Agricultural drying technology

The history of freeze-drying food began in 1100 by the Incas in Peru by taking advantage of the high position (4500m) of the Ande Mountains in South America. They use this high position with cold air and low pressure to freeze meat as well as some foods. Due to the effect of solar radiation and low pressure, food products after freezing immediately tend to freeze. Peruvian Incas have used this natural freeze-dried method for centuries. Food preparations have been preserved in the “mind” (a type of backpack) of military operations. With the good use of such natural freeze-dried food products, the live stockpiles of vegetables with enough capacity to feed 25-30 thousand people have been replaced by freeze-dried. nature on this plateau since 1533.

Then, in 1906, two French physicists, Bordas and Darsonval, invented a device to condense steam ice with snow mixed with acetone during the drying process. Next, other French and American scientists continued to perfect the method of plasma lyophilisation, which was fine in the US-Japan War (1941). In 1942-1943, there were complete freeze-drying devices with large capacity for the pharmaceutical industry. Researchers have spent another 15 to 20 years exploring the application of industrial freeze-drying for foods. Nowadays, freeze-dried food has become an industrial-scale food in the world, especially in Europe and America. People produce freeze-dried foods for export because it is the highest quality processed product.

Frozen products also retain almost intact fresh properties, biological activities, special activities such as freeze-dried products, but the subsequent storage and transportation processes need to be kept closed in cold circuits. continuously in a negative temperature environment (-18 ÷ -25o C). Therefore, if transported beyond 300km, or stored for more than 6 months, the total cost of frozen products will be higher than the total cost of freeze-dried products.

On the other hand, freeze-dried product has many advantages in long-term storage at normal temperature (other than freezing, must be kept at temperatures below – 18oC continuously) easy transportation and convenient use compared to the product. Frozen.

For 20 years in Italy, there have been large-scale freeze-dried food factories with modern equipment to process freeze-dried agricultural products such as Cicada beans, peas, cauliflower and tomatoes. , cabbage, onions, garlic, oranges, tangerines, bananas, apples, pears, strawberries and meat soups, crab soups, fish …. Italy’s freeze-dried foods have captured many markets. in central, northern Europe and America. Experts from the World Health Organization and the World Food Program have paid close attention to freeze-dried foods in food aid programs, food for under-fed countries and places affected by natural disasters (earthquake). , volcanoes, drought …). They concluded: freeze-dried food has many advantages over frozen products, can be stored normally for many years even in tropical climatic conditions, its volume is up to 12 times lower than volume of frozen or canned foods.

Explanation and principle of freeze-drying

Freeze drying (scientists called Lyophilization) uses vacuum and freeze to remove water – The cause of food or pharmaceutical spoilage. After being freeze-dried, products can be stored at room temperature for a long time without being damaged, before the expiry date indicated on the packaging, when necessary, the product is given water to restore the status quo.

Basics- Sublimation

Vacuum freeze drying is a preeminent product drying technique. First, the material is suddenly frozen at the triple-point temperature, causing the water in the product to become solid, then undergoing vacuum processing (sublimes) into a vapor form and condensing into water and discharged, the product becomes dry form

The principle of vacuum freeze-drying is to sublimate water to steam. In the chart, the three straight lines OA, OB, OC are called Liquefied Lines, Gasification Lines and Sublimation Lines, which show the relationship between steam and temperature at 3 stages: Ice- water, water – steam Water, ice-steam. The point O is called the junction point, the reciprocal temperature is 0.01 ℃, the steam pressure is 6.11 mbar 4.58 mmHg ， 611 Pa, at this temperature, 3 factors: water, ice and steam can coexist and in equilibrium. When the temperature or pressure changes, there will be a lot of changes or the water will melt or solid matter will become a gas – sublime. In a high vacuum state, with the principle of sublimation, ice in the product does not disintegrate but directly evaporates and then escapes, reaching the purpose of drying the product.

Therefore this process is called sublimation drying. Theoretically, the sublimation process only needs to be done at the junction point temperature, but in reality that condition is more stringent, usually requires a vacuum of 0.5 ~ 1.5 mbar and a temperature of about -25oC to ensure warranty proceeds smoothly.

Steps for conducting a vacuum freeze-drying process:

– Pre-freezing: Preparing the product for the next sublimation process, leaving the product at a cold temperature of 10 ~ 20 ℃

– Drying: During this process, the sublimation tape does not melt

– Heating and drying: in this process, the key is to remove residual water, let the product dry completely, this step plays a very important role in stable product storage

Comparison of advantages and disadvantages of freeze-drying:

The shape, color, nutrition, aroma, ingredients and taste of the material only change slightly.

The product is not polluted, very little residual water (1 ~ 3%), less bacteria, easy storage, convenient transportation at normal temperature.

Product structure is constant, easy to crush, easy to restore the status quo! Important!

Expensive equipment, difficult operation, difficult to operate in temperatures below – 60oC, need a high-efficiency vacuum machine.

When packing, care must be taken to note that fragile goods are very hygroscopic, so they need to be sealed

Application of freeze-drying:

– Mushrooms, carrots, sweet corn, spinach, celery, peas, cabbage, shiitake mushrooms, red peppers, onions, leeks, wood ear (cat mushrooms), coriander, asparagus.

– Beef slices, pork slices, chicken, duck meat

– Shrimp, squid, fry, crab, scallop

– Tofu, miso soup, soy sauce, vinegar powder, tea powder, coffee powder

– Royal jelly, flower powder, herringbone, garlic essential oil, Ganoderma oil, shiitake oil, yeast, antibiotics

– Strawberry powder, banana powder, pineapple powder, kiwi powder

– Microorganisms (types of bacteria), enzymes, blood plasma used in medicine, biological specimens, antibiotics