Abstract
Dates are subjected to postharvest losses in quality and quantity caused by water loss, fermentation, insect infestation, and microbial spoilage during storage. Cold storage is the main element in the postharvest quality management used for fruit preservation. Although cold storage is used for dates, precision control of the relative humidity (RH) using ultrasonic applications is not used thus far, or it is applied to other fruits on a small scale. Therefore, we designed and constructed an ultrasonic humidifier (DUH) for RH control in the cold storage room (CSR) of dates. The optimum air velocity of 3 m s−1 at the outlets of the DUH ducts produced a mist amount of 6.8 kg h−1 with an average droplet diameter of 4.26 ± 1.43 µm at the applied voltage of 48 V and frequency of 2600 kHz of the transducers. The experimental validation was carried out by comparing a CSR controlled with the DUH with two conventional CSRs. The three tested CSRs were similar in dimensions, cooling system, and amount of stored dates. The time required for cooling 800 kg of dates in the controlled CSR from 25 °C to the target temperature of 5 °C was approximately 48 h. The DUH precisely controlled the RH at the maximum RH set point of 80% in the tested CSR at 5 °C. The controlled RH at 80% has a positive impact on the physicochemical characteristics of the stored dates. It significantly reduced the weight loss of the fruits and preserved fruit mass, moisture content, water activity, firmness, and color parameters. However, no significant effect was observed on fruit dimensions, sphericity, and aspect ratio. The microbial loads of mesophilic aerobic bacteria, molds, and yeasts fell within the acceptable limits in all tested CSRs. Both stored date fruits and artificially infested dates showed no signs of insect activity in the controlled CSR at the temperature of 5 °C and RH of 80%. The DUH proved to be a promising technology for postharvest quality management for dates during cold storage.
Highlights
Surface drying of stored date fruits by the air temperature and relative humidity (RH) in CSRU1 and CSRU5 increased weight loss and led to color changes that were undesirable and resulted in a shorter shelf life of the fruits
Surface drying of stored date fruits by the air temperature and relative humidity (RH) in traditional cold storage rooms increases weight loss and leads to color changes that are undesirable and result in a short shelf life of the fruits
The validation of the designed humidifier was investigated based on the generated droplet size and its ability and precision in controlling the RH at the target value
Summary
Substantial food losses and waste occur during postharvest handling, processing, storage, distribution, and consumption [3]. The use of innovative technologies to preserve food will reduce postharvest losses that eventually lead to food insecurity [4]. According to the standards in [5], freedom from live insects, dead insects, eggs of insects, mites, and pathogenic microorganisms is one of the most important quality indices of dates [6]. Dates are subject to microbial infection during harvesting and handling processes through potential spoilage microorganisms such as bacteria, yeasts, and molds. Stored fruits under unsuitable storage conditions of temperature and humidity are accompanied by microbial spoilage [7,8]. The major microorganisms causing pathological disorders of dates through fermentation and decay include yeast of the genus
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