FREEZE DRYING, TEMPERATURE, AND NUTRITION LOSS

The freeze-dried foods are healthy similar to their fresh forms. Almost 97% of the nutrition is retained in freeze-dried food. Freeze-drying food items serve as the best method to preserve foods for longer durations while retaining their nutritional value. Freeze-drying provides the opportunity of an alternative method to obtain foods, especially fruit products in which a major amount of bioactive compounds that are usually sensitive to heat can be preserved.

The freeze-drying involves sublimation of frozen food substances at low pressure and the food materials are not exposed to very high temperatures. Heat is applied to shorten the process of sublimation. Still, the heat use does not impact the product quality. This not only preserves the flavor, and taste of the products but the nutrients having a biological activity that is sensitive to heat can be protected (1).

THE EFFECT OF SHELF TEMPERATURE ON THE NUTRITION LOSS

Various variables can transform the features of the product obtained from freeze-drying. Among them, shelf temperature is one such variable with the highest impact on product quality as it influences the processing time (2). When the shelf temperature is increased up to 40 ◦C, it decreases the drying time by more than fifty percent without having a major effect on the nutritional content (3). The secondary stage of drying speeds up due to higher temperatures because the water removal from the food material requires more energy (4,5). 

Usually, when it comes to determining the nutrient quality of food, vitamin C is taken as a reference because of its labile nature in comparison to other nutrients present in food (6). Therefore, if vitamin C is retained after the drying process then it indicates that other nutrients will be preserved as well.

Keeping this in view, the vitamin C content was determined in freeze-dried orange juice by a study at two different shelf temperatures, 30 and 50 °C for 48 or 18 hours. After freeze-drying, an increase in the content of vitamin C was observed. The change in the vitamin C content was bigger when the drying temperature was higher (7).

One more study determined the vitamin C content in orange puree at three shelf temperatures, i.e., 30, 40, and 50°C. This study also confirmed that vitamin C was preserved in the food when the shelf temperature was higher. The vitamin C content was found to be more in samples that were heated at 40, and 50°C but less in samples heated at 30°C (8).

THE SCIENCE BEHIND THE PRESERVATION OF NUTRIENT

The higher content of vitamin C at higher temperatures could be due to the inhibition of the enzyme known as ascorbate peroxidase. Vitamin C is utilized by this enzyme in its metabolic process as a cofactor and since this enzyme is blocked due to high heat, it cannot use vitamin C (9). Moreover, at higher temperatures, the length of the freeze-drying process is shorter. This way vitamin C is exposed to oxygen for less time and more vitamin C content is obtained in the freeze-dried product at higher temperatures (8).

THE TAKEAWAY

Shelf temperature plays an important role in nutrition loss during freeze-drying. The same food substance freeze-dried at different temperatures can yield different percentages of nutrients. Hence, it is very important to optimize the shelf temperature to get the maximum benefit from freeze-dried substances.

References

1. Serna-Cock L, Vargas-Munoz DP, Aponte AA. Structural, physical, functional and nutraceutical changes of freeze-dried fruit. Afr J Biotechnol. 2015 Feb 18;14(6):442–50.
2. Shishehgarha F, Makhlouf J, Ratti C. Freeze-Drying Characteristics of Strawberries. Dry Technol. 2002 Nov 1;20(1):131–45.
3. Egas-Astudillo L, Silva-Espinoza M, Uscanga M, Martínez-Navarrete N, Camacho M. Impact of shelf temperature on freeze-drying process and porosity development. In 2018.
4. Wu X fei, Zhang M, Bhandari B. A novel infrared freeze drying (IRFD) technology to lower the energy consumption and keep the quality of Cordyceps militaris. Innov Food Sci Emerg Technol. 2019 Jun 1;54:34–42.
5. Nireesha RG, Divya L, Sowmya C, Venkateshan N, Niranjan Babu M, Lavakumar V. Lyophilization/Freeze Drying - An Review. Int J Nov Trends Pharm Sci. 2013 Jul 10;3(4):87–98.
6. Moraga G, Martínez-Navarrete N, Chiralt A. Water sorption isotherms and phase transitions in kiwifruit. J Food Eng. 2006 Jan 1;72(2):147–56.
7. Uscanga MA, Salvador A, Camacho M del M, Martínez-Navarrete N. Impact of freeze-drying shelf temperature on the bioactive compounds, physical properties and sensory evaluation of a product based on orange juice. Int J Food Sci Technol. 2021;56(10):5409–16.
8. Silva-Espinoza MA, Ayed C, Foster T, Camacho M del M, Martínez-Navarrete N. The Impact of Freeze-Drying Conditions on the Physico-Chemical Properties and Bioactive Compounds of a Freeze-Dried Orange Puree. Foods. 2019 Dec 30;9(1):32.
9. Cuastumal H, Murillo B, ordoñez-santos L. Effects of heat treatment on the concentration of vitamin C and surface color in three tropicale fruits. Rev Lasallista Investig. 2016 Jun 1;13:85–93.