of preserving food by reducing its water activity and inhibiting microbial growth and enzyme activity. However, drying also affects the nutritive value of food in various ways. Depending on the type of food, the drying method, the drying temperature, and the pretreatment, some nutrients may be lost, retained, or even concentrated during drying.
Loss of nutrients
Some nutrients are sensitive to heat, light, or oxygen and may be degraded during drying. These include:
- Vitamin C: This water-soluble vitamin is easily oxidized and destroyed by heat and light. Drying can reduce the vitamin C content of fruits and vegetables by 50 to 80 percent . For example, a fresh plum has about 6 milligrams of vitamin C, while a dried prune has only 1.3 milligrams.
- Vitamin B1 (thiamine): This water-soluble vitamin is also heat-sensitive and can be lost during drying. For example, drying can reduce the thiamine content of rice by 20 to 30 percent.
- Vitamin A (carotene): This fat-soluble vitamin is susceptible to oxidation and light degradation. Drying can reduce the carotene content of fruits and vegetables by 10 to 20 percent.
- Folate: This water-soluble vitamin is also heat-sensitive and can be lost during drying. For example, drying can reduce the folate content of spinach by 40 to 60 percent.
Retention of nutrients
Some nutrients are more stable and resistant to heat, light, or oxygen and may be retained during drying. These include:
- Protein: This macronutrient is relatively stable during drying and can be retained in most foods. However, some amino acids may be lost or modified by heat or oxidation.
- Fat: This macronutrient is also relatively stable during drying and can be retained in most foods. However, some fatty acids may be oxidized or hydrolyzed by heat or moisture.
- Fiber: This indigestible carbohydrate is unaffected by drying and can be retained in most foods. Fiber can help improve digestion, lower cholesterol, and regulate blood sugar levels.
- Minerals: These inorganic elements are also unaffected by drying and can be retained in most foods. Minerals are essential for various bodily functions, such as bone health, fluid balance, and nerve transmission.
Concentration of nutrients
Some nutrients may be concentrated during drying due to the removal of water. This can increase the nutrient density of dried foods compared to fresh foods. However, this also means that dried foods have higher calories and sugar content per weight than fresh foods. Therefore, portion control is important when consuming dried foods. Some examples of nutrient concentration are:
- Calories: Drying can increase the calorie content of fruits and vegetables by three to five times . For example, a fresh plum has about 35 calories, while a dried prune has about 193 calories.
- Sugar: Drying can increase the sugar content of fruits and vegetables by three to five times . For example, a fresh plum has about 8 grams of sugar, while a dried prune has about 38 grams.
- Iron: Drying can increase the iron content of fruits and vegetables by two to three times . For example, a fresh plum has about 0.1 milligram of iron, while a dried prune has about 2 milligrams.
Drying is a useful method of food preservation that can extend the shelf life and facilitate the transport of nutrient-dense foods. However, drying also affects the nutritive value of food in various ways. Some nutrients may be lost, retained, or concentrated during drying. Therefore, it is important to choose appropriate drying methods, temperatures, and pretreatments to minimize nutrient loss and maximize quality retention. It is also important to consume dried foods in moderation and balance them with other fresh foods to meet the dietary requirements.
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