Freeze-drying is a popular food preservation method for fruits and vegetables. It is an effective way to remove water from the food while retaining most of its nutrients. This method creates a lightweight, crunchy, and long-lasting product that is perfect for snacks or cooking. However, the quality of the freeze-dried product depends on several factors, including the initial quality of the food, drying temperature and time, and pre-treatment method.
A recent study published in Drying Technology focused on the freeze-drying of carrot slices and how ultrasonic pretreatment can significantly improve the quality of the final product . The researchers found that this pre-treatment method can reduce the drying time, improve the rehydration ratio, and enhance the color and texture of the freeze-dried carrot slices.
The researchers used fresh carrots from a local market and sliced them into 2-3 mm thick pieces. They randomly divided the slices into four groups: control, ultrasonic, traditional freeze-drying, and infrared freeze-drying. The control group was freeze-dried without any pretreatment, while the ultrasonic group was immersed in distilled water and subjected to ultrasonic treatment for 20 minutes before freeze-drying. The traditional and infrared freeze-drying methods were performed using different types of equipment.
After evaluating the properties of the freeze-dried carrot slices, the researchers found that the ultrasonic pretreatment reduced the drying time of both traditional and infrared freeze-drying. The ultrasonic group also had higher rehydration ratios, brighter color, and better texture compared to the control group. In addition, the infrared freeze-drying method resulted in a higher retention of vitamin C and total phenolic content compared to traditional freeze-drying.
The ultrasonic pretreatment involves subjecting the carrot slices to high-frequency sound waves, which cause mechanical vibrations and cavitation within the food. These vibrations and cavitation lead to the formation of small bubbles and cracks within the food, which in turn increase its surface area and make it more permeable to water. This means that when the carrot slices are later freeze-dried, they absorb water more easily and quickly, resulting in a higher rehydration ratio and a more tender and juicy final product.
Moreover, the ultrasonic pretreatment helps to reduce the drying time by breaking down the cellular structure of the food, making it easier for water to escape during the drying process. This means that less time is required for the drying process, resulting in a faster and more efficient production process. Additionally, the ultrasonic pretreatment can help to reduce the energy required for the drying process, making it a more environmentally friendly option.
In conclusion, this study found that ultrasonic pretreatment is a promising method to enhance the quality of freeze-dried carrot slices. It can also be an effective way to reduce the drying time and energy required for the freeze-drying process. Furthermore, the use of infrared freeze-drying technology can help to preserve the nutritional value of fruits and vegetables during the preservation process. With the growing popularity of freeze-dried fruits and vegetables, this study provides valuable insights into how to improve the quality and efficiency of the freeze-drying process.
 Dongcui Fan, Bimal Chitrakar, Ronghua Ju & Min Zhang (2021) Effect of ultrasonic pretreatment on the properties of freeze-dried carrot slices by traditional and infrared freeze-drying technologies, Drying Technology, 39:9, 1176-1183