Infrared-assisted freeze-drying (IRFD): effect of the infrared radiation wavelength section on energy consumption and quality of freeze-dried fruits

Abstract

Infrared radiation-assisted freeze-drying (IRFD) is a promising method for time optimizing. The present thesis aimed to determine which section of the infrared radiation (IR) wavelength is ideal for drying fruits. For this, infrared lamp was attached to the top of the drying chamber. The temperature and weight reduction were monitoring during the time using thermocouples and a scale, respectively, both coupled to the data acquisition system. The research was divided into two papers, in article 1 the drying and final quality of the açaí pulp powder were evaluated. In this article, the treatments for pulp drying were freeze-drying control treatment, FD; freeze-drying and continuous near infrared radiation heating, NIRFD; freeze-drying and near infrared radiation heating after 20% weight reduction, NIRFD20%WR; freeze-drying and continuous far infrared radiation heating, FARFD; freeze-drying and far infrared radiation heating after 20% weight reduction, FARFD20%WR. IRFD significantly reduced the total drying time, with NIRFD and FARFD taking 49.42% and 33.40% less time than FD, respectively. The structure of powdered using IRFD was more open and with larger pores. FARFD20%WR had collapsed cells that compromised the functional properties of the powder (wettability, dispersibility). As for bioactive compounds, phenolic content was not affected by IRFD, ranging from 963.44 to 1025.60 mg/100 g. Antioxidant capacity increased (59.91 to 88.06%) when far infrared was used due to a Maillard reaction that generated antioxidant compounds and darker product. Content of anthocyanins reduced in samples dried by far infrared. In article 2, the experiment consisted of drying whole blackberry (Rubus spp.) using FD treatments; NIRFD; FARFD, and incorporation of IRFD after 40% in weight reduction: NIRFD40% WR and FARFD40% WR. The NIRFD treatment was also the fastest due to the penetration capacity of this wavelength section being higher, with a 42.51% reduction in drying time (17.2 h), followed by FARFD with 36.5% (18.9 h) compared to FD (29.9h). Blackberries had good final appearance and characteristic red color, p>0.05, L* 17.5, a* 17.6, b* 11.3 and ho 32.9. Samples obtained using long-wave had higher hardness (10.13 N) compared to FD (6.98 N). This result influenced negatively rehydration ratio. Also, these treatments reduced the anthocyanins content. On the other hand, NIRFD samples had the highest values for RR throughout the entire time evaluated. Higher antioxidant capacities (AC) were obtained in treatments with shorter drying times, with NIRFD and FARFD having AC values of 77.91 and 77.37%, respectively. Finally, both articles showed that IRFD drying exhibited good potential for the production of powdered or whole dried fruits when using the short wavelength, NIRFD, presenting products with good appearance, high nutritional content and maintenance of technological properties.