Fruit drying is one of the main processing and preservation methods of fruits because of its low production cost, simple technology, difficult deterioration and convenient eating and carrying. Litchi, longan, jujube, grapes, persimmons, apricots, plums, apples and other dry goods are very popular, and some of them are famous specialty products.
Food drying has a long history. China Northern Wei Dynasty's Qi Yao Min Shu mentioned the method of food drying, and ancient books such as Compendium of Materia Medica and Qunfangpu also talked about the drying of peaches, jujubes and other fruits, all of which belong to natural drying. Manual drying According to the records of W.V.Cruess, the tunnel drying method was first used in the United States in 1890.
Dry preservation principle
The preservation function of drying is to reduce the water content and water activity of the fruit itself (at the same temperature, the ratio of the water vapor pressure on the surface of food to the water vapor pressure of pure water). The more water in food, the greater the water activity. When the water activity is close to 1, all kinds of microorganisms can grow and reproduce with high enzyme activity; Food has low water content, high solute content and low water activity, so it is difficult for microorganisms to survive. Dried fruits are kept dry by packaging, which makes microorganisms and insects lose their living conditions, reduces their own enzyme activities and inhibits enzymatic and non-enzymatic chemical reactions. Generally, the water content 1.5 ~ 20% of dried fruits has a water activity value of 0.60 ~ 0.65. Except for some permeable yeast and a few dry molds, other microorganisms can't grow and reproduce, and the chemical reaction in the fruit is inhibited.
The moisture of fresh fruit can be divided into free water, bound water and bound water. Free water and partially bound water are removed by drying. The loss of moisture in the drying process includes the transfer of moisture from the middle of the fruit block to the outside and the evaporation of surface moisture. The typical drying process is divided into constant speed stage and slow speed stage. In the early stage, the surface moisture is sufficient, and the drying speed depends on the temperature, relative humidity and flow rate of the drying medium, which is fast and constant. In the later stage, the surface moisture is lost, and the drying speed depends on the internal moisture diffusion, and the drying speed decreases. The relative humidity of the external medium is lower than 100%, and the main limiting factor is the water on the evaporation surface.
The drying speed of fruit is influenced by four factors: ① Fruit characteristics. For example, the drying speed is slow when the texture is tight or the waxy skin is thick, and the drying speed is slow when the sugar content is high. ② Treatment methods. Such as the size, shape, alkali soaking treatment, etc. Proper cutting and alkali leaching are beneficial to improve the drying speed. ③ Characteristics of drying medium. If the flow rate is fast, the temperature is high, the relative humidity is low, and the drying speed is fast; ④ The characteristics of drying equipment have different influences, and the loading capacity of truck or conveyor belt is inversely proportional to the drying speed.
After drying, the volume of the fruit is reduced by 65-80%, the weight is reduced by 80-90%, and the shape is different due to uniform heating. After drying, the transparency increases: due to the evaporation of water, the contents of sugar and acid increase relatively; Non-enzymatic browning and enzymatic browning occur in different degrees according to different species and varieties; Vitamin c is lost to varying degrees; Chlorophyll and carotene changed, the color of olive became darker, and the yellow fruit partially faded.
process flow
Natural drying and artificial drying are slightly different, mainly reflected in the process of raw material selection and pretreatment, drying and post-drying treatment.
Selection and treatment of raw materials
Raw materials of dried fruits require high dry matter and soluble solids, less crude fiber, good texture and rich flavor. It is required to reach the maturity of fresh food. Pretreatment includes sorting, grading, cleaning, peeling, slicing, coring, blanching, sulfur treatment, alkali treatment, etc. The purpose and method of most treatments are the same as canning (see canned fruit). Sulfur treatment can promote drying, improve the color of dried products, maintain vitamin C and extend the shelf life of products. There are two methods: soaking and fumigation. The soaking method is to soak the cut dried fruit in a certain concentration of acidified sulfurous acid or sulfite solution for a period of time, and then take it out. The sulfite concentration is 1 ~ 3% and the time is 10 ~ 20 minutes. Fumigation refers to fumigation of fruits by burning sulfur in a closed container or room. The concentration of fumigation room is maintained at 1.5 ~ 2.0%, 1 ton of sulfur powder for fruits is 2 ~ 4 kg. Soaking in alkali before drying is helpful for drying. According to the type and maturity of raw materials, the concentration of alkali liquor is 0.2 ~ 5%, and the time varies from a few seconds to dozens of seconds.
Drying method
Including natural drying and artificial drying. Natural drying is to dry the processed fruits in a drying tray or other utensils, and some of them need to be piled up and spread flat. The final moisture content of dried fruits varies slightly with varieties and processing methods (see table).
According to different media, there are many manual drying equipment and methods:
Air convection drying
Specific methods include oven drying, drying room drying, tunnel drying equipment, conveyor belt drying equipment, airflow drying equipment, fluidized bed drying equipment and so on.
Photo 1 Drying box and drying room are still the main equipment for drying litchi, longan and jujube in China. The baking oven is directly heated with charcoal as fuel, and the pit shape is dug on the ground, on which purlins are placed and screen mats are laid, and fruits are directly heated and dried by the fuel on the screen mats. There are many architectural styles of drying room, mainly including drying room main body, heating equipment, ventilation and dehumidification equipment and material loading system (Figure 1).
Tunnel drying equipment is the most widely used in fruit drying, and all dried fruits that keep the shape of fruit blocks can be applied. The main body of the equipment consists of drying tunnel, heating part, ventilation and dehumidification device and carrying device (Figure 2 and Figure 3). According to the direction of air flow and dry materials in the equipment, there are three types: downstream, countercurrent and countercurrent (that is, the combination of downstream and countercurrent, also known as composite flow or mixed flow). The forward direction of materials on the downstream drying car is consistent with the flow direction of dry hot air. The fruit first comes into contact with high temperature and low humidity air, so the temperature of the fruit rises quickly and the drying speed is fast. With the drying entering the low temperature and high humidity environment, it is difficult to control the low moisture content at the end of the final drying. Suitable for fruits with high moisture content. Countercurrent drying of fruits changes from low temperature and high humidity medium to high temperature and low humidity medium, with low moisture content at the end, which is suitable for most fruits. Countercurrent drying combines the advantages of downstream and countercurrent drying and overcomes their respective disadvantages.
Figure 2
Fig. 3 In the continuous conveyor belt dryer, the materials run in the conveyor belt, and the conveyor belt is designed in a multi-layer form, so that each layer of materials can be turned over when changing. The whole machine is heated by steam and installed between each conveyor belt. Fresh air enters from the bottom and moisture is discharged from the upper outlet, which is suitable for continuous production (Figure 4).
Other fluidized bed drying equipment, airflow drying equipment and cabinet drying equipment are also used for fruit drying.
Fig. 4 Rolling drying
The principle is that the material is spread into a thin layer on the surface of a slowly rotating and constantly heating drum, and the water evaporates after one rotation, and the dried material is scraped off with a fixed scraper, which is only used to concentrate juice, puree and pulp.
vacuum drying
That is, in a closed container with a heating rack, decompression and dehydration are suitable for drying special dried fruits with low water content. Apples, peaches, apricots, plums and so on. The tunnel drying method is usually used for drying, and the water content of this method is about 5%.
freeze drying
Under the condition of freezing, the product is kept at a low temperature below the triple point for decompression, so that the water directly changes from solid to gas and is dried. Because there is no need for heating, the product quality is high, but the cost is high.
Dry aftertreatment
After drying, the finished product is selected, graded and packaged. Dried fruits that need to be soaked (also called sweating) can be stored in a closed container or warehouse for a period of time, so that the water inside the fruit block and the water between different fruit blocks (grains) can be diffused and redistributed to achieve consistency. In order to prevent pests during storage and maintain quality, the finished product should be fumigated before packaging to keep a certain amount of sulfur dioxide in the product. Generally, plums 1530 ~ 2600 ppm, peaches 1320 ~ 2460 ppm, pears 1033 ~ 1607 ppm, and raisins 805 ~ 6544 are needed. It can also be treated with methyl bromide or fumigated with carbon disulfide. Insect control also uses low temperature, heat treatment or other methods. Some dried fruits, such as raisins, need to be washed, dried and packaged.
The packaging of dried fruits requires that the packaging materials should be airtight, light, firm, durable, beautiful, cheap, non-toxic, tasteless and easy to carry and open. Commonly used are plastics and composite films, such as polyethylene bags, polyethylene-aluminum foil-polyester film composites, paper-polyethylene-aluminum foil-polyethylene and so on. Dried fruits are sold in the above bags, some in cartons, and then in corrugated boxes. There are also large packages used as raw materials for processing other foods.
Dried fruits should be stored at low temperature (0 ~ 5℃) and low humidity (50 ~ 60%), and attention should be paid to avoiding light, oxygen and insects.
Main points of dried fruit processing
Litchi and longan
Both of them are China specialties. Among them, dried longan is called longan, and the raw materials are varieties with large fruit, thick meat, small stone, high dry matter and high sugar content. Litchi requires moderate shell thickness and maturity. Longan is dried by cutting the stem, selecting and polishing the peel, and then heating and drying it in a charcoal oven. At that time, it turned 1 time every 4 hours. After drying, some products are dyed by spraying turmeric water. The drying rate of longan is about 30 ~ 33%. The drying process of litchi is the same as that of longan except that it does not need to polish the peel and dye. The average drying rate is about 25 ~ 30%.
jujube
The dried jujube products in China include red dates, black dates and southern dates. Raw materials require large fruit, thin skin, much meat, small stone and high sugar content. Jujube is dried with ripe fruits, blanched with moderate hot water, and exposed to the sun. During this period, attention should be paid to harvesting at night and drying again during the day. The drying rate is about 30%. The processing of black dates needs artificial blackening. Select reddish or white jujube fruits, blanch them with hot water 1 ~ 2 minutes, fill the laundry list or pile them together and cover them with straw mats or linen, and spread them out to dry or dry them after about 2 hours.
grape
The raw material is mature spike, thin skin and thick flesh, medium size, soluble solids above 20, preferably seedless. When drying, after removing rotten fruit and secondary fruit, the whole ear fruit is treated with 1.5 ~ 4.5% sodium hydroxide solution for 1 ~ 5 seconds, washed, dried in the sun for 3 ~ 5 days, dried for 2 ~ 3 days, and dried in the shade. In Turpan, Xinjiang, China, grape ears are also hung in a ventilated room to dry, and the product quality is high. The initial temperature of countercurrent tunnel drying equipment is 45 ~ 50℃, the final temperature is 70 ~ 75℃, the relative humidity is 25%, and the water content is 65438 08%. Products need softening, stem removal, cleaning, sorting, drying and packaging.
persimmon
The product is persimmon. Choose a round fruit with the same size, the pulp is sticky, and the maturity is to turn yellow and red in color, but the meat is hard but not soft. Selecting fruits free from pests and diseases, grading, removing sepals, keeping fruit bowls and fruit stalks, stringing or not stringing the fruit stalks with hemp rope, airing for about 20 days, kneading until soft, repeatedly airing for 2 times, spreading flat for half a month, doing indoor activities at night, covering with straw mats, and repeating for 2-3 times will cause frost.
apple
Selecting varieties with the same size, no hair loss, strong fragrance and light browning, sorting, peeling and removing cores, cutting into discs with a thickness of 5-7 cm, steaming for 2-4 minutes, and fumigating with sulfur for 1-2 hours. The starting temperature of countercurrent drying is 80 ~ 85℃, the ending temperature is 50 ~ 55℃, and the relative humidity is 65438 00%.
peach
Choose fruits with stones, yellow flesh and low crude fiber content, cut them in half, remove the stones, peel them with alkali liquor, cook and bleach them for 5 minutes, and fumigate them with sulfur for 65438 0 hours. The initial temperature of artificial drying is 55℃, the final temperature is 65℃, and the relative humidity is 30%. When drying naturally, extend the sulfur fumigation time and dry in the sun.
apricot tree
Choose large and mature fruits with low crude fiber content or medium-sized dark yellow fruits, cut them in half, remove the core (or not), smoke sulfur in the sun for more than 3 hours, and dry them in the shade after drying them to 50% to 70%. The initial temperature of artificial drying is 50℃, the final temperature is 75℃, and the relative humidity is 65438 00%. Dried apricots containing Huaqing rope should be dried in the sun.