Heat-pump drying system can be used to dry fruits at lower temperatures for a shorter time. This post will explains briefly the mechanisms of drying. It then points out the basic difference in working principles between heat-pump drying and conventional hot air drying. From there we can know why heat-pump technology helps process dried foods of better nutritional and sensorial quality.
Please watch the video clip above. The full subtitle is given below. Enjoy your watching!
Extra links: Application of heat-pump drying technology for manufacture of dried mango, dried jackfruit, and dried passion fruit.
What is drying?
Drying is a process in which a major part of water in the food material is evaporated and removed away by a strong air flow blowing over the food surface and therefore the product becomes dried or it has low moisture content which then, after proper packaging, will not spoil and be stable at room temperature for a long time.
The air flow for drying should be ‘dry’ or, in another way of speaking, it should have low relative humidity so that it can uptake moisture from food easily. Relative humidity is a parameter indicating how much vapor molecules the air already contains compared to the maximum molecules of vapor that air can hold.
Conventional hot-air drying
Temperature of air and its relative humidity correlate to each other. You can look at the psychrometric chart, the one that indicate properties of air. Increasing air temperature will reduce its relative humidity. And this principle is applied in conventional hot-air drying method.
In such a machine, the input air is heated to more or less 60 degree C, which is equivalent to 140-degree Fahrenheit, and correlatingly, the relative humidity becomes lower as needed. The heated air is blown into a drying chamber to dry the food material. The food uptakes heat from the air to evaporate water while the air uptake the vapor from the food. As the result, the temperature of the air is reduced while the exhausted air has now higher relative humidity. Part of it can be recirculated to reduce energy consumption.
In a heat-pump drying system,
Aside from the drying chamber, the system is equipped with one or two mechanical refrigerators depending on the capacity. The refrigerators serve to reduce the temperature of the air coming out of the drying chamber and the water vapor is condensed into liquid water which is then removed. The air is then warmed up again and now it has much lower relative humidity which means it becomes much drier. It is blown back into the drying chamber to take moisture from the food faster.
This advanced technology is called low-temperature drying. The whole drying process is also shorter because drier air withdraws moisture from food faster. Because of those facts the product preserves better the nutrients and sensorial quality including the taste, the color, and the flavor.
So basically, in order to dry food, we need to lower relative humidity of the air. And the main difference is: conventional drying reduces relative humidity of drying air by heating it up, while heat-pump drying cools down the air to condense vapor away before warming up to reduce the relative humidity.
The heat-pump drying system is significantly more expensive than the conventional drying machine. Operational costs can be also higher and all these together make it a challenge for many factories to invest in this new technology.
