UNIVERSITY OF GAZİANTEP FACULTY OF ENGINEERING DEPARTMENT OF FOOD ENGINEERING
JUNE , 2002
CONTENT
ACKNOWLEDGEMENT
I wish to express my gratitude to Assos. Prof. Dr. Medeni MASKAN for his interest, direction and guidance throughout the preparation of this project.
ABSTRACT
Fruits and vegetables are the most important part of human diet and they are main sources of some minerals and vitamins for nutrition. But they are easily spoilaged by microorganisms and also enzymatic activity.Therefore, storage of fruits and vegetables have an importance in food industry. Their quality and nutritional values can be preserved by different storage method.
Different storage methods for fresh fruits and vegetables were studied in this project.
ÖZET
Bazı mineral ve vitaminlerin ana kaynağı olan meyve ve sebzeler insan beslenmesinde önemli bir yere sahiptirler. Fakat mikroorganizmalar ve enzimlerin faliyetleri tarafından kolayca bozulabilmektedirler. Bundan dolayı meyve ve sebzelerin depolanmalarının gıda endüstrisinde büyük önemi vardır. Kaliteleri ve besin değerleri değişik depolama teknikleriyle korunabilirler.
Bu projede taze meyve ve sebzelerin depolanmaları için değişik depolama teknikleri çalışılmıştır.
CHAPTER 1
PRINCIPLES OF FRESH FRUIT AND VEGETABLE STORAGE
1.1 General Information
Storage of fresh fruits and vegetables prolongs their usefulness and, in some cases, improves their quality; it also controls a market glut. The principal goal of storage is to control the rate of transpiration, respiration, disease, and insect infestation and to preserve the commodity in its most usable form for the consumer. Storage time can be prolonged by harvesting at proper maturity, control of postharvest diseases, regulation of atmosphere, chemical treatments, irradition, refrigeration, controlled and modified atmospheres and by several other treatments. The main goals of storage are;
●to slow the biological activity of fruits and vegetables without chilling injury
●to slow the growth of microorganisms
●to reduce transpirational losses [1].
In practice, it is noticed that the quality of fresh harvest products varies considerably; even for products of the same batch. In addition, during storage several types of random fluctutions occur which affect the quality of the stored products and result in a large variability during commercialisation. The accumulation of small uncertainties also results in direct economic consequences for the postharvest industry and their auctions. In parallel, an economic side-effect can occur because the uniformity of the quality of the end products is not guarantueed [2].
1.2 Factors Effecting Storage
1.2.1 Temperature
Storage temperature should normally be maintained at the desired temperature for commodities being stored. Temperatures below the optimum range for a given fruit or a vegetable will cause freezing or chilling injuries, temperatures above, depending upon produce, will reduce storage life. A wide temperature fluctuation can result in rapid weight and water loss depending upon maturity of produce.
1.2.2 Relative Humidity
For most perishable fresh fruit and vegetables, the relative humidity should be maintained between 90 to 95%. The relative humidity below this range will result in a moisture loss from the produce. Thus the produce will be shrivelled and limp. Relative humidity if higher than 90% may cause excessive growth of microorganisms. A 5 to 10% loss in weight of produce results in shrivelling, which makes the produce look stale and unattractiveto sell. By using high relative humidity during storage, care must be taken to prevent the growth of surface microorganisms.
1.2.3 Atmospheric Composition
As the perishable fruits and vegetables undergo respiration, they consume O2 and release CO2. This effect can be successfully used to control the desired concentration of these gases in storage.The atmospheric composition in a storage room is controlled by addition of gases allowing the commodity to produce or consume gases or by physically or chemically removing undesirable gases from the storage room. High concentration of undesirable gases are removed by scrubbing devices. For example, CO2 can be absorbed in water or lime; C2H4 and other volatiles can be removed by potassium permanganate, catalytic oxidition or UV light; and O2 can be removed by using it in a combustion process or by a molecular sieve. In certain cases, external concentrations of gases are desirable and the accumulated gases can be adjusted by ventilation.
1.2.4 Light and Other Factors
Exposure of vegetables, such as potato, to light in storage can synthesize glycoalkaloids which are toxic to humans. Likewise, other factors such as herbicides, fungicides, pesticides and growth regulators may affect the produce and may have harmful affects on humans [1].
1.3 Plant metabolism
1.3.1 Respiration
Various substrates used in important synthetic metabolic pathways in the plant are formed during respiration. Aerobic respiration (for the sake of simplicity, the word respiration will be used throughout this paper to designate aerobic respiration) consists of oxidative breakdown of organic reserves to simpler molecules, including CO2 and water, with release of energy. The organic substrates broken down in this process may include carbohydrates, lipids, and organic acids. The process consumes O2 in a series of enzymatic reactions. Glycolysis, the tricarboxilic acid cycle, and the electron transport system are the metabolic pathways of aerobic respiration.
The ratio of CO2 produced to O2 consumed, known as the respiratory quotient (RQ), is normally assumed to be equal to 1.0 if the metabolic substrates are carbohydrates. The total oxidation of 1 mol of hexose consumes 6 mol of O2 and produces 6 mol of CO2. If the substrate is a lipid, the RQ is always lower than unity, because the ratio between C and O in lipids is lower than the ratio in carbohydrates. If the substrate is an acid, the RQ is higher than unity. Therefore, normal RQ values in the literature are reported as ranging from 0.7 to 1.3 The RQ is much greater than 1.0 when anaerobic respiration takes place. In fermentative metabolism, ethanol production involves decarboxylation of pyruvate to CO2 without O2 uptake.
1.3.2 Factors affecting respiration rate and respiratory quotient
The internal factors affecting respiration are type and maturity stage of the commodity. Vegetables include a great diversity of plant organs (roots, tubers, seeds, bulbs, fruits, sprouts, stems and leaves) that have different metabolic activities and consequently different respiration rates. Even different varieties of the same product can exhibit different respiration rates ). In general, non-climacteric commodities have higher respiration rates in the early stages of development that steadily decline during maturation Respiration rates of climacteric commodities also are high early in development and decline until a rise occurs that coincides with ripening or senescence.
Wounding plant cells and tissues causes the respiration rate to increase. Wounding induces elevated C2H4 production rates, that may stimulate respiration and consequently accelerate deterioration and senescence in vegetative tissues and promote ripening of climacteric fruit The wounding may be due to mechanical damage or cutting of the product. The respiration rate may gradually increase over time until a maximum value is reached and then start decreasing again to either the value before the wounding or to a higher value
.Temperature has been identified as the most important external factor influencing respiration. Biological reactions generally increase two or three-fold for every 10 °C rise in temperature within the range of temperatures normally encountered in the distribution and marketing chain. At higher temperatures, enzymatic denaturation may occur and reduce respiration rates. If temperatures are too low, physiological injury may occur, which may lead to an increase in respiration rate Other external factors are O2 and CO2 concentrations.
Respiration is widely assumed to be slowed down by decreasing available O2 as a consequence of reduction of overall metabolic activity The reduction of respiration rate in response to low O2 levels is not the result of the cytochrome oxidase activity, which has great affinity to O2, but due to a decrease in the activity of other oxidases, such as polyphenoloxidase, ascorbic acid oxidase and glycolic acid oxidase, whose affinity is much lower The influence of CO2 is not so clear in the process, and depends on type and developmental stage of the commodity, CO2concentrations and time of exposure [3].
1.4 Fresh Fruit Storage
Fruit for fresh storage have to be autumn or winter varieties and be harvested before they are fully mature. This fruit also has to be sound and without any bruising; control and sorting by quality are mandatory operations.Sorting has to be carried out according to size and weight and also by appearance; fruit which is not up to standard for storage will be used for semi-processed product manufacturing which will be submitted further to industrial processing.Harvested fruit has to be transported as soon as possible to storage areas. Leaving fruit in bulk in order to generate transpiration is a bad practice as this reduces storage time and accelerates maturation processes during storage.
Once fruit is harvested, any natural resistance to the action of spoiling micro-organisms is lost. Changes in enzymatic systems of the fruit also occur on harvest which may also accelerate the activity of spoilage organisms.
Means that are commonly used to prevent spoilage of fruits must include:
care to prevent cutting or bruising of the fruit during picking or handling;
refrigeration to minimise growth of micro-organisms and reduce enzyme activity;
packaging or storage to control respiration rate and ripening;
use of preservatives to kill micro-organisms on the fruit.
A principal economic loss occurring during transportation and/or storage of produce such as fresh fruit is the degradation which occurs between the field and the ultimate destination due to the effect of respiration. Methods to reduce such degradation are as follows:
refrigerate the produce to reduce the rate of respiration;
vacuum cooling;
reduce the oxygen content of the environment in which the produce is kept to a value not above 5% of the atmosphere but above the value at which anaerobic respiration would begin. When the oxygen concentration is reduced within 60 minutes the deterioration is in practice negligible [4].
1.5 Fresh Vegetable Storage
The vegetables can be stored, in some specific natural conditions, in fresh state, that is without significant modifications of their initial organoleptic properties. Fresh vegetable storage can be short term; this was briefly covered under temporary storage before processing. Also fresh vegetable storage can be long term during the cold season in some countries and in this case it is an important method for vegetable preservation in the natural state.
In order to assure preservation in long term storage, it is necessary to reduce respiration and transpiration intensity to a minimum possible; this can be achieved by:
•maintenance of as low a temperature as possible (down to 0° C),
•air relative humidity increased up to 85-95 % and
•CO2 percentage in air related to the vegetable species.
Vegetables for storage must conform to following conditions: they must be of one of the autumn or winter type variety; be at edible maturity without going past this stage; be harvested during dry days; be protected from rain, sun heat or wind; be in a sound state and clean from soil; be undamaged.
From the time of harvest and during all the period of their storage vegetables are subject to respiration and transpiration and this is on account of their reserve substances and water content. The more the intensity of these two natural processes are reduced, the longer sound storage time will be and the more losses will be reduced.
For this reason, vegetables have to be handled and transported as soon as possible in the storage conditions (optimal temperature and air relative humidity for the given species). Even in these optimal conditions storage will generate losses in weight which are variable and depend upon the species [4].
1.6 Handling of Fruits and Vegetables
Much of the success or failure in storing fruits and vegetables depends on having sound material at the start. All cut, bruised, frosted or otherwise damaged or mis-formed specimens should be rejected or placed aside for immediate consumption. Furthermore, the stored material should be examined frequently during storage. If wilting is noted, steps should be taken to increase the humidity either by sprinkling the floor of the storage or moistening the sand in which the root crops may be buried. If rot or any form of breakdown or other disease is noted, no matter how slight, it is better that affected specimens be rejected or at least separated from the sound material. If left in contact with sound fruit, the healthy specimens are likely to become tainted or otherwise affected. What is more, rotting vegetation develops heat, making temperatures more difficult to control. Another important factor to keep in mind is that as soon as the fruits or vegetables are harvested and have been sufficiently prepared for storage they should immediately be reduced to storage temperatures. At higher temperatures, vegetation ages more than at lower temperatures. Thus, if cooled immediately, the storage life of the product is prolonged [5].
1.7 Storage Operations
Storage operations have evolved into skilled methods of efficiency with a wide range of variations depending upon the existing facilities, including nature, variety and quantity of produce to be stored.
Storage operations may be either temporary, short-term or long-term. Temporary storage operations are needed for highly perishable produce which requires immediate marketing. It may be installed with or without refrigeration. Temporary storage is extremely important for roadside stands, gardens, markets, railway stations, shipping yards and retail stores. The mid-term storage operation is aimedat checking the market glut without product deterioration. This may extend from 1 to 6 weeks depending upon the need, kind and maturity of the produce. Banana, cabbage, tomatos and cauliflower are transferred to short-term storage rooms, when their quality is still good, and held there until a resonable market price is attained. Fruits and vegetables like apples, oranges, pears, squash, potatoes, sweet potatoes, carrots, onions and garlic require long-term storage. Its operations are mainly influenced by economic factors. The produce is stored during their periods of production and sold continuously during the rest of the year when producers and dealers can obtain reasonably high prices.
Storage operations may be classified as either natural or artificial. The natural storage operation keeps the produce in situ without any treatment, whereas artificial storage may be further classified into four types;
•mechanical or structural
•controlled atmosphere
•chemical
•radiation
In the case of natural storage, the main purpose is to let the fruits and vegetables mature and ripen on plants as long as possible. On the other hand, artificial storage operations attempt to provide conditions to prolong the produce quality.
1.7.1 Natural Storage
Vegetable such as potato, sweet potato and garlic are kept underground for several months. They are harvested prior to the rainy season for the better market price. This harvesting does not involve extra expenditure and building for storage.
1.7.2 Artificial Storage
Pits or trenches are dug underground for storing beets, potatoes, onions, carrots, cabbages and sweet potatoes where they are covered with straw and soil until there is a market demand.
1.7.3 Ventilated Storage
Cellars are underground rooms with slanting roofs covered with sods and soils. Cellars are provided with heaters and dry atmospheres during winter months. Potatoes, carrots, and beets are stored with high relative humidity at 1.7- 4.4oC. Ventilation is essential for good storage.
This storage has several advantages over other types;
•special construction is not needed
•produce is easily handled
•grading, storing, packaging of fruits and vegetables is facilitated
•air may be humidified
•fans can be controlled manually or automatically with a thermostat.
1.7.4 Ice Refrigeration
An advence on the above-ground warehouses was the use of ice as a refrigerant. Lower temperatures can be obtained enable longer storage of perishable fruits and vegetables. However, removal of melted ice water is a disadvantage.
Storage does not improve the quality of any food. The quality of a food will also not decrease significantly during storage as long as the food is stored properly and used within the recommended time frame. Maintaining a food’s quality depends on several factors: the quality of the raw product, the procedures used during processing, the way the food is stored and the length of storage [4].
CHAPTER 2
COLD STORAGES OF FRUIT AND VEGETABLES
2.1 Principles of Cold Storage
The cold storage of fruit and vegetables has advanced noticeably in recent years, leading to better maintenance of organoleptic qualities, reduced spoilage and longer shelf-lives [3].
Low temperatures provide many advantages to extend shelf-life by;
• reducing respiration and internal breakdown by enzymes
• reducing water loss and wilting
• slowing the growth of disease organisms
• reducing the production of ethylene
• providing “time” for proper handling, packaging and marketing [6].
All fruits and vegetables have a specific temperature and relative humidity requirement for storage. Furthermore, same fruit and vegetables from different ecologic conditions may require different temperature and relative humidity. All fruits and vegetables have an definite shelf-life even though the best storage conditions havr been achieved. The products will loss their quality after a time which changes 5 days to 6 months. So shelf-life is limited in cold storage [7].
A success of cold storage mainly depends on temperature, relative humidity and air movement.
2.1.1 Temperature
The most important factor effecting cold storage is the temperature. The optimum storage temperature for most agricultural products is one slightly higher than the freezing point of the product and it is highly important that the temperature be held fairly constant in order to obtain good results, since large temperature fluctuations and variations tend to promote premature product deterioration [8].
Since the temperature is 1-2oC above the freezing point, the product never freeze in cold storage. This is the main differences of this technique from freezing in which fruits and vegetables are stored at –20o or lower at frozen state [7].
2.1.2 Relative Humidity
Relative humidity is the percentage of saturated water vapor at a given temperature. Relative humidity (%) can be determined from psychrometric charts on the basis of wet-bulb and dry-bulb temperatures. As the temperature ofn air increases so does its water-holding capacity.As the relative humidity of air decreases, so does its vapor pressure and as vapor pressure decreases, the capacity of the air for removing water from moist sources increases [9].
when the relative humidity is larger than the equilibrium relative humidity value, an increase in the ventilating air rate reduces the losses of the product during the period of its storage while larger losses occur when the relative humidity values are lower than the equilibrium oneFurthermore, when the relative humidity was larger than the equilibrium value, an increase in the ventilating airflow rate reduces the natural losses of the product during the period of its storage. Whereas when the relative humidity values are lower than the equilibrium one, an increase in the airflow rate causes larger losses of the stored product [8].
2.1.3 Air Movement
Air movement must be sufficient enough to remove respiration heat. It essential that all parts of the cold storage room are subjected to a uniform flow of air. This is accomplished by proper placement of blowers or ducts and stacking of fruits and vegetables to permit free air flow.The capacity of a cold storage room is based upon adding all the heat inputs including;
•heat conducted through walls, floor and ceiling.
•field and respiration heat of fruits and vegetables.
•heat from air filtration
•heat from equipment such as light, fan, forklift and personnel moving in and out [9].
2.2 Changes During Cold Storage
2.2.1 Chemical Changes
Since the harvested fruit and vegetables are living tissue, they are exposed to some chemical changes during storage in cold rooms. This changes can be slow down by means of low temperature, but can not be stopped completely.
High molecular weight carbohydrates, such as starch, are hydrolysed to sugars which are used in respiration. Partial hydrolysis can be seen in proteins. Break down of pectic substances cause softening of tissue. Discoloration can be seen due to chlorophyll degredation.Such changes causes some taste, color and aromatic problems.So fruits and vegetables are subjected to spoilage and stailed [3].
2.2.2 Storage Injurious
Some injuries may arise when fruit and vegetables are stored under the critical temperature which is depend on the types of product. Not only this critical temperature but also storing time are important for injurious. In general, fruit and vegetables are exposed to cold ınjurious under 8 to 12oC. Some symptoms are darkening in tissue and outer surface, ripening defects and similar abnormalities. Storage injurious do not only arise from cold temperature, but also higher relative humidity and leakage refrigerants [7].
2.3 Cold Storage Operation
Cold rooms are now designed to cool down produce more rapidly and keep it within narrower temperature and humidity ranges. Among the specific developments: refrigerating capacity per unit volume has risen, cold rooms have become smaller but more numerous, screw compressors have gradually replaced reciprocating compressors, and installations run much more smoothly, thanks to more accurate and reliable sensors [3].
2.3.1 Storage Construction
Independent of the type of cold storage selected, there are several construction factors to be considered when cold storage is needed. These factors are site selection, size of the storage, structure and thermal insulation
2.3.2 Site Selection.
Marketing strategy will determine the type of cold storage and its location. For example, for retail sales, the cold storage and sale outlet must be close to a major road, and the area must have some parking space available. Also, there must be some space available for the movement and storage of empty contain-ers, equipment, and supplies. Ideally, an area should be planned for any future expansion. The cold storage needs to be placed in a well-drained area. It needs drains to remove water from condensation, cleaning and sanitation operations Availability of water is critical. Water is necessary for preservative solutions and for cleaning and sanitizing the cold storage and handling area. The water demand must be satisfied and the quality must be adequate. Also, wastewater disposal must be considered.
2.3.3 Size.
To define the size of the cold storage, evaluate the following factors:
• Volume of product to store
• Product containers (boxes, hampers, buckets)
• Volume required per container
• Aisle space needed (mechanical or manual operation)
• Lateral and head space
• Available site space
2.3.4 Refrigeration Load.
Once the type and size of cold storage has been determined, the refrigeration needs or
load must be calculated according to the product and storage needs. The refrigeration load is critical to main-tain the desired product storage temperature. Some factors determining the refrigeration load are:
• size of the cold storage
• type of product stored
• amount of product stored
• temperature of product coming into the cold storage
• optimum storage temperature for the product
• type and amount of insulation used in the cold storage
• location of the cold storage
• characteristics of the refrigeration equipment
• management practices used in the operation of the cold storage
2.3.5 Field Heat
Field heat is necessary to reduce the product temperature from harvest temperature down to the safe storage temperature within a given time period. The hotter the product coming into the cold storage, the more energy needed to reduce the product temperature and the more time required to operate the refrigeration equipment.
2.3.6 Heat of Respiration
The heat of respiration is the energy released by the produce as it respires during storage. The warmer the produce, the more heat of respiration generated.
2.3.7 Conductive Heat Gain.
The conductive heat gain is heat gained by conduction through the building floor, walls and ceiling. It is directly related to the insulation installed in the facility. The better insulated the cold storage, the less conductive heat gain.
2.3.8 Convective Heat Gain.
The convective heat gain is heat that enters the facility during the mixing of other air with the cool inside environment. This load is directly related to the number of doors in the facility. The more doors in the facility, the higher the possibility of air currents. The amount of heat gained increases with the amount of times and periods that doors are open.
2.3.9 Equipment Load.
Equipment operating in the storage, such as fans and lights, generate additional heat load. Lighting does not need to be excessive, just bright enough to identify product and labels clearly and allow safe movement.. Field heat is the amount of cooling
2.3.10 Refrigerants
New regulations governing the use, disposal, and manufacturing of refrigerants require close attention for users of cold storage. Refrigerants are chosen for their heat exchange properties and cost. Among the best are chlorofluorocarbons (CFC) and hydrochlorofluorocarbons (HCFC), but both types damage the ozone layer and CFCs contribute to global warming
2.3.11 Maintenance
Equipment maintenance, monitoring, and management are integral parts of all the activities. More specifically, the aspects that need special attention when managing a cold storage system are condensation and humidity control, temperature control, sanitation, maintenance, container design and positioning, and storage compatibility of commodities.
2.3.12 Condensation and Humidity Control.
The relative humidity in refrigerated storage must be within the optimum range for the commodity. When relative humidity for fresh plant material is less than 90 percent, a humidifier should be used
2.3.13 Temperature Control.
A thermostat is an integral part of the cold storage. The thermostat controls the operation of the refrigeration unit so a preset temperature can be maintained in the storage. The thermostat’s thermometer should not be used to monitor the storage temperature. A separate recording thermometer should be installed for this purpose. This type of thermometer will be helpful in determining whether the cold storage is maintaining ideal conditions. A thermometer that fixes the maximum and minimum temperatures could be an alternative for the recording thermometer
2.3.14 Sanitation.
Every commodity requires special sanitation procedures. To prevent contamination, the cleanliness and sanitation of the cold storage and contain-ers must be assured. The removal of condensation water, dirt, residuals, and trash from the cold storage will help maintain flower quality and extend the life of the facility. If mold is present on the walls, floor or containers, they must be disinfected with a solution of 0.50 percent sodium hypochlorite.(6)
Table 2.1 Storage conditions of some fruit and vegetables [7]
Temp.(oC)
R.H.(%)
Storage Time(day)
Fruits
Pear
-1 – +2
90-95
60-180
Quince
0 – +2
-90
90
Strawberry
0 – +2
90-95
5
Apple
0 – +5
90-95
60-180
Plum
-1 – +1
90-95
7-35
Apricot
-1 – 0
-90
21
Cherry
0 – +2
90-95
14
Peach
-1 – 0
90-95
14-42
Grapes
-1 – -2
-95
7-42
Morella cherry
-1
90-95
7
Vegetables
Bean
0
90-95
21
Okra
+7 – +8
90-95
10
Pea
0
85-90
7-21
Pepper
+8 – +9
90-95
21
Tomatoes(half mature)
+12 – +15
85-90
21
Tomatoes(mature)
+8 – +10
80-85
4-6
Dill
-10
-95
28
Artichoke
0
90-95
42
Green bean
+7 – +8
90-95
10
Carrot
+1
-95
150-180
Cucumber
+7 – +10
90-95
10-14
Spinach
0
-95
14
Celery
-0.5
-95
49
Garlic
0
65-75
180-210
Cabbage
0 – +2
90-95
60-90
Mushroom
+1 – +2
-95
14
Parsley
-2 – -1
-95
56
Potato
+6 – +8
90-95
240
Egg-plant
+8 – +10
90-95
14
Leek
0
-95
60-90
Green onion
-2 – -1
-95
49
Dry onion
0
70-75
240
CHAPTER 3
Controlled and Modified Atmosphere Storages of Fruit and Vegetables
3.1 Principles of Controlled and Modified Atmosphere Storages
Controlled atmosphere storage (CA) and modified atmosphere storage (MA) techniques used in the long-term storage of fruit and vegetables using low oxygen atmospheres. These techniques indicate removal, or addition, of gases resulting in an atmospheric composition in fruits and vegetables that is different fromthat of air (78.08%N2, 10.95%O2, and 0.03%CO2). Usually this involves reduction of N2 or elevation of CO2 concentrations.
MA and CA differ in the degree of control ;CA is more exact than MA. MA is often used interchangeably with CA. While MA storage, e.g., packaging in film bags, also requires a decrease in O2 and an increase in CO2 or N2, there is no attemp to control the atmospheres at specific concentrations. They differ only in degree and comtrol methods [1].
The shelf-lifes of fruit and vegetables, which go on respiring long after they have been harvested, can be extended using ‘modified atmosphere packaging’ (MAP). Some gases pass through these plastics more easily than others. Thus, MAP allows the carbon dioxide given off by the produce to build up, while the level of oxygen falls. This slows down respiration, and keeps goods fresh.
Maximizing a product’s shelf-life is a matter of creating the right atmosphere. If oxygen levels become too low, or carbon dioxide levels too high, goods start to give off a nasty smell. Also, the produce and the atmosphere change over time — food scientists need to consider the whole process, not just the initial state [5].
This technology seems straightforward as it uses permeable films and the respiration rate of the product at a specific temperature to change the concentration of carbon dioxide and oxygen around the product. However many users under estimate the complexity of this seemingly simple packaging system [10].
Having a suitable storage strategy for a perishable crop after harvest is a key factor in bringing agricultural production through from the farm to a worthwhile retail outlet. Often the market window for best value to producers is narrow and measures need to be in place to prolong holding periods to avoid times of glut or to extend the period of crop availability. Such measures are also required for the successful transportation of produce without loss of nutritive quality or flavour over the long distances covered by international trade. Controlled atmosphere storage is a technology which offers the potential of providing a means of bridging the gap between economic success or failure in the marketing of perishable commodities. It is also a technique under investigation for disinfestation or storage life protection of the durable commodities covered in stored product research [11].
The main aim of modified atmosphere packaging (MAP) is to change the composition of the atmosphere around the product so that the storage life of the product can be extended. Most fruit and vegetables age less quickly when the level of oxygen in the atmosphere surrounding them is reduced. This is because the reduced oxygen slows down the respiration and metabolic rate of the products and therefore slows down the natural aging process [10].
The demand for quality, well-stored and chemical free produce is ever increasing. Once the atmosphere required for storage of the fruit and vegetables is established, the ability to accurately measure and monitor the levels of oxygen, carbon dioxide, temperature, humidity, ammonia and ethylene becomes critical [1].
The difficulty with using modified atmosphere packaging is the establishment of a stable atmosphere inside the plastic bag. MAP is a dynamic system that is not controlled. As currently used there is no feedback system that can cut in if one of the factors listed above changes. Therefore it is important to use MA packaging only as recommended by the manufacturer [10].
Initially, levels of carbon dioxide in controlled-atmosphere storages were maintained by simple venting to the ambient air. As the technique was refined, carbon dioxide needed to be controlled independently of oxygen. Scrubbers containing caustic soda (sodium hydroxide dissolved in water) were used to remove carbon dioxide from the storage air before it became too concentrated. However, these scrubbers were expensive to build and hazardous to operate. In the mid-1950s a water scrubber was developed. Although costly to build, this type of scrubber was not hazardous [2].
Modified atmosphere packaging is a cheap and convenient packaging system that has the capacity to extend the shelf life of some commodities if it is used properly. If the complexity of the MA packaging is understood then it is more likely that the product will be handled with the proper care. The risks of this seemingly simple packaging system must always be considered.
There is always a risk/benefit when using modified atmosphere packaging, particularly
when a low oxygen atmosphere is providing the benefit. The following diagram illustrates this point. The greatest extension of shelf life occurs at the lowest possible oxygen concentration before anaerobic respiration is initiated. This point also carries the greatest risk. For example, if the respiration rate increases as a result of a small change in temperature then the oxygen level will fall below the critical level and off flavours will be produced. The same is true for atmospheres where the main benefit is high carbon dioxide. If respiration increases due to an increase in temperature then the level of CO2 may rise above the critical level and the product will also be damaged and made unsaleable. There are two ways to minimise the risk of spoilage. Firstly you could use a package that provides slightly more oxygen, and so provides less benefit in terms of shelf life but the package would also have a reduced risk of spoilage. Secondly ensure that the cool chain is maintained. If you can’t guarantee the temperature then you will be taking a very big risk with this type of packaging system. If the temperature rises by more than a few degrees then damage could be avoided by opening the bags to ensure adequate oxygen for the product. This is not often feasible but some packers recommend that the MA bags are opened once the product arrives at the wholesale market to ensure there is no risk of spoilage.
Figure 3.1 The Risk/Benefit associated with using Modified Atmosphere Packaging [10]
While basically a simple system for commercial usage it is critical that considerable care is paid to several factors the most important of which is temperature control.The factor that causes most problems in a commercial situation is temperature. Unfortunately the cool chain for fresh produce is not always continuous throughout the marketing system. Breaks in the cool chain such as during loading or unloading of trucks or packing of warehouses mean that the cool product can warms up. Warming of only a few degrees can be enough to cause the respiration rate of the product to rise and the oxygen within the package to fall below the recommended level. If the oxygen level falls too low then anaerobic respiration can be initiated. If this happens alcoholic off flavours develop within the product, making it unmarketable [10].
Quality optimisation and loss reduction in the postharvest chain of fresh fruits and vegetables are the main objectives of postharvest technology. Temperature control and modification of atmosphere are two important factors in prolonging shelf life.
Modified atmosphere packaging (MAP) of fresh produce relies on modification of the atmosphere inside the package, achieved by the natural interplay between two processes, the respiration of the product and the transfer of gases through the packaging, that leads to an atmosphere richer in CO2 and poorer in O2. This atmosphere can potentially reduce respiration rate, ethylene sensitivity and production, decay and physiological changes, namely, oxidation.
MA packages should be carefully designed, as a system incorrectly designed may be ineffective or even shorten the shelf life of the product. The design should take into consideration not only steady-state conditions, but also the dynamic process, because if the product is exposed for a long time to unsuitable gas composition before reaching the adequate atmosphere, the package may have no benefit. The design of an MA package depends on a number of variables: the characteristics of the product, its mass, the recommend atmosphere composition, the permeability of the packaging materials to gases and its dependence on temperature and the respiration rate of the product as affected by different gas composition and temperature. Thus, respiration rate modelling is central to the design of MAP for fresh fruits and vegetables [12].
3.2 The Theory of MA packaging
When a given weight of produce is sealed within a plastic bag, it uses oxygen and produces carbon dioxide. As the oxygen concentration inside the package falls, below about 10% the rate of respiration (oxygen use) starts to decrease. At the same time, oxygen moves into the bag through the walls of the plastic bag and carbon dioxide moves out. Oxygen and carbon dioxide move across the film in proportion to the drop in concentration of oxygen and rise of carbon dioxide concentration inside the plastic bag.
This seems simple however the rate of oxygen consumed is dependent on the following
factors;
• The weight of the product in the bag
•The temperature and
• The respiration rate of the commodity. Respiration rate may vary among cultivars, seasons and growing conditions.
•The rate of oxygen and carbon dioxide movement through the wall of the bag The rate of oxygen movement through the plastic bag depends on the surface area, thickness and chemical properties of the plastic film. The permeability of the film can be increased by adding holes [10].
3.3 Biochemical Consideration
3.3.1 Retarded Respiration
Respiration is a metabolic process that provides the energy for plant biochemical processes. The CA storage can influence respiration at three levels:
•aerobic respiration
•anaerobic respiration
•a combination of these two
Aerobic respiration occurs when the O2 supply is normal and results in the liberation of CO2 and water. Anaerobic respiration, taking place in atmospheres completely devoid of O2, produces CO2 and ethyl alcohol fermentation [9].
Fruit continues to respire (use oxygen and produce carbon dioxide) while in storage, the higher the temperature the faster the respiration. In controlled-atmosphere storage, the level of carbon dioxide is increased to about 5 percent from 0.03 percent and the level of oxygen is reduced such that the sum of carbon dioxide and oxygen equals about 21 percent of the atmosphere. This alteration lowers the rate of respiration and so increases the storage life of the fruit.
Raising the level of carbon dioxide to levels of 2 % or more can also be beneficial. Elevated CO2 levels can reduce the products sensitivity to ethylene, it can also slow the loss of chlorophyll which is the green colour of fruit and vegetables. High CO2 can also slow the growth of many of the postharvest fungi that cause rots. All these effects can help to extend the storage and shelf life of fresh produce.
3.3.2 Acid Accumulation
Modifications in conventional atmospheres result in specific alterations in metabolism, and some of these induced changes appear to reflect in the improved quality of the fruit. The CO2 atmospheres seemed to check the normal decrease in acidity usually experienced in storage. This suggests that the possible reasons for acid accumulation during CA storage might be lowered respiratory activity, increased CO2 fixation, or the presence of a less active enzyme that converts malic acid to pyruvate or oxaloacetate. The CO2 concentrations in the atmosphere are increased, CO2 fixation also increase.
3.3.3 Acetaldehyde Formation
Acetaldehyde is formed in large quantities by cells of higher plants in certain mixtures of CO2 and O2. Often high concentrations of it are accompanied by browning of the cells.
3.3.4 Increase in Sugars
Many biochemical changes result from the use of CA. The CA-stored fruits contain more sugar after ripening. Conventional storage produce relatively no changes or increase in sugars.
3.3.5 Pectin Changes
Pectin changes correlate with changes in the storage atmospheres. The CO2 storage may have an effect on protopectin hydrolysis. The soluble pectin content is much higher where higher temperatures or no CO2 are involved. The rate of pectin degradation is affected by both time and conitions of storage.
3.4 Adverse and Toxic Effects of CA
Controlled atmosphere storage has great promise, but also can have adverse effects on fruits and vegetables. The biochemical constituents of fruit show changes in an unusual manner during CA storage. Injury to fruit tissue can occur from decay and abnormality of metabolism induced by high CO2 and low O2 concentrations. Some of these disorders are in the form of browning of the fleshy mesocarp, tissue breakdown, and the accumulation of certain organic acids.
Tissue browning is another common disorder of fruits. One of the main causes of this discoloration is biochemical changes in tannins. The tannins are a complex assemblage of polyhydroxy phenols which often have no close structural relationship. Most phenolic acids are glycosides or esters. During storage, these compounds are hydrolyzed; toxic products could be accumulated in the tissue, possibly resulting in loss of cells and serving as substrates for enzymatic browning reactions.
The presence of several different phenolases, peroxidases and other enzymes can produce discoloration. However, browning does not occur until the plant cell is ruptured.
3.5 Physiological Effects of O2 Concentration
3.5.1 Varied Ripening Rate
While low O2 invariably delays ripening, O2 levels higher than those found in air may or may not hasten ripening. At O2 concentrations higher than in air, no change is observed in avocados. An increase in O2 content of the atmosphere from 30% to 50% above that of normal air accelerated ripening of bananas [9].
Temperature is the most effective environmental factor in prevention of fruit ripening.Ripening and ethylene production rates increase with increase in temperature. To delay ripening, fruits should be held as close to 0oC as possible. Prevention or delay of ripening in fruits has an indirect effect on microbial decay, since ripe fruits are more susceptible to attack by postharvest pathogens [13].
3.5.2 Reduced Decay at Low O2 Levels
Levels of 5 to 0% O2 reduce the decay incidence consistently. Growth of fungi may be retarted also at low O2 concentrations, although, there seems to be variation in sensitivity of different fungal species. The development of some disease is related to O2 tensions.
3.5.3 Fermentation at Very Low O2 Levels
At low O2 concentrations, the CO2/O2 ratio may increase as a result of fermentative reactions and off-flavors develop in many fruits and vegetables. It is generally recommended not to use concentrations below 2%O2.
3.6 Physiological Effects of CO2 Concentration
When the CO2 level increases in the storage atmosphere, the quantity of CO2 dissolves in the cell or combines with some cellular constituents. High levels of CO2 within the cell generally leads to the following physiological changes:
•decrease in the synthetic reactions of ripening
•inhibition of some enzyme activities
•decrease in production of volatiles
•disturbed organic acid metabolism
•slackening in the breakdown of pectic substances
•inhibition of chlorophyll synthesis and fruit degreening
•alteration in the proportion of various sugars.
3.6.1 Off-flovor Production at High CO2
In some cases, off-flover may be due to an accumulation of ethanol and ethanal. Concomitant with the development of undesirable flavor, off-color may also be abserved.
3.6.2 Induction of Some Physiological Disorders
High CO2 may induce many kinds of tissue browning in fruits and vegetables. Harvesting early in the season and long storage periods at high temperatures predispose the fruits to high CO2 injury. In some cases even low CO2 concentrations may be dangereous if the duration of treatment is very long at rather low temperature.
3.6.3 Humidity and C2H4 During Storage
The presence of C2H4 and water vapor should be given special attention. The degree of ripening induction of C2H4 is a function of its concentration, temperature, stage of fruit development and atmospheric composition. To obtain maximum retardation of ripening, C2H4 should be removed from the storage rooms or sealed packages. Adding KMnO4 is one of the more effective means of absorbing C2H4. To hasten ripening or degreening after CA storage, C2H4 may be subsequently introduced and O2 concentrations simultaneously increased.Fungi may develop even if CO2 is in exess. Use of fungicides may be recommended under these conditions. An effective fungicide may obviate lowering of humidity during storage.
3.7 Long CA Storage Treatment
Injuries may occur even at the recommended O2 and CO2 levels if the length of the CA storage period is unduly prolonged. However, some fruits store well in reasonably good condition for long periods compared to those stored in air. The storage life may be 40 to60% longer than in air, and sometimes more, according to temperature and variety. Additional tests are needed to determine the best conditions for long CA storage.
3.8 Short CA Storage Treatment
It may be useful to know the limits of susceptibility of fruits to CO2 and O2 concentrations during short periods because it is then possible to use these conditions transport or as pretreatments prior to processing. Fruits may withstand high CO2 or very low O2 concentrations for a short time.
3.9 Potential Benefits of CA and MA Storages
•Retardation of senescence and associated biochemical and physiological changes
•reduction of fruit sensitivity to ethylene
•controlling certain physiological disorders
•controlling postharvest diseases and decaying by inhibiting pathogens and insects.
3.10 Potential Harmful Effects of CA and MA Storages
•start or increase of blackheart in potatoes, brown stains on lettuce, brown heart in apples and pears
•irregular ripening of bananas, pears and tomatoes due to low concentration of O2
•development of off-flavors due to low O2 causing anaerobic respiration
•increased susceptibility to decay due to low O2 and high CO2
•stimulation of sprouting and retardation of suberization delaying in periderm formation in potatoes.
Table 3.1 CA and MA Conditions for Storage of Fruits [9]
Atmosphere
Injurious atmosphere
Marketable life (day)
Fruits
Temp.(C)
%O2
%CO2
%O2
%CO2
Air
CA
Apples
0
3
3
1
5-
200
300
Apricots
0
1-2
2-3
–
–
14
50
Blueberries
0
10
11
–
–
14
42
Cherries
0
1-3
5-10
–
–
21
28
Nectarines
0
1-3
5
0.25
6
21
42
Peaches
0
1-2
5
0.25
6
21
56
Grapefruit
7
2-5
2-5
1
–
28
42
Lemons
15
3-5
0-5
–
6
130
220
Limes
10
5
7
1
–
21
42
Oranges
1
15
0
5
5
42
84
Mangos
13
5
5
1
6
14
21
Pears
0
2
1
–
2
200
300
Plums
0
1-7
0-7
–
–
21
28+
Raspberries
0
2-5
0-20
–
–
3
3+
Strawberries
0
4-10
0-20
1
–
7
7+
Table grapes
0
5
2-5
–
–
120
120+
Avocados
7
1
9
0.5
–
30
60
Bananas
13
2
5
1
8
21
60
Papayas
13
1-2
0
–
5
14
21
Pineapples
7
2-5
0
–
–
12
12+
Table 3.2 CA and MA Conditions for Storage of Vegetables [9]
Atmosphere
Injurious atmosphere
Marketable life(day)
Vegetable
Temp.(C)
%O2
%CO2
%O2
%CO2
Air
CA
Artichoke
0
2-5
–
1
–
21
21+
Asparagus
0-2
1-3
5-15
1
–
14
14+
Green bean
7
2-5
15
–
–
7
21
Broccoli
0
2-5
10
–
–
14
21
Brussels sprout
0
2-14
5-10
–
–
21
21+
Cabbage
0
1-2
5-10
–
–
90
180
Cauliflower
0
2-4
2
1
5
14
21
Cantaloupe
5
2
10
1
–
14
14+
Melon
10
2-5
–
–
–
14
14+
Celery
0
1-4
–
–
3
28
28+
Sweet corn
0
2-4
0-10
1
10
3
8
Cucumber
8
3-5
–
–
–
14
14+
Lettuce
0
2-5
0
–
4
21
21+
Mushroom
0
1-2
10-15
0.5
–
5
8
Okra
10
–
10-12
–
–
7
7+
Green onion
0
2-5
5-10
–
–
7
7+
Green pea
0
5-10
5-7
–
–
7
7+
Sweet pepper
13
3-5
2-8
2
10
14
28
Tomato
–
Mature green
13
3-5
0
2
2
21
42
Breakers
13
3-5
0
2
2
14
28
CHAPTER 4
STORAGE OF FRESH FRUITS AND VEGETABLES IN POLYMERIC FILMS
The rapid development of semipermeable and perforated polymeric films and the growing use of packaging materials for prewrapping have led to consideration of establishing “controlled atmosphere” fresh fruit and vegetable packages. The studies for fresh fruit-vegetable packaging centerd on reducing moisture loss, providing protection from mechanical damage and improving produce appearance.
4.1 Fresh Produce Package System
A produce package is a dynamic system in which two main processes, respiration and permeation, all occur simultaneously. There is an uptake of O2 by the produce and evolution of CO2 ,C2H4 , H2O and other volatiles; at the same time, specific restricted permeation of these gases occurs through the packaging film. Among the variables that effect produce respiration are weight of the commodity, stage of maturity, membrane permeability, temperature, O2 and CO2 partial pressures, ethylene concentration and light. The variables that effect gas permeation into and out of the package are structure of the packaging film, thickness, area, temperature, and O2 , CO2 concentrations.
Steady-state conditions should be established within the intact packaging system, wherein equilibrium concentrations of O2 and CO2prevail, and respiration rate is equal to the permeation rate. A change in any of the packaging system variables will effect the equilibrium concentrations ranging from 3 to 9% O2 and from 2 to 12%CO2 are studied, depending on variety, type of film, thickness of the film, temperature, weight of the fruit and exess free volume. The films are perforated to reduce the CO2 level to avoid injuries. Unfortunately, perforation leads to an increase in O2 concentration which, in turn, becomes too high for effective reduction in the rate of respiration.
Considering the complexity of the packaging system with its many variables, the following factors should be considered when carrying out research in this area;
•the package design and selection of variables, e.g., type of film, weight of commodity, is not directed towards achieving the optimal gaseous composition within the package.
•very little research is conducted of the wide array of gas permeability properties of packaging films.This is ununderstandable, in view of the limited variety of low-cost commercial films.
•Perforation destroys the semipermeable nature of the film, as exchange of gas through large pores is primarily a physical process, and is largely independent of the chemical nature of the gas or its interaction with the polymer [9].
CONCLUSION
Fruits and vegetables plays an important role in our life with respect to their nutritional value. But they can be harmful to our health when they are spoiled. Storage under suitable conditions prevents their harmful effect. Cold storage, controlled or modified atmosphere storage and storage of fruits and vegetables in polymeric films are widely storage methods. Cold storage has an additional construction and control requirements. Controlled atmosphere storage is very useful with respect to its easy modification of O2 and CO2 concentration.
REFERENCES
1)http://www.onsitegenerating.com/asgs7combo
2)http://www.collections.ic.gc.ca/agrican
3)http://www.actahart.org/index
4)http://www.hgic.clemson.edu7factsheets
5)http://www.nature.com/nsu/feedback/
6)Karen, L.B., Rolonda, F., Alan, S., Cold Storage, Kansas State University Agricultural Experiment Station and Cooperative Extension Service, MF-1174, 1994.
7)Cemeroğlu,B., Yemenicioğlu,A., Özkan,M. Meyve ve Sebzelerin bileşimi, Soğukta depolanmaları, Volume 1, Ankara, 2001
8)Tashtoush,B., Natural losses from fruit and vegetable in cold storage, food control journal, Volume 11, Issue 6, page 465, December 2000
9)Salunkhe,D.K., Bolin,H.R., Reddy,N.R. Storage, Processing and nutritional quality of fruits and vegetables, 2nd eddition, Volume 2, CRC Press,1991
10)http://www.postharvest.com.au
11)Thompson,A.K., Controlled Atm. Storage of fruits and vegetables, Journal of stored product, Volume 38, Issue 1, page 93, 2002
12)Susana, C., Modelling respiration rate of fresh fruit and vegetables, Journal of food engineering, Volume 52, Issue 2, page 99, April 2002
13)Taşdelen, Ö., CA. Controlled atmosphere storage and edible coating effect on storage life and quality of tomatoes, Journal of Food Processing Preservation,page 303, 1998.
