Authors: Poonam Kumari, Sapna Panwar and Namita
Division of Floriculture and Landscaping
ICAR- IARI New Delhi- 110012
Correspondence address: firstname.lastname@example.org
Freeze drying was originally introduced in 1813 by William Hyde Wallaston at the Royal Society in London. It is the most effective method of flower preservation as compared to other traditional methods like sun drying, shade drying etc. Freeze-drying is a dehydration process that causes the vaporization of water directly from a solid ice crystal state to a vapour state without passing through the normal liquid state. The main advantage of freeze-drying is that it results in products that appear almost like the fresh originals. Physically, the original texture, structure, and highly volatile components including aroma, can be retained in many freeze-dried food products. Freeze-dried fowers are of sufficient value such that they justify the substantial cost of freeze-drying. Although the shape, size, and colour of freeze-dried flowers are similar to those of fresh ones, they are more fragile. The main disadvantages of freeze-drying are its high costs and precise processing techniques. The initial cost of equipment investment, electrical energy consumption, and equipment maintenance are relatively higher than those for other drying methods.
Freeze-drying is a relatively new preservation process for the preserved plant material industry. Other than those in trade journals and popular literature, few references define the requirements of freeze-drying flowers.
Principle: Freeze drying relies on the principle of sublimation, whereby ice held under conditions of partial vacuum (less than 4.58 torr) and low temperature (less than 0°C) will evaporate on heating without going through a liquid phase. The absence of liquid water during the dehydration process means that undesirable chemical reactions will not occur. Hence, colour and even fragrance are retained in the dried article (Dubois and Joyce 1989).
Basic consideration: When considering freeze-drying of any product, one must determine the proper dehydration conditions, including parameters such as freezing time and temperature, vacuum-drying time and temperature, and pre-processing treatments. Unfortunately, optimum freeze-drying procedures have been determined for very few products, including flowers. Processors of freeze-dried flowers often consider specific processing information proprietary because shortened processing times translate into more efficient use of equipment and greater profitability.
Tips for collecting planting materials are as follows
- Avoid collecting plants when they are wet or moist
- Use sharp knife or pruning shears to cut the flowers or planting material
- Select the insect or disease free planting materials
- Before freeze drying stem should be placed in water.
Procedure: In this process, the flowers are placed into a refrigerated chamber and the temperature of the chamber is lowered below freezing. A vacuum is then created in the chamber, causing the moisture in the flowers to sublimate, or change from solid to gaseous form. The water vapour is then collected in a separate chamber and the dried flowers are allowed to slowly warm to room temperature. This process takes several days (Trinklein, 2006).
High solubility owing to shape substantially maintained after drying
Oxidizeable substances are well protected under vacuum conditions
Long preservation period owing to 95%-99.5% water removal
Little contamination owing to aseptic process
Minimal loss of active ingredient
Minimal loss in volatile chemicals and heat-sensitive nutrient and fragrant components.
Minimal changes in the properties because microbe growth and enzyme effect cannot be exerted under low temperature
Transportation and storage under normal temperature
Flowers Suitable For Freeze Drying
The following types of flower can typically be freeze dried:
- Bird of Paradise
- Calla Lily
- Cattleya Orchid
- Dendrobium Orchid
- Lily of the Valley
- Phaleonopsis Orchid
- Rose (all varieties)
- Snap Dragon
Case studies: Freezing time and temperature was standardized in different varieties of roses and carnation to maintain the quality of flowers (Brown, 1999). Ruth (2000) reported that the bridal bouquets could be preserved without any damage to flowers by the freeze drying. Chen et al. (2000) investigated the effect of different freezing time (2 and 4 hours), freezing temperature (-35ºC) and vacuum drying temperature (27ºC, 37ºC, 47ºC) on colour, moisture content, stem and petal strength of roses and carnations. The results revealed that lower vacuum drying temperatures resulted in quality flowers with colour closer to fresh flowers. Bhattacharjee and De (2003) investigated that different cultivars of carnation flowers remained in their naturalistic state when cryo dried for 7 days at -20ºC temperature. Liang et al (2005) studied the effect of freeze drying and microwave drying on China rose flowers and noted that flowers retained good colour and appearance when freeze dried and pre-treated with tartaric acid solution before microwave drying.
Wilkins and Desborough (1986) investigated the effect of different pre-treatments (glycerine, clove oil, ethylene glycol, glycerine + dimethylsulfoxide, clove oil + dimethylsulfoxside, ethylene glycol +dimethylsulfoxside) on carnation flowers at a cryo drying temperature of -80ºC for 12 hours duration. After that the flowers were freeze dried for 7 days. The results showed that untreated flowers remained naturalistic in appearance while the pre-treated ones had lower aesthetic value. Sohn et al. (2003) investigated the effect of freeze drying for 14 days on the shape and colour of Rosa hybrida (cvs. Tineke, Golden Gate, Saphir, Roulette, and Rote Rose) and revealed that colour remained similar to fresh flowers but shrinkage was noticed after freeze drying.
Bhattacharjee SK and De LC 2003. Dried flowers and plant parts. In: Advanced commercial floriculture. Avishkar Publishers, Jaipur, pp 162-173.
Brown J 1999. Freeze drying: American Society of Agricultural Engineers, personal communications, e-mail: email@example.com
Chen W, Gast KLB and Smithey S 2000. The effects of different freeze-drying processes on the moisture content, colour and physical strength of roses and carnations. Scientia Horticulture 84(3/4): 321-332.
Dubois P and Joyce D 1989. Drying cut flowers and foliage. Farm note no 10/89. Western Australian Department of Agriculture, 3p.
Liang Ling Yun, Cheng YuLai and Zhang BaiQing 2005. Study on the application of freeze drying and microwave drying to cut flowers. Transactions of the Chinese Society of Agricultural Machinery 36(1): 71-74.
Ruth 2000. Pollyrosestationers Freezedried flowers. Website: http://www.pollyrose.com
Sohn KwanHwa, Kwon HyeJin and Kim EuiYoung 2003. Effects of drying methods on shape and color of Rosa hybrida. Korean Journal of Horticultural Science and Technology 21(2): 136-140.
Trinklein D 2006. Drying flowers and foliage for arrangements. Website: http://extension. missouri.edu/publications/Display Pub. aspxP=G6540.
About Author / Additional Info:
I am currently pursuing Ph.D in Floriculture and Landscaping from Indian Agricultural Research Institute, New Delhi.