Influence of mist-chilling on post-harvest quality of fresh strawberries Cv. Mara des Bois and Gariguette

The aim of this study was to assess the impact of mist-chilling on high-grade strawberry post-harvest quality (Cultivars “Gariguette” and “Mara des Bois”). Strawberries were chilled at 2 °C using three processes: air blast chilling at 0.3 m s⁻¹ or 1 m s⁻¹ and mist-chilling at 1 m s⁻¹. After chilling, fruits were submitted to different distribution chains characterised by different handling conditions and storage temperatures (2 °C or 7 °C) and by a 12 h retailing step at 20 °C. Strawberry quality was assessed by measuring 7 parameters: weight loss, commercial loss, firmness, sugar content, acidity, colour and sensory quality. Compared to air-chilling, mist-chilling did not reduce chilling time but it reduced weight loss by 20–40%. Mist-chilling had no detrimental effect on commercial loss defined as the percentage of fruit more than 1/3 of surface affected. It did not induce any major changes on strawberry quality. Temperature fluctuations undergone during cold storage and retailing had a detrimental effect on weight loss. The beneficial effect of packaging on weight loss was confirmed.     

Thawing of frozen strawberries

Frozen strawberries were thawed under different controlled conditions (natural thawing at room temperature, thawing in circulating air, thawing in a refrigerator, thawing in water and thawing in a convection oven). The temperature rise at the geometric centre of the strawberries was monitored until the temperature reached 6°C and thawing diagrams were drawn. The effects of thawing method on the weight loss in strawberries were determined. Strawberries thawed at higher temperatures showed greater weight loss. During thawing in circulating air, thawing time decreased with increasing air velocity.     

Interactions among cooling, fungicide and postharvest ripening temperature on peaches

Peach fruit (Prunus persica L. cv. ‘Miraflores’) harvested at the firm-ripe stage, treated or not with 2 g l⁻¹ iprodione, were cooled or not at 1°C and ripened at 15 or 20°C and 95% RH for 10 days. During ripening, weight loss, fungal development and changes in quality parameters (firmness, soluble solids content, titratable acidity, pH and ground and flesh color), and carbon dioxide and ethylene production were monitored. Cooling alone or combined with iprodione avoided Rhizopus nigricans decay during ripening at either ripening temperatures. A skin damage not previously reported on fungicide treated peach was observed at 20°C. Cooled fruit ripened at 15°C showed an anomalous respiration rate and ethylene production after the climacteric peak, a loss of firmness and a drop in titratable acidity after 7 days of storage, and reduced endo-polygalacturonase activity in presence of continuous pectinmethylesterase activity during the first week. Cooling before ripening at 20°C led to the best flavor without excessive total losses. These results helped in the optimization of warming cycles during cold storage used to avoid chilling injuries development on peaches.     

The temperature performance of domestic refrigerators

Chilled foods are stored for periods of between a few hours and many days in domestic refrigerators. However, there are little published data on the temperature performance of domestic refrigerators within the home. A survey has been taken in 252 households in the UK and some of the results are presented in this paper. The refrigerators investigated in the survey were found to have an overall mean temperature of approximately 6°C, which ranged from 11.4 to −0.9°C. Temperature ranges over the whole refrigerator varied from 4.5 to 30.5°C with 3.7% of the total being warmer than 20°C. On average 29.9% of refrigerators operated below 5°C and 66.7% operated below 7°C. Few refrigerators (7.3%) ran, on average, above 9°C. No refrigerator characteristic (apart from type) could be related to temperatures or temperature distribution in the refrigerators investigated.     

Theoretical and experimental freezing times of strawberries

Strawberries were frozen at different air velocities in an air blast freezer at −30°C. The freezing time was taken as the time required to lower the temperature at the geometric centre of the samples to −10°C. The freezing times measured in the experiments were compared with the values calculated using Plank's equation. The freezing times calculated by Plank's equation were fround to be higher than those found experimentally at any given air velocity. Freezing time decreased with increasing air velocity. This is attributed to the increase in heat transfer coefficient at increased air velocities.     

The primary chilling of poultry carcasses—a review

This paper reviews the published scientific studies that have been carried out on the chilling of poultry carcasses. The prime purpose of chilling is to limit the growth of both pathogenic and food spoilage microorganisms. There are a wide range of publications that show that, in general, the numbers of both types of microorganism, on the surface of poultry carcasses, is reduced during the chilling process. Immersion or spin chilling is not used in the production of ‘fresh’ chilled poultry in Europe, ‘dry’ air chilling being the preferred chilling method. Many people believe that there is some clear microbiologically based reason behind the selection of air chilling. However, the published data do not appear to support this belief, and if anything point to a microbial advantage of immersion systems. The rate of chilling has some influence on the taste, texture and appearance of poultry meat. Very rapid chilling can result in tougher chicken meat, while very slow chilling can produce pale soft exudative (PSE) muscle. However, in both cases the effect is not as marked as with red meat. No comprehensive published data has been found on the effect of a range of chilling systems, chilling conditions, carcass weight and shape on the rate of chilling, weight loss and heat loss. Without such data it is impossible to optimise the design of a poultry chilling system.     

Chilling injuries in peaches during conventional and intermittent warming storage

Firm-breaker and firm-mature peaches (Prunus persica L. Batsch cv. ‘Paraguayo’) were conventionally stored for 4 weeks at 2°C and 90–95% relative humidity or subjected to intermittent warming cycles of 1 day at 20°C every 6 days of storage at 2°C. Warming periods induced ripening (reduced flesh firmness, extractable juice and titratable acidity), while during continuous storage abnormal values of these parameters were found. After 2 weeks at 2°C and particularly after the subsequent 3 days at 20°C, woolliness and to a lesser extent, vitrescence and dryness of the cortical tissue were detected. Severe levels of these disorders were found more frequently in firm-breaker than in firm-mature fruits, which mainly developed vitrescence. Three cycles of intermittent warming prevented chilling injuries but increased weight loss and senescence symptoms. Compared with conventional storage, intermittent warming increased the shelf-life of firm-mature and firm-breaker peaches by 1 and 2 weeks, respectively. Some considerations on the commercial application of the intermittent warming technique are included.     

A theoretical study of a novel regenerative ejector refrigeration cycle

There has been a demand for developments of the ejector refrigeration systems using low grade thermal energy, such as solar energy and waste heat. In this paper, a novel regenerative ejector refrigeration cycle was described, which uses an auxiliary jet pump and a conventional regenerator to enhance the performance of the novel cycle. The theoretical analysis on the performance characteristics was carried out for the novel cycle with the refrigerant R141b. Compared with the conventional cycle, the simulation results show that the coefficient of performance (COP) of the novel cycle increases, respectively, by from 9.3 to 12.1% when generating temperature is in a range of 80–160 °C, the condensing temperature is in a range of 35–45 °C and the evaporating temperature is fixed at 10 °C. Especially due to the enhanced regeneration with increasing the pump outlet pressure, the improvement of COP of the novel cycle is approached to 17.8% compared with that in the conventional cycle under the operating condition that generating temperature is 100 °C, condensing temperature is 40 °C and evaporating temperature is 10 °C. Therefore, the characteristics of the novel cycle performance show its promise in using low grade thermal energy for the ejector refrigeration system.     

Analyse expérimentale et modélisation des cinétiques de température et de perte de poids de carcasses de lapin pendant la réfrigérationExperimental analysis and modelling of temperature and weight loss kinetics of rabbit carcasses during chilling

This study deals with the chilling of rabbit carcasses with constant or time-variable air properties, this latter situation being closer to industrial chilling conditions. The experiments have been carried out with carcasses with weights in the range of 0.8–2.0 kg (1) in constant chilling conditions, 55 trials using six carcasses each, covering the following range: air velocity 0.3–2 m s⁻¹, air temperature −3 to 10°C, relative humidity 70–90%, and (2) nine trials, each using two carcasses, have been performed with time-variable air velocity, between 0.5 and 2.0 m s⁻¹, or time-variable air temperature, between −5 and +5°C. The analysis of the kinetics was based on the slopes of the curves of the dimensionless temperature logarithm versus time. The influence of the carcass weight and of the air properties was assessed. A hidden variability of the cooling conditions was evidenced and explained by water evaporation. A model, based on analytical solutions relative to an infinite cylinder, was adapted to take into account the variation of the chilling conditions. A shape factor and an equivalent surface area have been fitted from the experiments performed with constant chilling condition. The calculated results were compared with the measurements for both constant and time-variable chilling conditions. The average difference between calculated and measured temperature kinetics was in any case lower than 1°C. The weight loss, calculated as a percentage of the initial weight, was predicted with an accuracy of 0.1%. The relative errors in chilling times were lower than 9%.     

Passive thermal energy storage in refrigerated warehouses

This paper investigates operational strategies that use stored products as thermal mass to shift refrigeration loads to more favorable operational periods (low energy cost periods, lower outdoor air conditions, etc.) allowing an opportunity to reduce system operating costs. An integrated model of the stored product, warehouse air, and warehouse structure is developed and thermal response characteristics are predicted for a selected warehousing facility. Simulated results are validated with experimental measurements. Food quality impacts associated with the temperature cycling caused by potential operating strategies are discussed. Results from this investigation indicated that a full load-shifting control strategy would save $82,000 (US) ($0.40/ft²/year or $4.28/m²/year) annually over the test facility's current operational strategy, representing 53% of the total cooling cost. Predicted maximum warehouse temperature variation is 5.6°C, which is not expected to cause significant product quality changes in the temperature range (below−18°C) studied.     

CO2-heat pump water heater: characteristics, system design and experimental results

CO₂ is one of the few non-toxic and non-flammable working fluids that do not contribute to ozone depletion or global warming, if leaked to the atmosphere. Tap water heating is one promising application for a trans-critical CO₂ process. The temperature glide at heat rejection contributes to a very good temperature adaptation when heating tap water, which inherits a large temperature glide. This, together with efficient compression and good heat transfer characteristics of CO₂, makes it possible to design very efficient systems. A heating-COP of 4.3 is achieved for the prototype when heating tap water from 9°C to 60°C, at an evaporation temperature of 0°C. The results lead to a seasonal performance factor of about 4 for an Oslo climate, using ambient air as heat source. Thus, the primary energy consumption can be reduced with more than 75% compared with electrical or gas fired systems. Another significant advantage of this system, compared with conventional heat pump water heaters, is that hot water with temperatures up to 90°C can be produced without operational difficulties.     

Comparative experiments on three-wall and one conventional road vehicle

In 1984 international research took place which compared the transport conditions of quick-frozen foods in thin-wall vehicles and conventional vehicles. This work was done at the request of the group of experts of the UNECE/TRANS/GE.11 in Geneva, Switzerland. In the Netherlands measurements have been carried out on three road vehicles — one conventional and two thin-wall vehicles — in the test station and on the road. The road transport involved taking a commercial cargo of frozen fish from Holland to Italy during the summer (July). Later on an additional test was performed in the test station with a fourth thin-wall vehicle at an ambient temperature of 30°C. The results show, that in the conventional vehicle, equipped with a bulk head and a ceiling air duct, and working on a temperature setting of −25°C, a maximum product temperature of −18°C could be maintained. The maximum temperature difference in the cargo was 6 K. Thin-wall vehicles appeared to have a temperature difference of 12 K between air inlet temperature and warmest product temperature. The maximum product temperature could be held below −12, −15 or −20°C, depending on the air distribution and, in particular, on the cooling power of the thermal appliance.     

Waste heat driven dual-mode, multi-stage, multi-bed regenerative adsorption system

Over the past few decades there have been considerable efforts to use adsorption (solid/vapor) for cooling and heat pump applications, but intensified efforts were initiated only since the imposition of international restrictions on the production and utilization of CFCs and HCFCs. In this paper, a dual-mode silica gel–water adsorption chiller design is outlined along with the performance evaluation of the innovative chiller. This adsorption chiller utilizes effectively low-temperature solar or waste heat sources of temperature between 40 and 95 °C. Two operation modes are possible for the advanced chiller. The first operation mode will be to work as a highly efficient conventional chiller where the driving source temperature is between 60 and 95 °C. The second operation mode will be to work as an advanced three-stage adsorption chiller where the available driving source temperature is very low (between 40 and 60 °C). With this very low driving source temperature in combination with a coolant at 30 °C, no other cycle except an advanced adsorption cycle with staged regeneration will be operational. The drawback of this operational mode is its poor efficiency in terms of cooling capacity and COP. Simulation results show that the optimum COP values are obtained at driving source temperatures between 50 and 55 °C in three-stage mode, and between 80 and 85 °C in single-stage, multi-bed mode.     

Experimental performance analysis and modelling of liquid desiccant cooling systems for air conditioning in residential buildings

The paper presents a new desiccant cooling cycle to be integrated in residential mechanical ventilation systems. The process shifts the air treatment completely to the return air side, so that the supply air can be cooled by a heat exchanger. Purely sensible cooling is an essential requirement for residential buildings with no maintenance guarantee for supply air humidifiers. As the cooling power is generated on the exhaust air side, the dehumidification process needs to be highly efficient to provide low supply air temperatures. Solid rotating desiccant wheels have been experimentally compared with liquid sorption systems using contact matrix absorbers and cross flow heat exchangers. The best dehumidification performance at no temperature increase was obtained in an evaporatively cooled heat exchanger with sprayed lithium chloride solution. Up to 7 g kg⁻¹ dehumidification could be reached in an isothermal process, although the surface wetting of the first prototype was low. The process then provides inlet air conditions below 20 °C for the summer design conditions of 32 °C, 40% relative humidity. With air volume flow rates of 200 m³ h⁻¹ the system can provide 886 W of cooling power.A theoretical model for both the contact absorber and the cross flow system has been developed and validated against experimental data for a wide range of operating conditions. A simulation study identified the optimisation potential of the system, if for example the surface wetting of the liquid desiccant can be improved.     

Simulation d'une machine frigorifique à absorption fonctionnant avec des mélanges d'alcanes

We propose in this article an absorption chiller operating with binary alkane mixtures as an alternative to compression machines. It is an installation using low-level energy at a temperature below 150 °C (waste heat or solar energy) and operating with environmentally friendly fluids. Ten mixtures are considered and compared with two cooling mediums of the condenser and the absorber: the ambient air at 35 °C and the water at 25 °C. For an air-cooled chiller, the COP reaches 0.37 for the n-butane/octane system. This value remains 27% lower than that of an ammonia/water installation operating under the same conditions. For a water-cooling chiller, the n-butane/octane and propane/octane systems give a COP of about 0.63, which is comparable to that of the ammonia/water system. When n-butane is used as refrigerant, the machine works at a pressure under 5 bars, which is an advantage compared with machines working with ammonia/water mixtures.     

Experimental investigation of silica gel–water adsorption chillers with and without a passive heat recovery scheme

We experimentally show that for the same heat exchanger inventory allocation, a four-bed adsorption chiller delivers a 12% higher ultimate cooling capacity than its two-bed counterpart. In addition it delivers a significantly improved quality of instantaneous cooling than a two-bed chiller at the same cooling capacity. The COP-enhancing feature of a passive heat recovery scheme that does not involve additional pumping action or valves is experimentally proven. It improves the COPs of a two-bed chiller and a four-bed chiller by as much as 38 and 25%, respectively, without any effect on their cooling capacities. The highest COPs achieved with a two-bed and four-bed chillers are 0.46±0.02 and 0.45±0.02, respectively. These are measured at a hot-water inlet temperature of 85 °C, cooling-water inlet temperature of 29.4 °C and chilled-water inlet temperature of 12.2 °C.     

Freezing time and rate of cauliflower florets under different freezing conditions

In this study, the freezing time and rate of 1 cm³ cauliflower floret samples were determined under different freezing conditions in an air blast freezer. Four different air temperatures (−20, −25, −30 and −35°C) and six different air velocities (70, 131, 189, 244, 280 and 293 m min⁻¹) were applied in the freezer, and the freezing rate and time of cauliflower pieces were determined under each condition. The freezing time of cauliflower samples frozen with cold air at −20°C and 280 m min⁻¹ was similar to that of samples frozen with cold air at −35°C and 70 m min⁻¹. When the velocity of air was increased from 70 m min⁻¹ to 293 m min⁻¹, the freezing time was approximately halved.     

Application of thermal battery in the ice storage air-conditioning system as a subcooler

This article experimentally investigates the thermal performance of a thermal battery used in the ice storage air-conditioning system as a subcooler. The thermal battery utilizes the superior heat transfer characteristics of two-phase closed thermosyphon and eliminates the drawbacks found in convectional energy storage systems. Experimental investigations are first conducted to study the thermal behavior of thermal battery under different charge temperatures (−5 °C to −9 °C) in which water is used as the energy storage material. This study also examines the thermal performance of the subcooled ice storage air conditioner under different cooling loads. Experimental data of temperature variation of water, ice fraction, refrigerant mass flow rate and coefficient of performance (COP) are obtained. The results show that supercooling phenomenon appears in the water and it can be ended when the charge temperature is lower than −6 °C. The system gives 28% more cooling capacity and 8% higher COP by the contribution of the thermal battery used as a subcooler.     

Supersonic two-phase flow of CO2 through converging–diverging nozzles for the ejector refrigeration cycle

CO₂ is environmentally friendly, safe and more suitable to ejector refrigeration cycle than to vapor compression cycle. Supersonic two-phase flow of CO₂ in the diverging sections of rectangular converging–diverging nozzles was investigated. The divergence angles with significant variation of decompression were 0.076°, 0.153°, 0.306° and 0.612°. This paper presents experimental decompression phenomena which can be used in designing nozzles and an assessment of Isentropic Homogeneous Equilibrium (IHE). Inlet conditions around 6–9 MPa, 20–37 °C were used to resemble ejector nozzles of coolers and heat pumps. For inlet temperature around 37 °C, throat decompression boiling from the saturated liquid line, supersonic decompression and IHE solution were obtained for the two large divergence angles. For divergence angles larger than 0.306°, decompression curves for inlet temperature above 35 °C approached IHE curves. For divergence angles smaller than 0.306° or for nozzles with inlet temperature below 35 °C, IHE had no solution.     

Description and experimental results of a semi-indirect ceramic evaporative cooler

Evaporative cooling is used in industrial and air conditioning processes to reduce temperature in different fluids. Direct evaporation systems can lead to environmental problems such as Legionnaire's disease, and indirect systems reduce system efficiency.This work presents the manufacture, test bed set up and trials carried out on a ceramic evaporative cooling system which acts as a semi-indirect cooler. Depending on air characteristics, it may act as a sensible or enthalpic exchanger. The water cooled in a cooling tower, using the return air coming from the conditioned room (22 °C and 50% comfort conditions) goes through the ceramic pipes, exchanging sensible and latent heat with a current of outdoor air.The use of this recovery system is mainly in climates with a high temperature and humidity such as tropical environments where the system yields a decrease in supply air humidity, using the cooling power of return air.The tests presented show the system behaviour for various supply air conditions.