How to use vacuum drying technology to dehydrate sweet potatoes?

Vacuum drying is a common method for making dehydrated sweet potatoes. This method is a drying method that puts the material under negative pressure conditions and appropriately heats it to the boiling point under negative pressure or cools it down to make it solidify and then dries it through the melting point.

There are two main methods of vacuum drying:

(1) Drying by melting point: During the drying process, the wet material (or solution) is first frozen into a solid state at a relatively low temperature (-10--50°C), and then the solid water in it is directly sublimated into a gaseous state and removed under a high vacuum (130-0.1Pa). This drying process is also called sublimation drying. The necessary condition for achieving this vacuum drying is that the pressure of the drying process should be lower than the saturated vapor pressure of ice at the operating temperature. It is usually controlled at 1/2-1/4 of the saturated vapor pressure of ice at the corresponding temperature. For example, when drying at -40°C, the operating pressure should be 2.7-6.7Pa.

(2) Drying by boiling point: During the drying process, liquid water vaporizes in two ways: axial evaporation and boiling. The vaporization rate of water when boiling is much faster than that when evaporating. The evaporation of water into steam can occur at any temperature. The boiling of water into steam can only occur at a specific temperature, but when the pressure is reduced, the boiling point of water also decreases. For example, at an air pressure of 19.6kPa, the boiling point of water can be reduced to 60°C. A vacuum dryer provides a heat source under a vacuum state, and supplies enough heat to the water in the material through heat conduction, heat radiation and other heat transfer methods, so that evaporation and boiling can occur simultaneously, accelerating the vaporization rate.

What are the characteristics of this vacuum drying?

(1) The equipment investment and power consumption are higher than those of normal pressure hot air drying, and the volatile liquid of this drying method can be recycled, but the production capacity is small, intermittent operation is required, and the drying speed is fast.

(2) The dried product can form a porous structure, appear crispy sponge-like, easy to crush, have good solubility and rehydration, and have good color and taste.

(3) It can reduce the chance of material contact with air and avoid pollution or oxidation deterioration.

(4) The vacuum degree and heating temperature range used during drying are large, with good versatility, low drying temperature, no overheating, easy evaporation of water, and short drying time.

(5) Vacuum drying is suitable for heat-sensitive materials, materials that are easily oxidized at high temperatures, or materials whose exhaust gases are valuable, toxic, or combustible.