Dried fruits are essentially fresh fruits that remove water, which are compact, lighter, easier to transport, preserve and some of the nutrients in the fruit are dried more.
A 2005 study published in the American College of Nutrition found that the antioxidants in raisins and prunes were 2 times more than fresh fruits. Dried fruits are rich in minerals, vitamins and enzymes that are good for the body (except for vitamin C, which is usually destroyed when it is dried).
Dried fruits not only have a very special flavor but also have many health benefits such as helping to clean the blood and good for the digestive system, suitable for heavy people like sports, thanks to their high carbohydrate content. Cardiovascular disease, high blood pressure.
General principles when drying fruits and vegetables
Drying is a measure of evaporating some of the water in vegetables to reduce the weight of vegetables. But in drying, a large amount of vitamin C is destroyed, depending on each type of fruit and each drying method that vitaminC losses can be up to 90%.
There are two methods: cold drying (sublimation drying) and hot drying.
+ Cold-drying: is freezing the fruit and vegetables at a temperature of – 200C, the water in the fruit and vegetables freezes into ice in solid form, then makes the water from solid to vapor, evaporate without going to liquid state. This method is called sublimation drying.
+ Heated: Use hot air to dry vegetables. It is the process of diffusing water from the outer layers to the surface, and the water from the surface of the product evaporates into the surrounding environment. In industry often dried fruits and vegetables by tunnel kilns. This method has the advantage of resolving large volumes with forced blower, making the evaporation rate fast and the temperature right. For families can only use the oven, manual drying oven or sun drying. The sun has the advantage of saving fuel, making use of solar energy, but reducing up to 80% of vitamin C and carotene, prolonged drying time, inferior color quality, encountering moldy products when it rains. In addition, the sun also requires a lot of area, difficult to solve the large output. In turnip season, the weather can be very dry.
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1. Blanching (steaming)
Before drying often blanching vegetables in hot water or steaming to protect product quality and shorten the drying time. When blanching, due to the effect of heat and moisture, the physical and chemical properties of the material are altered in favor of drying moisture loss: microorganisms are destroyed and the enzyme system in the material is suspended. operation (inactivation) to avoid damaging the product.
In vegetables and fruits, there are many types of yeasts, the most stable type of yeast is the Peroxidaza yeast. Inactivation (suspending activity) of this enzyme is inactivating other enzymes. In order to inactivate the enzyme Peroxidaza, it is necessary to heat fruits and vegetables at temperatures above 750C. For fruits rich in carbohydrates (potatoes …): blanching increases the porosity of the vegetables, due to the hydrolysis of pectin causing the link between the cell membranes to be broken.
Starch gelatinization when blanching also accelerates the drying process. For vegetables containing pigments (carrots, peas, plums …), blanching has the effect of keeping color, limiting discoloration or fading.
For fruits and vegetables with a thin wax layer on the surface (plums, litchi …), blanching loses the wax layer, creating tiny pains on the surface, thereby increasing the process of moisture exchange between fruits and lips ambient field, resulting in a shorter drying time.
2. Chemical treatment
To prevent oxidation during drying, it is common to use antioxidants such as sulfuric acid, ascorbic acid, sulfuric acid and sodium salts of sulfuric acid (such as metabunsulfite, bisunite, sulfite, etc.).
Citric acid inhibits non-glaze discoloration. The sulfuric acid and its sodium salts have a strong reducing effect, which acts on the oxidizing enzyme activity group and slows down the enamel-colored tint reaction. In addition, they also work to prevent the formation of melanoidin (the substance that causes darkening) and stabilize vitamin C, preventing vitamin C from being digested. The minimum content of SO2 for oxidation resistance is 0.02% (by mass).
3. Drying temperature
Vegetables and fruits are poor heat-resistant products: above 900C, fructose starts to caramelize, melanoidin-producing reactions, polymerization of polymers are strong. At even higher temperatures, vegetables may burn. Therefore, for drying vegetables and fruits often use the moderate drying mode. Depending on the type of material, the drying temperature should not exceed 80 – 900C. For blanched fruits and vegetables, to kill the yeast, the initial drying can be brought up to 1000C, after a few hours of lowering to the appropriate temperature.
The drying process depends on the rate of temperature increase of the drying material. If the rate of increase of heat is too fast, the surface of the fruit will harden and prevent the moisture from escaping. Conversely, if the growth rate is slow, the moisture drainage intensity is weak.
4. Air humidity
To improve the air’s ability to absorb moisture, it must reduce its relative humidity. Drying is the method to increase the hygroscopic ability of the air by increasing the temperature.
Normally when entering the kiln, the air has a humidity of 10-13%. If the humidity is too low, the vegetables will soak or create a dry crust on the surface, which adversely affects the subsequent evaporation. But if the humidity is too high, drying speed will decrease.
When leaving the oven, the air carries the moisture of fresh vegetables, so the humidity increases (normally about 40 – 60%). If the exhaust air is too low, it will use energy; conversely, if it is too high, it is easy to get dew, causing damage to drying products. It regulates the humidity of the outgoing air by adjusting its circulation speed and the amount of fresh vegetables contained in the kiln.
5. Circulation of air
During drying, air can circulate naturally or forced. In kilns, the air naturally circulates at a small speed (less than 0.4m / s), so the drying time is often long, making the quality of the drying product not high. To overcome this drawback, one must use fans to forced ventilation at a speed of about 0.4 – 4.0m / s in the drying equipment. If the wind speed is too large (over 4.0m / s), it will cause heat loss.
6. The thickness of the drying layer
The thickness of the dried vegetable layer also affects the drying process. The thinner the material layer, the faster and uniform the drying process will be, but if it is too thin, it will reduce the productivity of the kiln. Conversely, if it is too thick, it will reduce the circulation of air, leading to the product “sweating” due to moisture.
Usually it is advisable to layer fruits on drying trays with a weight of 5 – 8kg / m2.
7. Packing and preserving dried fruits and vegetables
After drying, classification should be carried out to remove unqualified species (due to fire or not meeting the required humidity). Good dry type is poured into a large tray or basin to regulate the humidity. Then fan to cool completely before packaging to avoid sweating.
Depending on each item, storage time and object of use, there are different packaging specifications. In addition, the conditions of product transport and storage are also important in the choice of packaging.
The type of material commonly used to protect dry fruits and vegetables is cardboard and plastic (PE, PVC, cellulose …). Paper bags and cardboard boxes are lightweight, cheap, recyclable, but vapor permeable, uneven under the effects of water and mechanics. Plastic bags have the characteristics of transparent, elastic, easily sealed with heat, low cost but some are waterproof. Gas permeability (PE), poor resistance (PVC, PET).
Plastic bags used to preserve dry fruits may consist of only one plastic film or a combination of films. In addition, it is also possible to store dried fruits and vegetables in jars, jars or metal containers tightly closed.
8. Method of drying
The simplest method of drying is sun drying, which has the advantage of utilizing solar energy and saving fuel; but the drying time is long, reduces much vitamin C (up to 80%) and carotene (pre-vitamin A), the color of the product is poor, requires a lot of drying yard area, difficult to solve the large and dense volume. Especially depends on the weather.
To proactively drying, use drying equipment. Drying is the process of separating water in a product by heat. It is the process of diffusing water from the inner layers to the product surface and the steam from the product surface to diffuse into the surrounding environment.
Conventional dryers use hot air as a drying agent, including: drying ovens, manual ovens or dryers with heating elements and blowers. In addition, there are other drying methods such as sublimation drying, infrared drying …
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