Storage technologies

A well-chosen storage technology helps maintain the appropriate quality of fruit and vegetables and also prevents the development of pathogens. The primary purpose of storage is to slow the ageing process caused by respiration and moisture loss for as long as possible after harvest. By delaying the related physiological processes, the shelf life and consumer attractiveness of fruit and vegetables are extended. Extending the period of suitability for consumption also has a direct impact on the profitability of horticultural production.

The following fruit and vegetable storage technologies are currently most commonly used:

• ordinary cold stores (NA);
• controlled atmosphere (CA) stores;
• ULO system (Ultra Low Oxygen) – controlled atmosphere stores with reduced oxygen content.

ORDINARY COLD STORES

In this technology temperature is the most important environmental factor, because it has the greatest influence on the rate of biochemical processes occurring in stored fruit and vegetables. Lowering the temperature by 10 K can reduce respiration intensity by as much as a factor of 2–3. The composition of air in such a cold store is: 21% O₂, 78% N₂ and practically 0% CO₂ (table).

SYSTEMATICS

This system began to be used more than 80 years ago in the United Kingdom and the United States for storing apples and pears. In the 1930s the first studies on the use of this method in Poland were carried out, mainly in vegetable storage. In the early period of its implementation the gas composition of the atmosphere was obtained by using the natural respiration process of the stored fruit or pure nitrogen, and in the 1960s oxygen absorbers were applied that rapidly absorbed excess oxygen present in the gas-tight chamber after it was closed. A new stage in the development of CA began after the introduction of controlled atmosphere generators. Membrane separators and molecular sieve gas separation devices began to be used for rapid reduction of oxygen concentration in the gas-tight chamber. For this type of chamber the technology provides for lowering temperature, reducing O₂ content and increasing CO₂ in the atmosphere. Two methods of generating the atmosphere are used: spontaneous, by ventilation with atmospheric air (lengthy and rarely used), and with the use of equipment for reducing CO₂ content.

ULO SYSTEM

In later years intensive research was conducted on the use of ultra-low oxygen concentration (ULO) in the storage of various species of fruit and vegetables. Application of a controlled atmosphere with both O₂ and CO₂ concentrations below 2% made it possible to achieve better storage results compared with a standard gas atmosphere composition. The main benefit of implementing this system, relative to technologies based only on lowering product temperature, is extension of the storage period while maintaining high commercial quality of the products. This effect is achieved by reducing the rate of biochemical and physiological processes leading to ripening and over-ripening of products and by lowering sensitivity to ethylene. ULO system conditions do not favour the development of pathogens. Rules for operating this type of facility must be strictly observed, because very low O₂ and CO₂ content creates a danger to the life of people servicing them (Photo 1). Photo 1. CO2 and O2 parameters in modern facilities are monitored automatically.

TECHNICAL REQUIREMENTS

CA technologies require the use of gas-tight cold storage chambers in which uncontrolled gas exchange between the chamber interior and the external atmosphere is impossible. All chamber elements must pass a gas-tightness test; in addition to walls, ceiling and floor (Photo 2a) this also applies to door and inspection window frames (Photo 2b) and to all structural elements of the chamber passing through partitions, including pipes, cables, sections, hangers, heat exchangers, etc.

The capacity of the installed refrigeration equipment should ensure that the required temperature inside the chamber is reached within 24 hours. Chambers should be equipped with under- and over-pressure safety valves and compensation bags to compensate for small pressure changes (Photo 3). The ventilation system must be designed so as to prevent the inflow of fresh air from outside (this is only permissible during the initial cooling period). Energy considerations require the use of high-class thermal and vapour barrier insulation.

MEASURING INSTRUMENTS

Storage parameters require continuous control and regulation of atmosphere composition, temperature and humidity in the chambers. Strict observance of the calibration schedule for sensors and measuring instruments is essential. Correct location of measuring sensors in the chambers is also important. The following equipment is recommended in this respect:

• CO₂ content measured by the volumetric method or by measuring the degree of absorption of infrared radiation by CO₂, measurement accuracy approx. 5% of the measured value;
• O₂ content measured by the paramagnetic or electrochemical method, measurement accuracy 0.1%;
• temperature measured using resistance temperature sensors (e.g. Pt1000); at least three temperature sensors are required: after the cooler (supply air), before the cooler (return air), among the stored products (in the centre of the chamber about 1 m above the floor), measurement accuracy 0.1 K;
• relative humidity measured by the hygroscopic or condensation method using humidity sensors, measurement accuracy approx. 3%.

Measurement of the amount of water condensed on the coolers is also recommended. This is done using automatic condensate quantity counters or tanks with level gauges, as well as the use of air humidifiers.

ATMOSPHERE CONTROL

Equipment used for control:

• CO₂: carbon scrubbers (Photo 4), molecular sieve scrubbers, mixer absorbers, membrane devices;
• O₂: gaseous or liquid nitrogen, catalytic converters, atmosphere generators,
• nitrogen generators (Photo 5), oxygen absorbers; ethylene: catalytic ethylene burners, ethylene absorbers.

Modern single-stage scrubbers are characterised by a short time to reach set parameters in the range down to 1% CO₂ and O₂ concentration, high durability of the activated carbon bed, microprocessor control and self-diagnostics capability. The new generation of two-stage CO₂ scrubbers, based on ILOS-Plus technology, enables operation at extremely low CO₂ and O₂ values (concentrations in the range of 0.4% CO₂ and O₂ are achieved). Contemporary nitrogen generators are designed to produce high-purity nitrogen (98%). They are characterised by high nitrogen generation efficiency with low electrical energy consumption. Catalytic ethylene burners (operating at about 240 °C), in addition to their primary function of destroying ethylene, help remove microbiological hazards. In the air that flows through them, spores of pathogenic fungi and other pathogens not resistant to such high temperature are destroyed.

INNOVATIVELY

As a result of strong market demand for extending product storage periods while maintaining high quality, a number of modern storage technologies have appeared in recent years. Among the most widespread is Dynamic Controlled Atmosphere (DCA) technology. This is a low-oxygen technology consisting in creating atmosphere conditions in the facility with an oxygen concentration below 1%. Based on chlorophyll fluorescence measurement, the dynamically determined minimum safe oxygen concentration is established, i.e. one that during storage allows quality to be preserved and maintained at a high level and limits the occurrence of external and internal product damage. The ILOS-Plus system also belongs to low-oxygen storage systems. The main parameter indicating the quality of stored products is the ethanol concentration in the flesh of the stored fruit. Dynamic Control System (DCS) is another example of innovation in storage. In this system ethanol concentration is examined on the basis of a sample of storage atmosphere taken from a special container (Photo 6). Further innovative technologies are Advanced Control of Respiration (ACR) and Dynamic Fruit Respiration (DFR) systems. They enable control of the conditions under which fruit is stored on the basis of the respiratory quotient – RQ. This coefficient is the ratio of the rate of CO₂ production to the rate of O₂ consumed in the respiration process.

CHEMISTRY IN STORAGE

In addition to low-oxygen methods of storing vegetables and fruit, chemical preparations based on 1-methylcyclopropene (MCP) are used in storage. Molecules of this compound attach to ethylene receptors in cell membranes, thereby inhibiting its synthesis. The storage method consists in pre-cooling the fruit and carrying out an appropriate procedure for applying the preparation in a tight storage chamber. The preparation is placed in the chamber by a generator. After the cycle is completed the generator is removed from the room, and the products may be stored in both normal and controlled atmosphere. This method ensures maximum effects only when recommendations concerning harvest date, cooling and other post-harvest operations are observed.

PACKAGING

Research work has been conducted for many years on the use of packaging that modifies the gas composition of the atmosphere and allows extension of the fruit and vegetable storage period while maintaining quality. Modified atmosphere packaging, i.e. MAP technology, is gaining popularity and is increasingly widely used in practice. In this technology passive and active atmosphere modification can be distinguished. Passive modification occurs when the desired atmosphere composition is obtained as a result of product respiration and diffusion of gases through films with appropriately selected selective permeability. Films are made, among other things, of low- or ultra-low-density polyethylene, polyvinyl chloride (Photo 7), ethylene vinyl acetate, oriented polypropylene or polyurethane. The active form of modification, on the other hand, consists in replacing the normal atmosphere in the package with an appropriate gas mixture already during packaging. In addition to the term MAP, packaging technology with modified atmosphere uses designations specifying the properties of a given package.

The following types of packaging can be listed:

• EMA (Equilibrium Modified Atmosphere) – packaging with an equilibrium atmosphere that establishes itself during product storage, made of film with selective gas permeability;
• MIP (Modified Interactive Packaging) – interactive packaging made of polyurethane film with optimum permeability for O₂ and CO₂, enabling water vapour to migrate outside and preventing its condensation inside the package;
• APS (Active Packaging System) – active packaging in which the desired gas atmosphere composition is obtained and maintained thanks to various additives to the package. The following additives to packages or packaging material are used for this purpose: O₂-absorbing substances, substances generating or absorbing CO₂, ethylene-absorbing substances, humidity regulators, odour absorbents, inhibitors of pathogenic microorganism development.
• IPA (Intelligent Packaging Application) – “intelligent” packaging containing indicators of product suitability for consumption. These indicators may react to the presence in the package atmosphere of ethanol vapours arising as a result of product spoilage or to the presence of pathogenic microorganisms.

Authors: Leszek Bednarczyk, MSc Eng. Dorota Niedojadło, MSc Eng. Bogdan Szczepański, MSc Eng.