Physical Treatments

Cold Storage Management Refrigerated produce (the primary refrigerated produce are bananas, meat, citrus fruit, fish, and seasonal fruit) are widely stored and transported worldwide, and in the last years, the fruit trade from Mediterranean countries toward extra EU countries (especially Japan, United States, Canada) grew continuously, with increased volume of shipped foodstuff. Produce temperature is the most important factor affecting the quality of fruits and vegetables, which remain alive after harvest. The vegetable tissue metabolic activity requires environmental oxygen and produces carbon dioxide and heat. At any point of the cold chain, produce should be held at its lowest recommended storage temperature, and the rapid cooling after harvest (precooling) is essential to maintain the quality of fresh fruits and vegetables during postharvest handling and distribution (Opara and Zou 2007). In order to optimize the cool chain management and to promote rapid and uniform cooling of produce, several factors are fundamental: (1) the power of the cooling equipments should be adequate to the cooling time required, (2) air circulation around the produce should be adequate to the product type and the packaging material used, (3) accurate control of temperature and RH in the storage room, and (4) the refrigeration plant energy efficiency that dramatically affects the cost of storage. Furthermore, precooling to remove the “field heat” is an effective strategy to reduce the period of high initial respiration rate prior to storage and transportation. Authors (Di Renzo et al. 2011) carried out several empirical studies to optimize the precooling step on various citrus species. A pressure cooling plant,

Fig. 1 Heat transfer coefficient calculated during the trials for the “control” (room cooling system) and operating the fans at frequency of 35, 40, and 50 Hz. When operating the fan at 50 Hz frequency, a “h” (heat transfer coefficient) equals to

0.86 Wm-2 K-1 was calculated, five times greater than “h” calculated for the cooling room storage

equipped with a temperature control system, was used to study the cooling rate on Tarocco blood oranges.

In the pressure plant, the fan reduces the pressure level to 96–98 Pa in the space between the pallet rows, and the air velocity around the fruits is generally maintained around 0.8–1 m/s. Results showed that cooling rate is strictly dependent to both fan speed and pressure level. When operating the fan at 50 Hz frequency, a “h” (heat transfer coefficient) equal to 0.86 Wm-2 K-1 was calculated, five times greater than “h” calculated for the cooling room storage (Fig. 1).

In a recent research, a tunnel-type forced-air cooler was used to rapidly cool oranges, stacked in pallet, before simulating a container cold transport. For this purpose, a refrigerated container (reefer, 400 High Cube) was used, available at the Oranfrizer Company (Scordia, Catania, Italy). The refrigeration unit was located on the end wall of the container, defrosting operated automatically every 25 h, and each cycle held for about 30 min. Cold air flows around the fruits in the container through the gratings in the floor and then drawn off again below the container ceiling. The circulating fans force the air through the air cooler, which also acts as the evaporator in the cold circuit, and back through the gratings into the cargo. The container was loaded gradually, starting with the pallet to be placed in the end side of the container, and every time a pallet was loaded on the container and before the final placing, also temperature probes were placed into the fruits, making a little hole (about 0.3 cm diameter) in the fruits. Data showed a good result in terms of temperature homogeneity, which varied in a short range (±0.5 oC) depending on probe location inside the container; temperature reached a minimum level of about -0.8 oC during the early stage when cooling system starts. This minimum level can be considered too low for orange fruits, due to the risk of cold damage rising (about -1.0 oC).

Generally, also a 90–95 % relative humidity (RH) is needed to obtain the best shelf life of most fruits and vegetables, except for few species (bulb onions, garlic, winter squashes, ginger). Low RH around the produce causes wilting or shriveling, reducing marketability. When moist air contacts with a cold surface that is at a temperature below the dew point of water vapor in the air, condensation will occur, and frost begins to form if surface temperature is below the freezing temperature of water, so frost growth on heat exchangers placed in the cold room is a common problem for refrigeration systems, and it affects the thermal performance of heat exchangers in several ways (Chen et al. 2003).

Frost growth on heat exchanger surfaces increases the thermal resistance between the fins and the airflow (Na and Webb 2004) and decreases the cooling capacity of heat exchangers used in refrigeration systems and reduces the airflow through heat exchangers and increases the air pressure drop (Chen et al. 2003).

In a recent study (Altieri et al. 2007), authors evaluated the performance of a frosted finned tube heat exchanger for different cooling capacities and heat transfer mechanisms, with the aim to design a new defrosting system based on indirect measure of frost layer present on the heat exchanger surface.

Air temperature, brine temperature, air speed through the cooler fins, and the electric power absorbed by the fans were measured. Data collected during the trials showed that reducing the heat load inside the cold storage room, i.e., increasing the refrigeration power with respect to the same heat load, requires a more frequent defrosting operation and increases energy consumption.

Experimental trials showed that frost deposition is the fastest when the heat transfer mechanism is the free convection. Furthermore, fan power absorption seems to be an optimal marker to control frost on finned tube heat exchanger placed in high-humidity environments. The presence of the frost on the exchanger surface decreased air velocity through the air cooler and caused an increase of fan power absorption around 7 % from the beginning to the end of the test (Fig. 2). The direct correlation between frost thickness and absorbed power makes this parameter suitable to continuous control of defrosting process.

Modified and Controlled Atmosphere Storage A controlled atmosphere (CA) or modified atmosphere (MA) around the produce is created by alterations in the concentrations of the respiratory gases in the storage atmosphere; these alterations

Fig. 2 Frost on the exchanger surface decreased air velocity through the air cooler (a) and caused an increase of fan power absorption (b) around 7 % from the beginning to the end of the test

include elevation of carbon dioxide (CO2) level, reduction of oxygen (O2) tension, or both. Whereas the term CA storage generally implies precise control of O2 and CO2 concentrations in the atmosphere, the term MA storage is broader and may indicate any synthetic atmosphere, arising intentionally or unintentionally, in which the composition of its constituent gases cannot be closely controlled. Carbon dioxide is the only gas used inducing a significant level of antimicrobial activity and survival on the produce. These induced environmental conditions have a marked effect on product physiology, starting from altered primary metabolism and respiratory pathways, and involve changes in gene expression, protein accumulation, and metabolite concentrations (Kanellis et al. 2009). During long storage, application of low concentration of O2 is able to delay the decay on cherries, blueberries, raspberries, strawberries, figs, and pomegranates. While storage under CA (12 % O2 + 12 % CO2) for up to 8 weeks controlled significantly postharvest disease and maintained the quality of organic table grapes, storage under CO2 at high concentration (90 % or more) is used in order to quickly remove astringency from kaki (or Japanese persimmon), a high nutritional fruit very popular in the Mediterranean area. Di Renzo et al. (2013) carried out a simultaneous CO2/ethylene gas treatment of persimmon, as opposed to the common sequential application of ethylene following CO2. The influence of both treatments on the fruit quality was evaluated in terms of weight loss, color index, firmness, total soluble solids, tannin content, and juice titratable acidity, immediately after the treatment, after 7 and 21 days, to simulate the shelf life period (fruits were stored at 6 oC and 85–95 % RH). Results showed the efficacy of the simultaneous CO2/ethylene treatment that within 7 days after the treatment allows picking up immature persimmons to complete the simultaneous treatment within 24 h to have ready to eat fruits for market, optimizing the chain of such a short-season produce.

Modified atmosphere packaging is a nontoxic method for keeping quality and extending shelf life of fruits and vegetables (Kader et al. 1989), by reducing respiratory activity, delaying softening and ripening, and reducing the incidence of various physiological disorders and pathogenic infestations (Caleb et al. 2013) due mainly to the relatively low oxygen and high carbon dioxide levels inside the package.

Ultraviolet-C (UV-C) irradiation light is part of the electromagnetic spectrum, with wavelengths between 200 and 280 nm; due to its antimicrobial effect and low cost, this treatment is attractive to the food industry (Shim et al. 2012). The effectiveness of this treatment for microbial inactivation depends mainly on radiation dose and the structure and topography of the surface of the product.

Heat treatment has been recognized as a feasible postharvest treatment for fruits and vegetables with potential to delay ripening and decay, since it is easily applied, leaves no chemical residues, and can reduce the initial population of microorganisms. These effects include changes in tissue respiration, hormone production, particularly ethylene and enzyme activities, and other changes that impact on fruit and vegetable quality. Heat treatments include hot water dips, hot water brushing, and hot air treatments (vapor heat and forced air). The type of hightemperature treatment and its duration affects fruit or vegetable ripening or senescence as well as nutritional and quality attributes.

< Prev   CONTENTS   Next >