Storage Systems

C H A P T E R

12 Storage Systems Atef Mohamed Elansari*, Elhadi M. Yahia†, Wasim Siddiqui‡ *

Department of Agriculture and Bio-systems Engineering, Faculty of Agriculture, Alexandria University, Alexandria, Egypt †Faculty of Natural Sciences, Autonomous University of Queretaro, Queretaro, Mexico ‡Department of Food Science and Post-Harvest Technology, BAC Sabour, Bhagalpur, India

12.1 INTRODUCTION The definition of the term “storage” as employed to fresh produce virtually and implicitly means the maintenance of fresh produce under planned and controlled conditions. This may involve small nonrefrigerated installations or the large-scale refrigerated storage of different horticultural commodities in order to comply with a systematic and continuous domestic need, offer a level of price stability, provide year-round supply, and reduce loss and waste. The essential distinction between very perishable and less perishable crops is in their metabolic rate, which is exhibited in their relative water activity and respiration activity. Fresh perishable produce has elevated water activity and rates of respiration, and therefore, is commonly characterized by a fairly short postharvest life. Hence, postharvest handling including storage of fresh produce is challenging. The goal of the fresh produce supply chain is to deliver safe, sound, healthy and highquality foods to the end consumer. Therefore the storage of perishables is one of the essential and critical factors for all fresh produce for the broadening of processing season and effective marketing. It is well recognized that the factors affecting consumer perception of fresh produce are abundant; many of these factors are intrinsic to storage variables. Consequently, it is vital to design and choose the best storage conditions that eliminate the linked biological and chemical kinetics or physical reactions. In the cold chain management, a structured cold atmosphere is maintained immediately after harvesting until consumption in order to delay the ripening and senescence processes. Within the postharvest storage systems, there are many alternatives that can be used to achieve the goals of maintaining quality and extending the storage and shelf life of fresh produce. Effective storage systems have been developed with a wide scope of adaptations depending upon the available facilities. Adaptations factors include (but not limited to); type,

Postharvest Technology of Perishable Horticultural Commodities https://doi.org/10.1016/B978-0-12-813276-0.00012-2

401

# 2019 Elsevier Inc. All rights reserved.

402

12. STORAGE SYSTEMS

variety and quantity of the produce to be stored and handled; the duration of storage; the end use of the produce; and marketing type. Low temperatures or cold storage have been widely applied to hinder the biological activity of horticultural commodities, to enable their handling through different supply chain components such as storage, transport, distribution over longer periods and distances, and marketing. However, the application of low temperatures or cold storage is bound by cost and benefit concerns. Principally, all fresh produce can benefit by cold storage at an optimum low temperature that extends storage life and maintains quality. Despite this, such benefits occasionally do not compensate for the cost of mechanical refrigeration along with its energy and maintenance, such as in the case of extremely low price of the produce. Another obstacle is how to generate full utilization of refrigerated space over a long period of time each year. The reason is that many horticultural crops are highly perishable and can only withstand very short storage time (a few days), while several others retain a longer storage life varying from less than one month to several months. Low-temperature storage in some tropical and subtropical countries, where refrigeration is crucially needed could be expensive with very limited electricity resources and limited infrastructure that make energy consumption unaffordable in such countries, if it exists. The alternative, although not as effective, to the cold storage method in some cases includes storage in shade, deep wells, cold water, evaporative cooling, radiant cooling, underground storage or in caves, high-altitude storage, night air ventilation, and “zero-energy” cool chamber, among others. There are several renewable energy solutions, which can be incorporated within the cold chain such as the use of mobile solar powered vans/solar cooled containers. In the following sections details will be given on the most feasible methods and systems applied for the storage of fresh produce.

12.2 OBJECTIVES AND GOALS OF STORAGE The main objective of postharvest storage as a component of the value chain is to maintain produce quality as superior as possible for as long as possible. Storage techniques and conditions are important postharvest factors marking quality maintenance of fresh produce. These operations are primarily directed at minimizing the rate of metabolism of harvested produce. Due to its capital and running expenses, cold storage adds to the cost of production, increasing the price of the produce; the bigger the involvement of the storage system, the higher the added cost. Short-term storage is applied so as to present some marketing flexibility, although it is not worthwhile to store perishables if the price increases, plus storage would lower quality and shelf life. However, the main reasons for the storage of produce are not only associated with marketing, but also with maintaining the quality by considering the following: – Minimizes decay by slowing down microorganism’s progression. – Lowers transpiration or water losses that otherwise promote unfavorable effects, such as wilting, elongation, rotting, greening, sprouting, and toughening. Such activities affect appearance, quality and texture.

12.3 BUILDING OF STORAGE SYSTEMS

403

– Slows down the biological activity of fresh produce, such as the case of reducing the production and action of the natural ripening agent ethylene. – Minimizes the surplus sale in the market, thus guaranteeing good returns to the farmers. – Assures the accessibility of the produce during the off-season. – Reduces waste and spoilage of produce. – Normalizes the price of the produce during the season, as well as during the off-season.

12.3 BUILDING OF STORAGE SYSTEMS There are several varying technologies of fresh produce storage, the adoption of which will depend on capital and running costs, in addition to the type of produce to be stored. Such methods range from fairly simple and low capital cost in situ to expensive controlled atmosphere stores. Nevertheless, before considering a storage system there are several aspects that need to be considered. The maximum storage life of a harvested crop depends on its production history, quality, maturity at harvest, and its marketing plan. These factors determine the economical feasibility of storage and business plan. It should be noticed that not all fresh produce are responsive to immediate storage. Some produce require some prestorage postharvest treatments such as curing or waxing. Among the factors that should be considered before selecting a particular storage system are the ambient temperature of the store, the conditions of the produce before moving into the store, the regular working hours, and the decision of which produce will be stored, whether it is cut flowers, fruits, vegetables, or mixed commodities. An investment plan or feasibility study should provide an answer-whether the products that are going to be stored would be for local markets, grocery stores, restaurants, hotels, food processors or for export. This is in order to design a facility that can meet the customer’s needs of hygiene, quality, volume, and time of execution. Therefore, the establishment of an investment plan and a feasibility study for the storage system should be the first step to be conducted. Such a plan should include the justification for the selected location, the position of the store with respect to the farm or the production area, transportation, shipping means, and distance to market. Cold stores should be designed for different types and varieties of produce. They can also be designed with some small-volume rooms instead of large-volume ones to facilitate the accommodation of smaller growers. Small-volume rooms are better for storage of fresh produce due to the following purposes: • Various fresh produce require a specific combination of temperatures and relative humidity (RH). • Some produce are incompatible regarding their requirements of temperatures, RH, and other factors. • Ethylene, carbon dioxide, and odors produced by vegetables and fruits during storage need to be expelled repeatedly. • Perishable storage rooms need regular sanitation. • Small storage rooms can consume more energy compared by large storage rooms.

404

12. STORAGE SYSTEMS

Examples of important factors that should be considered for the site selection include • • • • •

Electricity accessibility and credibility. Reliable network of accessible roads. Sustainable supply of clean water. Closeness to the production areas. Proximity of local markets, food processing plants, market distribution centers, institutional bodies (food service companies, hotels, restaurants, or other possible customers). • Future expansion. • Is it going to be part of a packinghouse or a stand alone cold store?

12.4 ON-FARM STORAGE SYSTEMS On-farm storage system may not require any electrical energy or power to run. It may involve the use of bricks, sand, bamboo, or other locally accessible materials. Some of the on-farm storage systems can be structured on-site using supplies that are made or sold close by. Consequently, effective storage systems have been developed to include a wide range of alternatives governed by the prevailing capabilities, including nature of the produce, its variety and quantity. On-farm storage systems can be temporary, short-term, or long-term, where temporary storage systems are required for abundant perishable fruits and vegetables that demands immediate marketing. The position and design of the storage system have an effect on system performance and effectiveness even when an industrial or commercial refrigeration system is used. For example the altitude lowers ambient temperature by 10°C for every 1000 m of elevation. In addition, using shaded areas could help in dropping thermal variances between field air and storage temperatures. On-farm perishable storage systems may be classified as naturally refrigerated or mechanically refrigerated systems. For the natural refrigerated storage system the goal is to let the produce develop and ripen on mother plants, then apply some kind of supplementary method to keep it further and as long as possible, wherein the mechanically refrigerated storage system uses controlled environments to lengthen the produce shelf life. The operation of naturally refrigerated systems includes in situ, heap or clamps, pits, evaporative cooling, among other methods.

12.4.1 In Situ In this system of perishable storage, harvesting is delayed until the produce is requested. This system is applied for citrus, yam, potato, sweet potato, garlic, and ginger, which can be sustained in situ for few months and are taken away from the storage locations prior to the wet and rainy season to eliminate rotting, sprouting, freezing, and chilling injuries. This system does not comprise excessive expenditure, structure or fabrication for storage. The shortcomings of the in situ storage system include intensive labor, varying climatic environments, and possible hostile weather such as freezing and humidity. The difficulties of pest

12.4 ON-FARM STORAGE SYSTEMS

405

FIG. 12.1 In situ storage systems for garlic and onions.

and rodents control are problematic in this type of storage system. These conditions form ideal circumstances for the development of possible disorders. In situ storage systems minimize losses via storing for up to 4 month. This system can be established by shielding fresh produce from direct sunlight and wind in order to lower exposure to heat and to prevent contamination (Fig. 12.1). Utilizing either roofing or fabric tenting for granting deep shelter over all working areas and gathering points is recommended. A cavernous overhanging cover extension (with a minimum of 1 m) can supply shade for openings either windows or doorways. Also, a light color or reflective cover can lower both surface temperature and temperature under the shelter by up to 20°C.

12.4.2 Root Cellars In cold areas, chilly winter temperatures and cold soils make root cellars a good method for storing produce, as it is easier to control temperature and humidity in a small cellar. A root cellar (Fig. 12.2) should be cold, dark, and damp and in a convenient location with an area of an optimum size that may be located by a walled-off part of a basement or garage with a window for ventilation. Water drainage is important for keeping out surface water in case of rain. Several types of produce can be stored in root cellars, including apples, beets, broccoli, Brussels sprouts, cabbage, carrots, Jerusalem artichokes, leeks, parsnips, pears, potatoes, rutabagas, turnips, and winter radishes. For maintaining high humidity inside the root cellar, a dirt floor may be installed; water is added as needed. Another method is the use of water container with a large surface area such as pans. One more choice is the use of a damp burlap over the produce. For ventilation, air needs to be circulated through the root cellar because warm air rises and cool air falls; an air intake fan may be installed down low and the outlet up high.

12.4.3 Basement Storage Rooms Basement stores (Fig. 12.3) commonly have a temperature between 10°C and 15°C with dry air, which makes them appropriate for some types of produce storage. However, in order to accomplish the cooling and moist conditions that are necessary for most produce, it may be necessary to create a separate room to have a furnace there. This individual room should be placed in the coldest part of the basement, far from the furnace if it does exist. The exterior

406

12. STORAGE SYSTEMS

Accumulated ice

Ground level

Root cellar

FIG. 12.2

The concept of a root cellars for the storage of vegetables.

FIG. 12.3

Basement stores for potatoes.

walls do not need to be insulated while the inside partitions should have an optimum thickness of insulation. Temperature control can take place in this storage area by opening and closing the outside window. Relative humidity (RH) can be maintained high by wetting the floor or by keeping wet burlap sacks or similar materials.

12.4.4 Night Cooling In some hot elevated temperature environments a significant difference in temperature between day and night occurs can be utilized to keep fresh produce cool. In dry desert

12.4 ON-FARM STORAGE SYSTEMS

407

environments the variance between daily maximum and minimum temperatures can be as great as 22°C during the summer. In these circumstances, night cooling using nighttime ventilation as a source of refrigeration can be effective for cooling some fresh produce. Nighttime cooling is commonly used for unrefrigerated storage of potatoes, onions, sweet potatoes, hard-rind squashes, and pumpkins. Rooms used for this purpose should be well insulated, where a fan built into the store is switched on when the temperature outside at night becomes lower than the temperature within the room. The fan may be equipped with sensors with a differential thermostat via a simple control system that allows for automatic switches on and off until the temperatures equalize. This method (Fig. 12.4) is applied for the bulk storage of onions. Night ventilation successfully preserves a given produce temperature when the external air temperature is lower than the product temperature for 5–7 h per day. Field heat can be absorbed out of the produce by the low nighttime temperature during early morning hours. Several ways are available for produce storage, where the length of storage time can be longer in specifically designed structures. The technology utilized depends on marketing issues, such as whether higher prices outweigh the costs. The most common methods are described in the following sections. 12.4.4.1 Natural Ventilation Natural ventilation (Fig. 12.5) takes advantage of the natural airflow around the produce to remove the heat and humidity generated by respiration and other factors. Assemblies that present some form of shield from the external surroundings and gaps for ventilation can be used. In this system, fresh produce is arranged in bulk, bags, boxes, bins, or pallets. The disadvantage of natural ventilation is that it leaves produce exposed to pests and diseases as well as to hostile climatic environments that can have a damaging effect on quality. 12.4.4.2 Forced-Air Ventilation Heat and gas exchange can be enhanced provided that air is forced to flow through and around the stored produce (Fig. 12.6). Such arrangements allow for a more effective operation of space for bulk storage of produce. Air conducts run underneath a perforated floor

FIG. 12.4 Forced ventilation of farm storage system for onions.

408

12. STORAGE SYSTEMS

FIG. 12.5

Natural ventilation storage of some crops.

FIG. 12.6

Forced ventilation for the stored onions.

(Fig. 12.6) and air is forced through the produce. As air passes along the route of least resistance, loading configurations as well as fan size (e.g., air flow rate and static head pressure) and conduct dimensions should be carefully designed to confirm that there is an even distribution of air throughout the stored produce.

12.4.5 Clamps or Heap Clamps or heaps (unventilated clamps) are an effective and inexpensive method for the storage of root crops by shielding them with hay and soil to protect them from weather and rodents. A clamp storage system (Fig. 12.7) involves covering piles of produce with suitable materials after the crop is sorted and the best quality that will withstand the storage process has been selected. Then, produce is stacked in a heap on a layer of grass and covered within layers of straw and soil. Ash, lime, and sawdust may be used for further effects such as discouraging reinfection. Clamp storage is a long-standing practice that helped immensely in eliminating waste in produce such as potatoes, sweet potato, turnips, rutabaga, carrots, red beets, salsify, and parsnips.

409

12.4 ON-FARM STORAGE SYSTEMS

Produce covered by soil layer

Soil covering both sides

Soil spade About 20 cm straw layer 180–240 cm

The height is have the width of the clamp

FIG. 12.7 Potato clamp covered with soil and straw.

This storage system requires a low capital investment, as it can be designed applying locally available materials for aeration and insulation, but it is labor intensive for construction, loading, and unloading, where the same disadvantages of the in situ storage are applied here. Clamps can be used indoors and outdoors, where for the latter, adapting buildings using ambient convective ventilation are suitable for this purpose. Clamps can be applied in countries with areas that have a sufficiently high altitude, such as in the mountains or on high plateau in the tropics or in winter in the subtropics. This practice has been a conventional method for storing potatoes in some parts of the world where a piece of land at the side of the field is used with a width of about 1–2.5 m. The dimensions are laid out and the tuber piled on the soil in an extended conical heap. Straw is usually laid on the ground underneath the potatoes. The steepest angle (angle of repose) of the tuber restricts the central height of the heap, which regularly represents one-third of the width of the clump.

12.4.6 Pits Pits are trenches tunneled underground for storing tubers, such as beets, potatoes, sweet potatoes, onions, turnips, carrots, cabbages, and parsnips, where they are shielded with straw and soil until there is a market need (Fig. 12.8). The tuber pit characterizes the oldest system of storage. Pit storage is commonly regarded as inexpensive for rural societies as it requires minimum materials. Submersion in the soil allows for the produce to stay cooler relative to the air temperatures. Pits must be well drained and protected from rodents. In some parts of the world, pit storage is mainly used for table potatoes due to the associated weather dependence, the poor control of climate inside the pit, and the associated quality hazards that increase the chance of spoilage. Also, storage in pits is both labor intensive and quite risky, as it is difficult to monitor quality; all stored crop can be lost before the grower is conscious that there is a problem. In this system the tubers are likely to be assembled loose and shaped into long heaps

410

12. STORAGE SYSTEMS

Exist of air

Storage space (wet air)

Vented base

Entrance of cold air

Produce loading and unloading

Exist of air

FIG. 12.8

A modified pit design for better ventilation.

on the field headland. It is vital that the soil under the pit stays desiccated, even in humid weather environments, which can occur during storage. Weight loss and decay are significantly lower in potatoes stored in pits as compared to those stored at room temperatures with no refrigeration. Poor sorting of potatoes prior to storage as well as poor ventilation of potato pits are possible causes of major losses. In order to

12.4 ON-FARM STORAGE SYSTEMS

411

eliminate condensation forming on the roof of many pits and dripping on the potato stack below a modified pit design is introduced, which includes a better ventilation to provide air space of 30 cm above the potato stack surface, promoting the use of two aluminum vents to promote air circulation, as well as a wooden rack on the floor to ensure air flow under the potato stack (Fig. 12.8). The basic downside to this system is that once the pit is open, it should not be resealed. An important general issue associated with the storage of root crops in pits is the missing of the mechanism that regulates dormancy. Vents, which provide adequate ventilation in the underground pit, have a significant effect in removing the inhibition of localized heat pockets from the pit, which tends to promote sprouting activity. For example, in Afghanistan the ventilation of traditional pit storage rigorously combined with improved plant selection in the field and better seed handling practices (e.g., separating seed from tubers destined for consumption) cut potato storage losses down from 30% to 5%.

12.4.7 Evaporative Cooling The theory of evaporative cooling (Fig. 12.9) is established by the fact that the evaporation of a liquid absorbs substantially more heat (latent heat) than the amount required for its temperature to rise by a few degrees (sensible heat). Therefore, the term evaporative cooling conveys the cooling obtained uniquely by the evaporation of water in air. For the water evaporating into air an assessment for weighing the efficiency and capability of the evaporative cooling system is the deficit between the air wet-bulb temperature and dry-bulb temperature. The larger the difference between the two temperatures, the greater is the evaporative cooling outcome. Therefore evaporative cooling is a physical phenomenon where latent heat is released and results into the cooling of the produce or medium with which the air comes into contact. One way to apply this method is by dragging dry air through wet padding or fine film of water. The dry air is cooled by giving up its heat content to the humidity of the pad. This method has been found to be competent and relatively inexpensive in lowering the temperature and enhancing the RH in an enclosed storage area, which has been widely tried for increasing the shelf life of fresh produce in some developing countries with arid and semiarid climates. Evaporative cooling extends the shelf life of fresh produce, such as potato, tomato, grapes, mango, orange, banana, sapota, aonla, plums, bitter gourd, capsicum, pineapple, cauliflower, green pepper, cluster bean, peach, brinjal, cucumber, chili, beat, ladies finger, peas, carrot, radish, and leafy vegetables. One common type of evaporative cooling storage structure is the double wall structure, with a gap between the walls that is loaded with porous water-absorbing materials (pads). These pads are maintained continuously wet by diffusing water. When partially dried air passes through the wet pad, mass and heat transfer occur, with the energy required for the evaporation process obtained from the air stream. This is the most economical way of reducing the temperature by humidifying the air. Evaporative cooling has some advantages compared to mechanical refrigeration; such as, it does not use refrigerant, so it is environment friendly, in addition to being silent (neither making sound nor accompanied by any sound), as it has no moving part. The system also saves energy with very limited initial investment where the operational cost is minor.

412

12. STORAGE SYSTEMS

2

6

7

1 3 5

1- Outside dry and hot air 2- Water distributor 3- Pad 4- Water collector 5- Water pump 6- Blower 7- Cool and refreshed air

4

FIG. 12.9

The concept of evaporative cooling.

The system is quickly and easily installed with minimum maintenance requirements and can be fabricated using locally available materials in remote areas. The storage size of evaporative cooling system can vary to accommodate few kilograms to several tonnes. However, this system has several disadvantages compared to mechanical refrigeration. The system is only suitable for the short-term storage of fresh produce soon after harvest. Additionally he achieved temperature reduction cannot be lowered beyond the wet-pulp temperature of the outside air. In such a method, and if the exhaust fan is not used to draw the air, there will be no power required to run the store to cool the fruits, and therefore, the term “Zero Energy Cool Chamber” is used (Fig. 12.10). These chambers are proper for short storage periods of time for hot-season produce, such as mangoes, tomatoes, cucumbers, eggplant, and peppers. The lowest outdoor air temperature vital for effective evaporative cooling operation is more than 32°C, along with a relatively low wet bulb temperature, preferably below 2°C. In other words the level of chilling depends on the original RH of the ambient air and the effectiveness of the evaporating surface. If the ambient air humidity is low while the evaporating surface is

12.4 ON-FARM STORAGE SYSTEMS

413

FIG. 12.10 Zero energy cool chamber.

humidified or wetted close to 100% RH a large drop in temperature will be accomplished, and cool moist conditions will be provided for the optimal storage of fresh produce within that system. Consequently the system is adequate for the area with a very dry air that can absorb a lot of moisture where a greater cooling take place; this is why such system is not workable in the humid areas.

12.4.8 CoolBot CoolBot is a kit (Fig. 12.11) that converts a standard air conditioning unit to a cold store by allowing it to drop the temperature of the room to as low as 1°C. The CoolBot was developed as an affordable way for small-scale producers to cool produce on the farms. The system transforms an insulated room and an inexpensive, readily available window air conditioner into a cool room that substantially reduces the cost of a cool storage environment for fruits, vegetables, flowers, and other products. This system was commercially effective in expanding the marketable life of the produce and offers a low-cost storage facility to the small and marginal farmers.

FIG. 12.11 The CoolBot kit installed and running for room cooling.

414

12. STORAGE SYSTEMS

12.5 MECHANICAL REFRIGERATION Temperature is the most important single factor that affects the life of fresh produce by directly effecting the rates of biochemical activities. The use of low temperatures via mechanical refrigeration for the storage of fresh produce refers to the perception of the thermal load. Thermal load characterizes the subtraction of heat emitted by the stored produce to lower its temperature to the desired level. In the storage of fresh produce the optimum storage temperature is closely linked to respirational intensity, as it can be lowered by reducing the temperature to a certain limit depending on the commodity. In other words the respiration rate depends on the storage environment; the lower the temperature, the slower the respiration rate, consequently slowing down the kinetics of biochemical reactions including those associated with senescence. Cold storage (Fig. 12.12) is a component of the “cold chain,” which is defined as the set of systems that confirms the maintenance of produce quality throughout the harvest stage up to the household consumer. The key elements in this chain include precooling systems, cold storage, inland refrigerated transportation, refrigerated rail and marine transportation, refrigerated displays in supermarkets, and domestic and industrial refrigerators. With the mechanical refrigeration system the chemical and biological processes in fresh produce are slowed down; therefore the accompanying deterioration and loss of quality and nutrients are minimized. As an example the taste and quality of sweet corn is influenced heavily by its sugar content. Up to 50% of its initial sugar content may be consumed in 1 day when stored at 21°C (converted to starch), while it only loses 5% if stored at 0°C. The vase life can be doubled if stored at 0°C compared with at 12.5°C. Also, fresh asparagus may lose 50% of its ascorbic acid content in a single day at 20°C, but in 12 days at 0°C. The precise temperature and RH control via the mechanical refrigeration system also extends the shelf life of fresh produce. For example the first form of unpleasant yellowing of broccoli may be hindered by three or more days of refrigeration. The principle details and the theory behind the mechanical refrigeration system is covered in Chapter 7 of this book.

FIG. 12.12

Cold store for fresh produce.

12.6 PRODUCE HANDLING INSIDE THE STORE

415

12.6 PRODUCE HANDLING INSIDE THE STORE The function of a cold store is to maintain perishable produce at low temperatures. There are two essential design issues: • Types of produce or mix of produce to be stored. • Type of handling system to be applied. Different horticulture products have special temperature, RH, ventilation, and air flow rate requirements and limits. Therefore they have different handling, hygiene, and storage guidelines, which sometimes are not identical, even for the different varieties of the same produce. Product storage requirements can also dictate the interfacing transport vehicles and environmental loading requirements. There are many handling systems in the cold storage rooms, where applications vary according to many factors that must be carefully defined, including dimensions and capacity of the warehouse, the actual dimensions and the weight of the product unit, and the allowed stack heights, as well as the loading and unloading source, whether through a loading dock equipped with conveyers or by forklifts. Handling methods have a direct impact on the economies of facility in terms of cooling capacities required, energy consumed, and the footprint of the project in addition to the size (the total cubic meters of the internal volume) of the labor required for operation. There are also some other factors that are not related to dimensions, such as speed of loading and unloading to and from the cold rooms. In addition to the nature of the stored produce and weather, another factor is the immediate reach to the stored product that might be needed for processing throughout the year. Therefore, the material handling systems affects the operation management and quality control required. Storage methods for holding fresh produce include bulk storage, pallet racks, and bins. The use of modern stacking systems promotes standardization in fresh produce load handling and brings storage space in synergy with modern packaging systems.

12.6.1 Bulk Storage This is a flexible, low-cost method of storage and handling (Fig. 12.13). Bulk storage consists of piles of unpackaged produce loose on the floor of storage buildings; it is generally inexpensive and commonly used for products that can be handled in bulk and piled deep, such as potatoes, turnips, and onions. The produce is piled in the store, where it exerts forces on the wall that must be resisted by the building’s design. These forces are significant and should not be misjudged in the store design. Other produce such as beets, carrots, and parsnips can also be stored in bulk, though in thin piles. The height of the pile depends on the mechanical properties of the produce, where the ones in the bottom layers are subject to a large pressure that may cause injuries. In other words, deep bulk piles tend to cause pressure, such as by bruising drawbacks in the case of storing the produce over 4 m high or if extremely ventilated by nonhumidified air. Usually, ventilation is carried out by passing humid and refrigerated air through the produce from a duct system placed under the pile, and the air intake usually in the roof space above the produce piles (Fig. 12.14). The applied refrigeration system is similar to what is

416

12. STORAGE SYSTEMS

FIG. 12.13

Bulk storage of potatoes: empty store (with slots on the floor and its details) versus loaded store.

FIG. 12.14

Duct system placed under the pile of bulk stored potatoes.

used for a conventional cold store, but cooling air passes through a chilled cascade of water to raise its RH. In the case of the bulk storage of potatoes after the crop is cured the store temperature must be maintained at 3–4°C. Despite the optimum RH for potato storage being 90%–95%, or as

417

12.6 PRODUCE HANDLING INSIDE THE STORE

high as possible without fully saturating the air, it is favorable to bring the storage humidity up to 95%–98% to accommodate storing other produce, such as carrots, parsnips, beetroot, or swede. Onions keep well under these storage temperatures, but RH should not be higher than 75%. As potatoes lose more water during their first month of storage than any other time in storage because of wound healing, maintaining an optimum level of ventilation is crucial. Some methods are available to increase humidity, such as the development of wrapping pallets in plastic films. The bulk store can also be divided into sections with plastic curtaining, using insulated tent systems to accommodate a mix of products. An optimum ventilation system provides the air flow rate necessary to satisfy the temperature and humidity required to safely store produce within the different layers of the pile in order to assure proper, even conditions. Poor control of temperature and/or RH can reduce shelf life and accelerate defects. Consequently, difficulties of bulk storage include challenges in passing cold humid air through the pile’s layers. However, highly insulated bulk stores with forced draft ventilation can provide relatively inexpensive and better storage, especially for larger quantities of produce.

12.6.2 Advanced Bulk Storage In the advanced bulk storage system for potatoes (Fig. 12.15), air is induced through the opening of hatches or louvers or a mechanized gate equipped with actuators. Then, the air flows through the pressure fans that are equipped with nonreturn valves (check valve) Cooling coil Cold store room

Louver with actuator allowing out side air in

Louver

Louver with acuter allowing exhaust air out

Temperature, carbon dioxide and relative humidity sensors Outside conditions monitoring unit

Humidification unit Heater for dehumidification

Movement stopper Cold store insulated door

Air pump with check valve and VFD

Pressure chamber Ventilations slots

FIG. 12.15 Bulk storage system for potatoes.

418

12. STORAGE SYSTEMS

toward the inside of the pressure chamber. The function of the pressure chamber is to equalize the pressure of the induced air that is being accelerated and forced to flow into the ducts underneath the piles on the storage deck or through a fully gridded floor, so that all parts of the store receive the same amount of ventilation. A humidifier is positioned right after the pressure fan in order to humidify the air before it enters the potato bulk. The air spreads through the produce along the height of the pile, then leaves the storage via the exit hatches. The check valves are to avoid the flowing back of the pressurized air in case one of the fan stops functioning. The ventilation systems should be capable of blending outside air with inside air as required based on calculation programs that allow for an easy prediction of ventilation requirements for the given levels of carbon dioxide or ethylene. In order to keep all physical conditions inside the store at the right level and also uniformly distributed, such systems should be fully controlled and capable of modulating 100% of the air, whether it is: (1) the fresh air induced from outside; (2) the internal air; (3) any required mixture of external and internal air, depending on the situation and conditions. Stores may be equipped with frequency drives that slow down the pressure fans, rather than setting them at fixed ventilation schedules in order to save energy and avoid drying. Proper sealing of the buildings may cause an accumulation of CO2 and ethylene, which can lead to problems for the crops as well as with safety. Keeping CO2 level under control by providing proper amounts of fresh air can delay or even prevent sweating when stored at a higher temperature. This is very important for processing crops that can accumulate sugars, if the CO2 level in the storage is too high. In addition to the refrigerated system, the cold store could be equipped with heaters as a part of the humidification for drying, curing, control of condensation inside the store and also to accommodate different storage regimes required for potatoes, onions, or other bulk produce. The system shown in Fig. 12.15 is a model for an advanced potato bulk storage system that includes mechanical refrigeration unit, pressure fans as well as a modern ventilation arrangements. Cost functions are derived via energy consumption by the system fans, the air conditions and weight losses of the product caused by dehydration. Using these cost functions, optimal control settings that minimize the total costs for a specific storage period can be achieved. By integrating the precise control of ventilation with refrigeration, humidification, and dehumidification systems, any climatic conditions can be accomplished in order to accurately dry, cure, and store potatoes, onions and other crops. A major disadvantage of the modern bulk storage system is that it requires a considerable capital investment in the warehouse, refrigeration, and control system, in addition to the equipment necessary to fill and empty the storage facility.

12.7 BULK BINS A bulk bin is a wooden crate used for handling a variety of fresh produce (Figs. 12.16 and 12.17), such as large volumes of potatoes, pumpkins, onions, cabbage, olive, squash, sweet potatoes, carrot, garlic, raisins, stone fruit, pecans, kiwifruit, avocado, melons, apples, and pears. The bulk bin is reusable and helps to unitize and protect loose produce for fork/

12.7 BULK BINS

419

FIG. 12.16 Bulk bin used for potato and carrot storage.

FIG. 12.17 Bulk bin used for apple storage.

platform truck handling. Using bulk bins during storage assists in the separation of different stocks, provided that the same storage conditions are combatable for all stored produce in terms of temperature, RH, ventilation requirement, ethylene, and odor generation and sensitivity. Bulk bin usage while storing fresh produce is very helpful, especially in case of having many handling steps required during storage or preparation.

420

12. STORAGE SYSTEMS

Bulk bins are being used for harvesting, shipping, and storage. Bin sizes vary, but there is a move to a standard at 1.2  1.6 m with a height up to 1.2 m. The capacity of these standard boxes is 1000 kg for onions and 1250 kg for potatoes. There is a tendency to use much smaller ones, around 100–400 kg; smaller bins are utilized for more perishable produce. The bulk bin is usually designed with a palletized bottom in one direction or two for easy handling. Bulk bin vents vary, but all of them should allow for a perfect alignment through the produce so they greatly speed up the cooling rate by admitting the cool and humidified air to flow uniformly. It is recommended that the stored bins should be stacked to form air channels 10–15 cm wide to direct the air flow passage, in addition to allowing a gap between these stacks and walls to grant refrigerated air a chance to absorb the heat of conduction through the walls. If a single bin is improperly oriented, then it can restrict airflow to downstream row. When bins are used for storage the floor (which is typically arranged in lanes) should be finished in a way that guarantees a perfect flatness, so that cold air supplied into the openings at the base of the pallet bins is allowed to pass through the openings, where it is drawn back either towards the recirculation vent (during refrigeration mode) or the exhaust air damper (when the refrigeration mode is off ). With a flat floor design, boxes can be stacked 4 or 5 high for safety; however, the maximum allowed stack height is restricted by the stackability of the bins and/or forklift scope. The floor should be designed to hold the obligatory loads from the storage equipment and the produce. The key advantages of using bulk bins rather than bulk storage are the low cost of bulk bins and their greater strength; a bin can last for up to 10 years if maintained well.

12.8 RACK SYSTEMS A fresh produce warehouse with a large-scale module is a dynamic environment with a fast produce rotation and rapid movement of forklift trucks; therefore storage rack systems are engineered structures designed to minimize losses during the operations of such a warehouse. Furthermore, some fresh produce should be stored in racks, as packaging limits the ability to stack them because of weight and volume consideration, such as in the case of cut flowers. With storage racks (Fig. 12.18), stored produce can be readily removed by a well-qualified lift truck handler. It can also improve volume utilization with good planning, therefore it can enhance storage productivity and security. The use of racking systems (compared with bulk storage, sacks, or plastic boxes) eliminates subjecting produce stack of falling down, minimizes produce contamination by lowering the traffic within the cold store, and increases the efficiency of the refrigeration system by minimizing the infiltration of outside hot air through the doors. In the storage rack system the largest unit of material used is the pallet, which is a wooden or plastic platform with enough clearance underneath its top surface to permit the placing of forks for later pickup purposes. The size of pallet is defined by its depth and its width, while pallet height is usually not quantified, though it is always compatible with railcar and truck trailer dimensions. Although the sizes of the pallets vary across regions in the world the international standard pallet dimensions for fresh produce are 120 cm (width) by 100 cm (length).

12.8 RACK SYSTEMS

421

FIG. 12.18 Simple single-deep pallet rack.

One of the most important considerations in the storage system of fresh produce is the density of storage. An empty cold store is cheaper to operate than a full store. There are several solutions that can maximize the density of the store in order to increase the feasibility. Additionally, filling volume in the most intensive manner cuts down refrigeration cost by eliminating the refrigeration of a vacant space. The concern regarding environmental issues is a further focus. The type of storage system should be a balance between the category and volume of stored produce and the frequency of transits (the speed of throughput), in addition to some other considerations, such as selectivity and applying the LIFO (last in, first out), FIFO (first in, first out), or FEFO (first, expired first out) principles. There are several options of racking systems aimed to offer high-density storage, all of which can increase cost efficiencies in perishable refrigerated warehouses. The following sections will discuses some of theses options.

12.8.1 Double-Deep Racking Double-deep racking (Fig. 12.19) is a technique that is applied during refrigerated storage, where pallets remain stationary or static in their storage positions until transferred from the system. Two produce pallets are stored, one at the back of the other at the same shelf position,

422

12. STORAGE SYSTEMS

Produce loading

Produce loading

Produce unloading Produce unloading

FIG. 12.19

Double-deep rack system.

freeing up the costly floor space or footprint and cutting down on the number of access aisles required for forklifts. This means a maximum of four rows can be placed back to back. It is used for the storage of several pallet quantities of the same produce, and the system is footprint efficient, as it minimizes the number of access aisles. With the appropriate forklift, produce pallets can be accessed by the same spot and can be recovered relatively fast. The double-deep racking system is appropriate for a high-stock rotation storage regime and is ideal for same produce storage, where several pallets with the same SKU (stock keeping unit) can be kept. A double-deep racking system is considered a

12.8 RACK SYSTEMS

423

selective option, where a particular produce pallet can be chosen for handling in a very short time, as half of SKUs are behind pallets when storage locations are at full capacity. Doubledeep racking is better suited to fresh produce with a medium to long-term storage life. The capability of a double-deep rack system to exclude unnecessary aisles saves more of the warehouse footprint that can be utilized for produce storage. This aspect makes doubledeep pallet rack systems one of the most cost effective and space efficient alternatives on hand. The most substantial drawback of the double-deep rack system is the need for a specialized forklift to access the further (i.e., second deep pallet) pallet on the shelf.

12.8.2 Drive-Through Racking System In the drive-through racks system (Fig. 12.20) the forklift is allowed to drive into the racking bays from either side to place or collect the produce pallet. Produce pallets are braced by rails fixed to upright frames, and forklifts are driven between uprights to reach the produce pallets. Therefore, produce pallets can slide backwards or forward on a continuous rail because it is an open system at both ends, permitting first-in, first-out pallet storage (FIFO method). Forklifts drive into the rack to pick up the first pallet they come to, while pallets in the center of the rack system are not called up as commonly as those on the ends. A drive-through system allows for the storage of large pallets of similar produce using a limited footprint. Subsequently, selectivity is preferable than storage density, as many pallets are stored and accessible through a single pallet spot. A drive-through rack system is a cost-effective solution to high-density storage. It requires fewer aisles and provides better volume utilization than selective racking (double-deep), as it allows for storing up to 75% more pallets.

FIG. 12.20 Drive-through racking system.

424

12. STORAGE SYSTEMS

12.8.3 Drive-In Racking System The drive-in racking system (Fig. 12.21) also permits the storage of high-density produce when compared with some other methods. Applying the LIFO (last in, first out) concept the drive-in racking system utilizes a sole pass and exit point where accessibility is only from one edge of the run of produce pallets. The pallets are picked up or loaded separately from the rack face. In other words, forklifts enter into the storage lanes of the drive-in racking to place or pull out produce pallets, which means that operating aisles are minimized, and a significant space or footprint is saved; thus it is ideal where floor space or footprint is expensive.

FIG. 12.21

Drive-in racking system.

12.8 RACK SYSTEMS

425

Therefore the drive-in racking system is applicable for the large volumes of same SKU with high density produce pallets, such as for citrus and dates. Although superior storage density can be accomplished by using a “drive-in” racking system, it may not be the case at higher levels, as the forklift’s driver steers via the narrow alley between rows.

12.8.4 Pallet Shuttle There are two ways to store various produce pallets in racking arrangements (i.e., deep lane pallet storage); these methods were explained previously and they are either drive in or drive through. There is a third system, the pallet shuttle, which is a semiautomatic compacted with systems that utilize an electric shuttle (Fig. 12.22) to place and pick up the produce pallets inside static shelves rapidly and accurately. A pallet shuttle is an ideal solution for the bulky static storage systems that are more likely to be coupled with low rotation inventories where manual stacking and retrieval is a feasible mode of operation. The shuttle is driven by an electric motor with rechargeable batteries that run on rails inside the storage channels of the rack system, eliminating the use of a forklift inside the racks (i.e., drive in and drive through) and substantially minimizing operating hours and enabling produce pallets to be consolidated by channels rather than the entire lanes. Through the pallet shuttle the forklifts load the produce pallets on the rails at the front direction, where the electric shuttle picks the load up and moves them towards the head unoccupied location in the channel, squeezing the load as much as possible.

12.8.5 Carton Live Storage or Gravity Racking System This system is made up of sloping roller tracks that secure the optimum loading and retrieval of produce, as well as ideal produce replacement or circulation (Fig. 12.23). The produce cartons or pallets are loaded in at the upper end of the lane and slide down smoothly by gravity, headed for the pick (lower) end at the other edge. This system is specified for produce distribution centers that have a large picking volumes to be prepared for each order in a limited time period. The advantages of the system include the full automation of the produce package replenishment in addition to applying the FIFO concept that allows for ideal turnover as well as a higher number of SKUs at the front of the racking system. Furthermore the required period to arrange for orders is minimized, leading to an increased and efficient storage capacity with better utilization of the store footprint.

FIG. 12.22 Pallet shuttle.

426

FIG. 12.23

12. STORAGE SYSTEMS

Carton live storage.

12.8.6 Pushback Racking System Pushback racking system is a footprint efficient method of produce pallet storage for a sound designed cold store where the system runs on a FILO concept (Fig. 12.24). The individual produce pallet is fed in chain towards wheeled carriers at different elevations, where it is pushed distant back into the bay by preceding deposits. Sloped steel lead channels guarantee that the pallets are possessed in position in order to utilize entirely the extent of each aisle. The pushback racking system allows for the wise utilization of the warehouse’s volume and footprint capacity, which is very feasible compared to other alternatives, such as acquiring extra storage floor space. The system relatively constrains selectivity to a degree, but selectivity can be increased, with each produce having its particular given lane. The system is suitable for the storage of bulky stocks that have few SKU and facilitates streamline procedures. In the pushback racking system, pallets are stacked in a row up to 10 pallets deep. Once a pallet has been

FIG. 12.24

Pushback racking system.

427

12.8 RACK SYSTEMS

picked up the last ones roll forward for easy access at the picking edge for the next picking cycle. At the same time and immediately after the picking up of a pallet the pallet is loaded into the racking system from the other side, where it is simply pushed further back into the racking by the weight of the new pallet being loaded into its place and so on. There are several advantages for using the pushback racking system. Since the pallets reach the aisle, pushback racks are much quicker to stack and discharge compared to the drive-in racking system. Furthermore, stock circulation and footprint utilization are significantly enhanced due to the possibility of using each level to keep a different produce in pushback racking system. At last, rack destruction is substantially eliminated, as the forklift driver does not have to move into the rack to unload pallets.

12.8.7 Mobile Racking System The mobile racking system (Fig. 12.25) is comprised of traditional pallet racking, where bays of pallet racks are installed onto mobile carriages that move along a floor track, therefore eliminating multiple fixed access aisles. The mobile bases have electric motors, sliders, and a range of necessary safety accessories to ensure safe and efficient function. It is a dense storage capacity system that maintains 100% selectivity with immediate access to each particular produce pallet, as it is a single deep system. On requesting a particular pallet, commands are conveyed to the mobile racking by radio remote control that moves the racks simultaneously. Once the operator pushes the button and carriage controls are fixed directly to the carriage, the unit opens spontaneously to supply direct access aisle to the intended pallet. This space is the only aisle required between each Cooling coil

Superflat floor

FIG. 12.25 Mobile racking system.

Changeable aisle

Floor track

428

12. STORAGE SYSTEMS

racking shelf as soon as the racking is called up. This exclusive design permits the opening and closing of any particular aisle. The system manufacturers ensure safety by a photoelectric beam system fixed on each movable rack and at the outer edges of the entire system. The mobile racking system and its higher capital price is only cost effective if the available footprint or space is limited and a lower selectivity is required. The mobile racking system allows the application of the FIFO principal while having an exceptionally dense storage capacity.

12.8.8 High-Rise Fully Automated Rack System A rapid and efficient pallet handling system is vital because the pallet handling system is the central element in the warehouse; distribution operations start from the receipt of produce throughout their subsequent delivery. With the advances of computerized controlled storage and retrieval systems a new invention of refrigerated warehouses is emerging. The high-rise fully automated rack system (Fig. 12.26) minimizes the need for manual labor along with the most advantageous use of the available footprint, which considerably increases the efficiency and stock capacity of the refrigerated warehouse. As applied for fresh produce a high-rise fully automated refrigerated warehouse is a rack-supported building that becomes very common where racks have a dual function; stacking as well as structure support. This design allows the utilization of fewer cranes (Fig. 12.27) than aisles, leading to lower capital investment costs. In the high-rise fully automated system the order picking is prepared by a computerized controlled storage sorting and retrieval system. The system is superior in saving space, as it is a narrow aisle with just enough room for a mini load retrieval system to place and retrieve items. A crane is used to access pallets that are stored on regular racks. The crane is powered by three individual electronic motors that permit movement in the x, y, and z directions.

FIG. 12.26

High-rise fully automated rack system.

12.9 VENTILATION

429

FIG. 12.27 Lifting crane of high-rise fully automated rack system.

The high-rise fully automated systems are characterized by their flexibility to accommodate produce pallets of various volumes and turnovers while offering complete access for each single load. Such aspects provide unlimited stock control with the highest level of safety, as each pallet position is assigned for a specific produce pallet.

12.9 VENTILATION Fresh produce are alive and require fresh air to allow respiration. Without ventilation during either storage or transport, respiratory gases can build up and harm the produce. Thus if the ventilation rate is too low, produce may suffer from a buildup of carbon dioxide or ethylene, depending on the type of produce. Ventilation also helps in removing excess moisture, which encourages mold growth, and minimizes odor accumulation. If no good ventilation is provided and continuously maintained, then hot and humid areas can build up in the store, which in turn affect the quality of stored produce and create ideal conditions for the development of diseases. Therefore most fresh produce storage systems as well as packages are characterized by ventilation arrangements. Likewise, controlled ventilation permits uniform cooling of the produce. For most of the nonrefrigerated storage systems and since it is not essential that the rooms be hermetically sealed, usually there are suffusion ventilation. Ventilation can be classified as natural and forced. The natural ventilation storage is the simplest system where the natural airflow around the product continuously removes the heat and humidity generated by respiration. The storage structure provides some form of shield from the exterior environment, where the gaps for ventilation are often used. For naturally ventilated storage systems the ventilation arrangements should be equipped with proper screens to keep out animals, rodents, and pests. In the forced-air ventilation, auxiliary fans are used to enhance heat and gas exchange, where air is forced to pass through the stored produce with a velocity of 10–13 m/s. This system allows for a more effective utilization of the footprint for bulk storage. The pressure fans

430

12. STORAGE SYSTEMS

are placed to generate air flow within and around the produce, as well as for the distribution of chemicals for sprout suppression or disease control, if required. Therefore air flow should be as uniform as possible throughout the stored produce, where a well designed warehouse should be properly designed to accomplish a homogeneity of air flow at its normal operating condition. Both conventionally and for the bulk cold storage of potatoes a ventilation rate of 72m3/h/ton has been favored in Great Britain, but with some quality consideration; there is a tendency to increase the rates to up to 144 m3/h/ton, so the air flow can remove heat more rapidly with minimum weight loss. For the ambient air storage of vegetables (e.g., cabbage, onions, and potatoes), a ventilation capacity of 0.7 m3/h/ton is commonly used and recommended. The same flow rate (0.7 m3/h/ton) is recommended for ventilated pallet bin storage as well; this is at a 210–250 Pascal static head gauge pressure for the fans. While ventilation is crucial for many fresh produce items and for any storage systems, unnecessary amounts of ventilation will increase energy use and affect the response and accuracy of temperature control provided. It is therefore vital to ensure that the correct amount of ventilation is supplied.

12.10 HUMIDIFICATION SYSTEMS There are many aspects that influence humidity levels in fresh produce storage systems, including insulation, refrigeration system type, and air circulation. The goal of the humidification system is to reduce commodity water loss and shrivel and quality loss. At full saturation conditions where 100% humidity is maintained, water droplets can condensate on the produce, which can lead to decay. A humidity level of less than 90% for some produce leads to weight loss, thus making the produce less tempting to consumers. Some of the produce that require environments of 90% RH or higher include asparagus, apples, broccoli, Brussels sprouts, cabbage, carrots, cauliflower, celery, collards, corn, grapes, kale, leeks, lettuce, parsley, pears, peas, radishes, rhubarb, rutabagas, and spinach. However, for certain produce a low level of RH is required, such as for onions and garlic, for which 60%–70% RH is required. The simplest practice of increasing the RH of the fresh produce storage air is to wet the floor or mist the storage containers with cold water and allow it to evaporate. There are two main systems for maintaining humidity in fresh produce storage facilities: active and passive systems. In the active system (Fig. 12.28), water molecules are introduced into the air by different systems. In humidifiers working with vaporization techniques, vapor is generated via a steam boiler located outside the store, where very fine released particles are introduced into the air, resulting in an efficient humidification system. A vaporization technique or steam humidifier’s leading advantage is the providing of water as a vapor directly into the air stream, which consequently does not cause any wetting on the produce surfaces. The make-up water for the boiler must be chemically treated, whereas demineralized water is normally utilized to minimize the buildup of minerals inside the boiler as well as inside the cold store. To avoid rust, stainless steel boiler vessels are utilized. The drawback of the vaporization technique is its energy requirements: the decalcification of the water, in addition to being a heat load on the refrigeration system. In another type of

12.10 HUMIDIFICATION SYSTEMS

431

FIG. 12.28 Active humidification system in a fresh produce cold store.

humidification system a compressed atomizing system releases a high-velocity air stream around a water orifice, where the high-velocity air stream generates a vacuum at the orifice end, which shears the water into droplet ranging from 30 μm in diameter, depending on the velocity of the atomizing air. Demineralized water should be used with the system to avoid dusting from mineral salts found in normal tap water that may block the nozzles. With the centrifugal spray humidification system a spinning disc is used for spattering water droplets, and the water is sprayed at low pressure. The harnessed centrifugal force is applied for atomizing the water, and very fine droplets of water are blown into the cold store. The system is limited to temperatures above 0°C because of icing risks. However, it is an energy efficient method with a low cost. Ultrasonic humidifiers (dry fog) operate by means of an electric transducer that rests in a thin water bath, where it converts high frequency electricity into mechanical oscillation, causing small droplets (about one micron in diameter) to break away from the surface of the water bath and quickly be absorbed into the air flow. Ultrasonic humidifiers do not add to the heat load of the refrigeration system, as the mist is generated by oscillation and not heat, therefore the water temperature is not raised. Demineralized water is typically applied both to avoid mineral buildup on the transducers and to wipe out the mineral dusting of the treated cold store. The passive system means that the cooling management is sufficient for maintaining an optimum level of RH within the store. As the difference between refrigerant temperature and air temperature (dT) becomes small, there should be no concern regarding humidity loss or decreasing. This is usually achieved by selecting an optimum refrigerating coil or evaporator with a large surface area, along with the temperature of the surface controlled by the right automatic devices. The advantage of a passive system is that the RH level is consistent throughout the store and kept at a constant level in the range of 92%–98%, depending on the specifications and the design of the system cell itself, in addition to the outside environment. Several factors need to be considered when selecting a humidification system for a fresh produce store, where each application will have a distinctive set of requirements that will affect system selection and design. Some of the factors include the humidification load, store construction, energy source, water quality, and level of control required.

432

12. STORAGE SYSTEMS

12.11 HYGIENE AND SANITATION The cleaning and disinfection of stores are important in maintaining the quality of fresh products from diseases and contamination, as well as to avoid economic and environmental potential problems and to reduce waste. There are specific procedures that need to be followed for effective cleaning and disinfection operations according to the stored produce, the type of store, and the nature of the storage process. In general, produce storage system must fulfill the codes and regulations for the precise hygiene rules and microbiological criteria of produce, as well as temperature controls and cold chain compliance. Additionally, it is advisable to consider: aspects of hygiene, a Hazard Analysis and Critical Control Points (HACCP) program (if needed), stored produce quality, and produce shelf life at an early stage in the design process of the storage system. Therefore the design should be based on minimizing the potential of the buildup of contamination as well as to maximize the simplicity of cleaning and disinfection. There are many potential sources of contamination inside storage warehouses. Fungi and bacteria are one of the main sources that must be actively eliminated during storage and handling in order to ensure the safety and wholesomeness of the produce. Produce coming from the field can be contaminated by fungi and bacteria during the growing season, as well as throughout harvesting and transportation. Field packages used during harvest, transport and storage can be a contamination source, as well as evaporators, cold storage walls, ambient air, and other sources of pollution. The presence of waste in refrigerated warehouses such as those produced by fruit sorting also allows microorganisms to rapidly multiply and become a major source of contamination. The accumulation of volatiles from sorting and grading processes provides sufficient carbon sources to support the growth of fungi and possibly some bacteria. Likewise, water used in cleaning of warehouses and washing of boxes, bins, and produce can be a potential source of contamination. The aim of providing a safe and sound environment for the storage of produce is mainly to achieve the following requirements: 1. 2. 3. 4.

To minimize the effect of using chemical substances and pesticides. To eliminate any microbial hazards that render the produce unsafe. To reduce water consumption required for cleaning purposes as much as possible. To lower the production of polluting solids for the environment as well as energy consumption.

Perishable cold stores usually present a challenge for the sanitation process, as they are naturally humid and frost will build up on the evaporators. In other words the dark and wet environment of the cold store is an optimum breeding ground for mold and bacteria. Accordingly the humid conditions encourage microbial growth on the evaporator surface, especially Listeria. Also, unfiltered return air will accumulate mold spores and bacteria on the moist evaporator coil surfaces and therefore can be transferred to the produce via the recirculated air, causing cross-contamination. In order to eliminate this possibility, it is recommended that the fin spacing within the evaporator surface be not more than 2.5 cm to help in accumulating the smallest amount of dust and mud. The use of ultraviolet irradiation in fresh produce cold store evaporators is a well-established practice to overcome this

433

12.11 HYGIENE AND SANITATION

problem. For the evaporators, it should be reachable for washing and sanitation on a weekly basis, where continuing cleaning of the evaporators and condenser surfaces can also help stabilize temperatures and extend the life time of the refrigeration system. For an effective cleaning-up scheme of fresh produce cold stores, there is a need to follow certain measures in a particular order: cleaning, sanitizing, washing, and dehydrating. The practices and materials applied to implement each step must be adjusted to treatment systems and cold store elements such as handling systems for fresh produce, floors, ceiling, walls, doors steel structure, and light fixtures, in addition to the constituents of the cooling systems and other treatment. Table 12.1 presents the procedures and materials required for each element of the store and intervals between these procedures.

12.11.1 Procedures of Sanitation Visual inspection of all incoming ingredients, packaging, and pallets for signs of infestation is essential, and any of these that are not free of dirt, litter, bird, insect, or rodent contamination should be rejected. Inspection should include the condition of delivery trailers for signs of pests. Loading dock areas should be kept clean inside and outside, especially around dock levelers, if they exist. Cleaning (Fig. 12.29) is the first step in maintaining a fresh produce store. Trash on the floor can generally be purged simply by sweeping. Conversely, robust brushing may be required TABLE 12.1

Disinfestation Procedures and Materials Used of Fresh Produce Cold Store

Element

Kits

Cleaning/ Sanitation Process

Walls and corners

Easy nylon brush, then pressurized hose if needed

Roofing or ceiling, including lighting

Cleaning Items

Regularity

Foam, sweep, rinse

Chlorine-quaternary ammonium based cleaner

Once per month

Easy nylon brush, then pressurized hose if needed

Foam, sweep, rinse, wipe up

Chlorine-quat-based cleaner

Once per month

Floors

Rigid-bristle broom (or straw for in-farm storage systems), floor scrubbers, low-pressure hose

Wash, rinse, wipe up

Chlorine-quat-or iodine-based cleaner

Daily

Insulated and noninsulated doors

Sponge, scouring pad, or cloth

Foam, brush, rinse

Chlorine-quat-based cleaner

Once per week

Hoist, overhead light fixtures

Cleaning pad

Wipe up, clean

Water, soft soap

Once per quarter

Evaporative coils

Pressurized hose

Rinse, sanitize

Water, sanitize with quat

Once per quarter and after tuber storage season

434

FIG. 12.29

12. STORAGE SYSTEMS

Cleaning process of the storage structure.

when debris has adhered to the walls, ceilings, and doors. Careful dusting is similarly needed to facilitate the subsequent steps. Vacuuming or wetting the surfaces are worthy methods of getting dust off them without spreading it into the surroundings. On the other hand, highpressure washing with a nonfoaming industrial detergent is often an effective method. For safety reasons, electrical arrangements that are not waterproof should be sheltered. Storage areas should be disinfected when no perishables are stored. In case of using corrosive substances, it is vital to shield electrical systems and metal piping especially if they are in use. Disinfecting the storage area should be done by using the right products using the suitable methods (Table 12.1). Exposing boxes, pallets, containers, and other equipment to the sun or strong wind will moderately disinfect them. Particular disinfectant materials give off odors that may contribute unfavorable flavor to perishables produce. Additionally, some of these chemicals can corrode both the metal materials constructing the warehouse as well as the storage equipment. Consequently, appropriate rinsing is essential when specific disinfectants are applied, as shown in Table 12.1. Rinsing involves comprehensively spraying all surfaces treated with disinfectants, first we apply rinsing to the elevated elements so that the deposits are flushed down towards the floor. Stagnant water must be purged completely. Potable water of drinking quality should be used so that the warehouse is not recontaminated. Drying is the final principal step in a perishable warehouse clean-up process. The aim is to exclude humidity, which offers ideal situations for the growth of molds and rot. If the disinfectant applied does not necessitate rinsing, then drying initiates directly after disinfecting; otherwise, it should be applied after the rinsing phase. This step involves removing excess water and allowing the storage areas a good quality airing. If outside surroundings are clean, hot, and dry, then the outside air can be induced inside the store for drying purpose.

12.11 HYGIENE AND SANITATION

435

Preserving a high temperature as well as low humidity rate inside the perishable warehouse can reduce the evolution of mold and bacteria. The intervals at which the warehouses need to be cleaned depend on the type of storage and intensity of its use. For short-term storage, such as for berries or stone fruits, hygiene procedures should be continuously in progress, as these produces are very perishable and encourage the growth of saprophytic microorganisms. Debris building up should not be allowed in view of the fact that it generates a constant source of contamination, which, if not excluded by preventive measures, can result in large economic losses. For the long-term storage of commodities, such as apples, potatoes, onions and carrots, cleaning should begin instantaneously after the storage season. All other phases involved in cleaning up the perishable store should be conducted before the beginning of each season, specifically if there were losses caused by microorganisms during the previous storage period. It is vital to accurately define the cause of contamination present in the store, which helps in establishing the preventive measures to be taken. In this case the level of contamination and its sources must be determined so that the cleaning process facilitates disinfectants with the appropriate detergents. The cleaning and disinfection process is very easy if the warehouses are designed and built in a manner that is easy to wash and is compatible with disinfection. Executing a perishable store clean-up process will not solve all the problems, but conducting such measures will significantly reduce losses, especially those caused by microorganisms. All these steps are indispensible to the warehouse clean-up process. Once the warehouse has been thoroughly cleaned, it is important not to recontaminate it with working boots and soil or agricultural machinery brought from the field. Similarly the store should not be recontaminated by raw produce coming directly from the field or by allowing any field equipment to enter the cold store from field such as containers or forklifts. Cleaning and disinfection systems should be monitored for effectiveness and should be repeatedly reviewed and adjusted to reflect changing situations. Cleaning procedures should include the removal of debris from the store surfaces, doors, and accessories; the application of a detergent solution; rinsing with water; and, where appropriate, disinfection. Optimum preparation must be done for the storage and removal of any waste. Waste must not be allowed to accumulate in the storage areas or the adjoining environment. Cleaning materials being used for cleaning should be specifically identifiable and kept or stored separately in secure storage facilities; they should be used according to manufacturer’s instructions for their intended purpose. Some sterilization systems are used for fresh produce cold stores, where ozone is used as an alternative to chlorine to reduce the spread of diseases during storage, as well as to preserve the value of products and the accompanying disinfectant of all insects, parasites, and bacteria leading to a prolonged storage life. The waste sanitation and disposal systems of the facilities have to be designed and located in such a manner to avoid any risk of contamination. It must be disposed away from the supply of potable water. Waste accumulation should not be allowed next to the raw or finished product or inside the warehouses and have to be disposed of permanently. Cleaning operations in accordance with a schedule, with sound operating standards and strict compliance with all requirements and safety precautions, contribute greatly to reducing economic losses and also contribute to the maintenance of these facilities for long periods of time, thus increasing their economic returns.

436

12. STORAGE SYSTEMS

12.12 FIREFIGHTING Fighting a fire inside a perishable warehouse space can be extremely difficult, as perishables are sensitive to smoke and can get contaminated even by a small amount of it, which can cause a large monetary loss. If present in the air, moisture will condense and accumulate in the interior of the piping, which can form an ice plug. Ice plugs are often found in the main feeding refrigerant line, where the pipe enters the store, representing an additional challenge. For nonrefrigerated storage systems, materials and methodology used for construction can be considered self-resistance walls that rarely evolve fires. Accordingly, such a system has no hazard sources to initiate or encourage fire. As a precaution, fire extinguishers of the correct type should be accessible in all buildings and should be adjusted and evaluated for functioning on a regular basis. Water is commonly used for firefighting, but sand or sandy soils are more efficient for extinguishing certain types of fires. Insulated cold storage with polyurethane panels, which are flammable, have all the typical exposures found in any warehouse, as well as many hazards. These challenges can be managed with an awareness of the special hazards introduced by insulation and the need for special sprinkler protection systems that are appropriate for protection. Fire loads associated with the storage of fresh produce are mainly due to cardboard packaging materials. Combustible materials also include wooden pallets, plastic wrapping, and other packaging materials. Insulated sandwich panels with combustible core materials also need to be accounted for the fire load density assessment, as this can vary drastically from noncombustible mineral wool core to highly combustible polystyrene. Generally the stored produce itself represents a lesser fire risk. In all cases, insulated panels with optimum fire rating should be selected. Cold stores should have sufficient escape openings to the outside. As most of the insulated panels are manufactured form petrochemical materials, dry powder or foam extinguishers are best for fire that originates from these sources and similar ones, including diesel, oil, and electrical sources. Ordinary fire detection equipment is generally not suitable for the cold environment found in cold storages of perishable produce. The use of aspirating smoke detection systems is considered the most appropriate for these conditions. The detection unit is located outside the cold storage and draws air through a network of pipes inside the storage compartment in order to detect smoke particles.

12.13 CONCLUSIONS Different alternative fresh produce storage systems have been developed over the years for both on-farm and nonrefrigerated methods that especially suit developing countries, which have some challenges with electricity supply, particularly in the remote areas. Theory and operation principles were discussed along with hygiene and cleaning requirements. The chapter introduced different handling methods inside the perishable stores that save energy and minimize loss and waste. Several choices are available for different circumstances. The selection criteria for each method were discussed based on its advantages and disadvantages.

FURTHER READING

437

Further Reading Abera, G., Haile, D., 2015. Yield and nutrient concentration of Anchote [Coccinia abyssinica (Lam.) Cogn.] affected by harvesting dates and in-situ storage. African J. Crop. Sci. 3 (5), 156–161. Bartsch, J.A., Blanpied, G.D., 1984. Refrigerated and controlled atmosphere storage for horticultural crops. In: Paper No. NRAES-22. The Northeast Regional Agricultural Engineering Services, Cornell University, Ithaca, NY. Bevan, J.R., Firth, C., Neicho, M., 1997. Storage of organically produced crops. In: The Henry Doubleday Research Association Ryton Organic Gardens. MAFF, Coventry, CV8 3LG, pp. 5–8. Catholic Relief Services, 2014. Introduction—Improved Seed Storage Briefs. Catholic Relief Services, Nairobi. https://www.crs.org/sites/default/files/tools-research/seed-storage-briefs.pdf. Cramer, M.M., 2013. Food Plant Sanitation, Design, Maintenance, and Good Manufacturing Practice, second ed. CRC Press, Taylor & Francis, Group, New York. Dubey, N., Raman, N.M., 2016. Effect of CoolBot cool room on shelf-life of cabbage and cauliflower. Am. Int. J. Res. Formal, Appl. Nat. Sci. 13 (1), 66–69. Elansari, A.M., Siddiqui, M.W., 2016. Recent advances in postharvest cooling of horticultural produce. In: Siddiqui Wasim, M.W., Ali, A. (Eds.), Postharvest Management of Horticultural Crops Practices for Quality Preservation. Apple Academic Press, Oakville. Elansari, A.M., Yahia, E.M., 2012. Cold Chain for Perishable Foods (in Arabic). FAO Regional Office for the Near East and North Africa, Cairo. http://neareast.fao.org.  AAbdalla, N., Taha, H.S., Fa´ri, M., 2015. Postharvest management of fruits El-Ramady, H.R., Domokos-Szabolcsy, E., and vegetables storage. In: Sustainable Agriculture Reviews. Springer International Publishing, Cham, pp. 65–152. Mrema, G.C., Gumbe, L.O., Chepete, H.J., Agullo, J.O., 2011. Rural Structures in the Tropics Design and Development. Food and Agriculture Organization of the United Nations, Rome. Munster, E.S., 2001. Methods and technologies in potato storage. Landtechnik 5, 328–329. Palou, L., Crisosto, C.H., Smilanick, J.L., Adaskaveg, J.E., Zoffoli, J.P., 2002. Effects of continuous 0.3 ppm ozone exposure on decay development and physiological responses of peaches and table grapes in cold storage. Postharvest Biol. Technol. 24 (1), 39–48. Pringle, B., Bishop, C., Clayton, R., 2009. Disease control in store. In: Potatoes Postharvest. CAB International, Oxfordshire. Schweige, J., Potts, M., Keith, D., Santibanez, M., Neukirchen, M., Yari, F., Be, M., 2013. Increasing potato yields through improved potato storage pits. In: UCDavis Manual. http://eafghanag.ucdavis.edu/other-topic/ postharvest/potato-cool-storage-manual. Thorsteinsson, A.R., Tomasson, B., 2016. Performance-based fire safety design of cold storages. In: SFPE’s 11th International Conference on Performance-Based Codes and Fire Safety Design Methods. http://www.efla.is/media/ images/UtgefidEfni/Abstract_Performance-based_fire_safety_design_of_cold_storages.pdf.