Written by: Martina Vavrusova Hedegaard, Ph.D. / Technologist / SD Freeze Drying
- Pumpkins, in general, are vegetables native to South America but nowadays they are grown in Europe, Australia, New Zealand, India, but also in Southeast Asia including Thailand, Indonesia, Malaysia and other countries.
It has been reported that pumpkin production has increased annually by 2.2 % and from 1992 to 2012 pumpkin production increased to 25 million tonnes.
- Pumpkins belong to the family Cucurbitaceae and the genus (family) Cucurbita. There are five domesticated species of which the three species Cucurbita pepo L., Cucurbita maxima Duchesne, and Cucurbita moschata Duchesne, represent the most economically important species cultivated worldwide.
- Generally, pumpkins are quite stable after harvesting (1 – 3 months) but after peeling they are more susceptible to microbial spoilage, moisture loss, colour changes and softening.
- Because of the large size of pumpkins and difficulties in removing their strong peels, there is a high demand for suitable processing techniques, which results in a final product with valuable nutritional qualities. Therefore, drying and powdering are suitable techniques for prolonging shelf life and maintaining the nutritional value of pumpkins.
- The pumpkin powder is an alternative to the fresh pumpkin with many nutritional benefits. It has storage and transportation advantages over fresh pumpkins due to its longer shelf life.
- The pumpkin powder is used as a supplement to cereal flour in bakery products like bread, cakes, and cookies. It is also used in soups, spices, and sauces. Pumpkin powder has a highly desirable and pleasant flavour, good sweetness, and deep yellow-orange colour.
- Therefore, in order to prolong the shelf life of pumpkins for retail markets and in order to best maintain nutritional qualities, freeze-drying is the most appropriate technique.
- Pumpkin flesh is an excellent source of β-carotene. β-carotene is a red/orange natural pigment (carotenoid) found in plants and fruits. It is known as a precursor of vitamin A, which the human body converts into vitamin A (healthy skin and mucous membranes, good eye health and vision). β-carotene has been studied as an antioxidant for its ability to inhibit the oxidation of other molecules known as radicals, which damage cells and eventually lead to cancer and other chronic diseases. Carotenoids in general, including β-carotene, can promote health benefits when taken at dietary levels.
- Pumpkin is a good source of B-vitamins (water-soluble) such as B6 (pyridoxine), B1 (thiamine), B2 (riboflavin), B9 (folic acid), vitamin K, vitamin C, and minerals such as potassium, phosphorus, magnesium, iron and selenium.
- Furthermore, pumpkin contains other biologically active compounds such as polysaccharides, phenolic compounds, terpenoids and sterols. Pumpkin is generally a very good source of fibre, and some studies also indicate that a pumpkin-rich diet may reduce blood glucose levels.
Freeze drying of the pumpkin powder
- Freeze drying is the most effective way of protecting nutrients during processing. Other techniques like spray-drying or higher temperature drying often result in complications related to the stickiness of the vegetable juice which needs to be reduced by adding additional carriers such as maltodextrin (increase in viscosity).
Furthermore, techniques using high temperatures compromise important qualities. Some examples:
Colour – Heat and oxidation during drying at higher temperatures result in pigment losses in carotenoids. As also shown in the study by Mujaffar et al. (2015), where five different pumpkin cultivars were studied, freeze-dried powders were superior in colour when compared to samples that were hot air-dried.
Nutrients – Traditional drying methods can lead to excessive loss of vitamin C and β- carotene degradation.
Texture – Elevated drying temperatures result in protein denaturation, which may decrease the solubility of the final powder. The study by Mujaffar et al. (2015) also confirms that freeze-drying of pumpkin purees resulted in a fine powder with noticeably improved colour and less granular texture than powders obtained by hot air drying.
Flavour – heat treatment often removes volatile flavour compounds and introduces additional flavour compounds (off flavours) through the autooxidation of unsaturated fatty acids, thermal decomposition, and Maillard reactions.
- It is extremely important to realize that freeze-drying is the best method for drying higher quality foods and that a higher selling price of these nutritive valuable products compensates for the higher cost of the drying process.
- Since freeze-drying uses low temperatures, it is generally recommended for drying foods containing heat-sensitive compounds, especially antioxidants such as carotenoids, phenolic compounds, ascorbic acid, and tocopherols.
- Generally, freeze-drying gives pumpkin powder a looser texture and a colour more like that of fresh fruit.
Example of the freeze-drying of pumpkin puree
The following example is a study by Dirim & Gülışkan (2012), which focuses on the effect of the freeze-drying process on the production of pumpkin (Cucurbita moschata) puree powder as a functional ingredient used for bread, cakes, soups and pasta products for improving their nutritional values and flavour, and the powder properties.
- Freeze drying (shortly) – The pumpkins were peeled, deseeded and ground in a blender to produce puree.
The pumpkin puree was frozen in a 3 mm layer in Petri dishes at -40 °C for two hours.
The samples were freeze-dried under vacuum (13.33 Pa absolute pressure), at -48 °C condenser temperature for 9 hours.
The temperature of the heating plate was set to +30 °C to accelerate the sublimation process (constant during the drying process) while avoiding melting the product.
- Results – Moisture content and water activity values of the pumpkin powders were within acceptable limits for safe storage of the final product.
The bulk density of the freeze-dried pumpkin powder was 0.113 g/ml.
The freeze-dried powder was reconstituted instantly with water at room temperature.
- Conclusion – This study confirms that freeze-drying of pumpkin puree results in the final powder product, which could be reconstituted within a few seconds (16 s) and with only slight physicochemical changes compared to fresh pumpkin, see Table 1.
Therefore, the final powder can be used as a valuable food ingredient for the purposes of flavouring and improving the nutritional content of foods.
Table 1. Results of the analysis were applied to fresh pumpkin puree and freeze-dried pumpkin puree powders. Adapted from Dirim & Gülışkan (2012)
- Dirim, S. N. & Gülışkan, G. (2012). Determination of the effect of freeze drying process on the production of pumpkin (Cucurbita moschata) puree powder and the powder properties. GIDA, 37, 1-8.
- Mujaffar, S., Isaac, W., Mohammed, M. (2015). Preliminary investigations into the production of freeze dried pumpkin powders. Conference paper, 1-11.
- Roongruangsri, W. & Brolund, J. E. (2015). A review of drying processes in the production of pumpkin powder. International Journal of Food Engineering – De Gruyter, 1-11.
- Henriques, F., Guiné, R. P. F., Barroca, M. J. (2012). Influence of drying treatment on physical properties of pumpkin. Croatian Journal of Food Technology, Biotechnology and Nutrition, 7, 53-58.
- Paiva, S.A. & Russel, R.M. (1999). Beta-carotene and other carotenoids as an antioxidant. Journal of the American College of Nutrition, 5, 426-433.