Avocado ripening technology
C.B. Maat and N.S.Y.M.C. Holswilder Interko
This article belongs to the book “Cultivo, poscosecha y procesado del aguacate� (Spanish version)
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2020
Poscosecha
Avocado ripening technology
C.B. Maat and N.S.Y.M.C. Holswilder info@interko.com INTERKO
Index 1. Why we ripen avocados ............................................................................... 2 2. Understanding the parameters .................................................................... 2 3. Why avocados are unique ............................................................................ 4 4. Perfecting the ripening process .................................................................... 5 5. Explaining the science .................................................................................. 6 6. Choosing a ripening room ............................................................................ 9 6.1. Appreciating the differences .................................................................. 10 6.2. Random access room .............................................................................. 11 6.3. Conventional cold room ......................................................................... 12 6.4. Side curtain system ................................................................................. 13 6.5. Tarp system............................................................................................. 14 6.6. Reversible fans ........................................................................................ 15 7. Interko ripening solutions .......................................................................... 17
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Avocado ripening technology
1. Why we ripen avocados Ripening avocados is all about fulfilling consumer expectations. Fresh fruit at the peak of its maturity can be eaten straight away and offers optimum quality, which leads to repeat purchases. Unripe fruit, meanwhile, can result in a poor eating experience, undoubtedly making people think twice about buying again. However, while some shoppers want to buy perfectly ripe fruit that is ready to eat, others prefer less mature fruit with a longer shelf-life to take home for another day. Furthermore, not all consumers are confident about knowing how to tell when certain fruits are ripe, while others have more experience of waiting for fruit to ripen at home. The key to ripening avocados is understanding your end consumer. There is no one-size-fits-all approach to ripening, just like there is not one type of shopper. By offering avocados at the right level of maturity, or ripeness, for your market, you will unlock repeat purchases and increase your sales.
2. Understanding the parameters Before we begin ripening avocados, there are three fundamental factors that determine the process that must be considered carefully: Consumer Expectations All consumers have different expectations when it comes to buying and eating fresh fruit. Their requirements depend on their eating habits, their culture and where they buy their produce. Whether a shopper wants to buy an avocado that’s ready to eat, or to ripen at home for later, you need
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to know what your end consumer requires before you start the ripening process. You don’t want to over-ripen or under-ripen the fruit, and cause disappointment. You must consider what your consumer expects from your avocados. Fruit Origin & Transportation Not all avocados will be at the same level of ripeness, nor the same temperature before they enter a ripening room after shipment from the source. Avocados harvested from the same tree will not all be equal. For example, fruit that was shaded for most of the day will be different from the fruit that received more sunlight. Also, the maturity level of the fruit at the time of harvesting can be unequal. Often, avocado maturity is defined by its dry matter or oil content. Generally speaking, the percentage of dry matter increases during the season. This means that fruit harvested earlier will differ from fruit harvested later in the season. Added to that, during transportation the box sitting at the bottom of the pallet may have received a slightly different temperature from the box placed at the top. You must understand all of the conditions experienced by your avocados. Fruit Size & Shape Avocados come in small, medium and large sizes. You cannot have very small and very large avocados ripening together in the same room. Avocados are also spherical in shape, which means there is a limit to the speed at which you can heat up or cool down an avocado without destroying the fruit due to the laws of thermodynamics. You must appreciate the inherent characteristics of your avocados.
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Avocado ripening technology
3. Why avocados are unique Looking closely at an avocado it is clear that the fruit differs considerably from other fruits, like bananas or mangoes. Whereas most fruits are sweet, and contain vast amounts of sugar, an avocado contains a lot of oil. This oil content is what gives the avocado its unique creamy texture, and what influences significantly the eating experience. So where does this oil come from? Like all plants, the avocado tree creates glucose and other monosaccharides through the process of photosynthesis. These are then linked in long chains to create polysaccharides, which form the different carbohydrates, such as starch and fiber. At this point, there is no difference between an avocado and a banana. However, the avocado uses a different, much denser, mechanism, to store all of this solar energy. While the fruit is still hanging on the tree, a large portion of its formed sugars are converted to fat – the human body uses a similar mechanism. Combined with the starch, the conversion of sugar to fat accounts for most of the dry matter content of an avocado. This is why there is a noticeable difference between the fruit at various stages during the season. Because the fatty acids that make up the oil content are produced before the harvest, the dry matter and oil content do not change much during ripening. The biggest change can be attributed to the loss of water.
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4. Perfecting the ripening process Although we cannot change the three fundamental factors that affect avocado ripening, if ripening operators understand these parameters we can respond appropriately: Consumer Expectations Know how ripe your consumer wants their avocado. Should it be ready to eat now, do they want it ripe within 2 days, or would they prefer to ripen it at home? Fruit Origin & Transportation Allow your avocados to reach the same temperature first. This is known as ‘equalization’. It takes time for avocados to warm up and equalize. You need to expose all the fruit to the same temperature for a period of time before you start ripening. Fruit Size & Shape Choose avocados to ripen together that are within a similar size range. And, don’t ripen too fast! The speed of the ripening process is crucial. If you go too fast, the fruit won’t warm up uniformly, and therefore the fruit will not ripen uniformly. This is why sometimes an avocado can be unripe on the inside, and completely ripe on the outside, or vice versa. It’s a complex process that requires experience on the part of the ripening operator.
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Avocado ripening technology
5. Explaining the science There are no chemical or physiological differences between fruit that has ripened naturally and fruit that has ripened within in a ripening room. Either way, the process comprises a series of chemical reactions which are influenced by internal and external factors, such as the levels of temperature, ethene (or ethylene) and carbon dioxide (CO2). The only difference is that ripeners can control these conditions within a ripening room. Temperature The reason why avocados must reach the same temperature before the ripening process begins can be explained by the chemical reactions that take place during ripening. When avocados ripen, long starch molecules are converted into shorter saccharides, or sugar, which softens the fruit, and makes it taste sweeter. This conversion is called a hydrolysis reaction, which is endothermic, and absorbs energy. The enzyme responsible for this reaction is β-amylase. Another reaction is the conversion of sugar to carbon dioxide (CO2) and water. This is called an oxidation reaction, which is exothermic. By burning off some of the fruit’s sugars, this reaction releases energy to maintain all of the chemical reactions within the fruit. (It’s important to note that during ripening the fatty acids within an avocado are not used for energy, which means they are not burned.) The level of sugars available to burn is determined by the state of the avocado at the start of the ripening process; e.g. the origin of the fruit, whether it grew in the shade or in the sunlight, and its temperature during transportation. This explains why not all avocados will react in the same way because to start with some fruit will have less sugar to burn. For this reason, we ‘equalize’ the temperature of the fruit at the start of
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the process. This ensures a similar conversion of starch to sugar, and, therefore, uniform ripening. Temperature also determines the rate of the reaction during ripening, plus it influences the effectiveness of the enzyme which serves as a catalyst for the reaction. A temperature that is too low or too high would cause the reaction to stop completely. Generally, a higher temperature will yield an increased reaction rate. But because of the heat released by the oxidation of the sugar during the exothermic reaction, the ripening operator must cool the fruit again to slow down the reaction, and then stop the reaction at the desired level of ripeness. Otherwise, the fruit will convert all of the starch, and burn all of the produced sugar until there is nothing left. You will see this in over-ripe fruit. They become very soft because all of the long polysaccharides have been broken down. The change in a fruit’s firmness can be measured using a penetrometer, which indicates the ripeness of an avocado. Ethene The ‘ripening hormone’ responsible for activating the enzymes within the fruit that produce the sugar is called ethene, or ethylene as it is commonly known. Ethylene is an odorless, colorless gas that is produced and released by most fresh fruits and vegetables as a natural ripening agent. In a ripening room, to ensure that all the fruit starts to ripen at the same time, a higher concentration of ethylene is released. But first, the fruit must be heated up in order to start respiring, and for the fruit’s enzymes to activate. This reduces significantly the concentration of ethylene required in the ripening room, and, therefore, lowers the operational costs. The combination of warm air and the release of ethylene into the room causes the fruit to respire, and the ripening process to begin.
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Avocado ripening technology
Carbon dioxide When the fruit begins to respire, the CO2 released into the air decreases the reaction rate. We often describe CO2 as the ‘evil empire’ of ripening because it suffocates the respiration of the fruit. CO2 also inhibits the binding of ethylene, which, in turn, stops the enzymes. The same effect can be observed when the concentration of oxygen (O2) is decreased, which is why ventilation during the ripening process is required to keep the reaction going. Fortunately, this is a parameter over which the ripener has more control because they can regulate how much CO2 and O2 is present within the room. What’s more, a high concentration of CO2 or a low concentration of O2 can be used to slow down the reaction rate. This is also the concept behind controlled atmospheres for fruit storage. Speed The reason why avocados must not be heated or cooled too quickly during ripening is because the shape of the fruit resembles a sphere. This means that the surface area of an avocado is relatively smaller than the total volume of the fruit, which creates a non-linear temperature curve within the fruit. This makes controlling the temperature and the flow of temperature from the skin of the avocado to its core during ripening both difficult and important. Some industry players have mastered the avocado ripening process to perfection. Even still, not every avocado on supermarket shelves today is uniformly ripened. The thermodynamics – the balance between heat and other forms of energy – have to be just right.
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Thermodynamics play a significant role in achieving uniform ripeness from the skin of the avocado, right through to its core. You don’t want the outer layer of the fruit to start ripening, and the core to remain cold and unripe. Equally, you don’t want the core to continue ripening for too long when the skin has already reached ripening maturity. To calculate how fast an avocado should be heated or cooled down, an energy balance is needed to show how much energy is flowing within a section of the fruit, as well as what is flowing out, and how much energy is being released by the chemical reaction within the fruit. In doing so, we can ensure that ripening is uniform throughout the avocado. So, what does all this chemistry and thermodynamics tell us? Essentially, it means that the process of ripening avocados is a complex mechanism which is dependent on not just temperature but also the concentration of certain chemical species, like ethylene, CO2 and O2. When used and timed correctly, we can steer the ripening process in the right direction to yield both the desired level of ripeness, as well as a uniformly ripened avocado from the skin right to the core.
6. Choosing a ripening room Just as consumer expectations, fruit origin and transportation, plus fruit size and shape determine the avocado ripening process, these three fundamental factors also define the size and design of the ripening room. On top of that, ripeners must consider the volume of fruit that they wish to ripen. A high-volume ripening room for bananas would be impractical for smaller loads of avocados. On the other hand, a large-scale avocado ripener would benefit from a more expansive room.
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Avocado ripening technology
The size, shape and design of the avocado boxes that will be stacked onto the pallets inside the ripening room is another important consideration. This is because the box has a significant influence on how the air flows through the boxes. Airflow depends on the box size, shape, the location of its ventilation holes, and whether it has a closed or non-closed bottom. Unlike the global banana industry, for which packaging is fairly standardized into three common box types, the avocado industry is still undergoing rapid expansion. As such, there are thousands of different box sizes, shapes and designs on the market, all of which affect the airflow to the fruit. Considering all of the above factors, we recommend talking to a ripening specialist to design the ideal ripening room for your avocados. There are multiple solutions on the market but only a bespoke solution that accommodates the intricacies of your fruit supply, as well as your unique set-up in terms of your available space, electrical supply and budget, will come close to achieving the perfect ripening environment. 6.1. Appreciating the differences Although ripening room designs come in all shapes and sizes, any airtight insulated cold room should feature the following basic components: -
A cooler to cool the room. This can be either a direct expansion evaporator, a pump evaporator, or a cold-water coil. Electric heating elements to heat up the room, or a hot water circuit. An ethene generator, or ethene injection, to supply ethene to the room. Ventilation to supply fresh air containing oxygen, and to remove carbon dioxide.
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-
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A humidifier to keep humidity high, and to prevent the fruit from drying out. An air guidance system to guide the air flow through the pallets as efficiently as possible to control the temperature of the fruit, and the concentration of the above-mentioned molecules. Sensors to measure temperature, humidity, carbon dioxide, ethylene and oxygen.
Once you have this standard framework, you can design different styles of avocado ripening rooms, such as a conventional cold room or a random access room, with either a tarp system or a side curtain system, plus the option of reversible fans where applicable. Next, we discuss the advantages and limitations of these types of ripening rooms for avocados. 6.2. Random access room A random access ripening room is the ideal solution for ripening small to medium loads of avocados. In a non-random access room, the pallets of fruit are stacked against each other. As such, the pallets can only be removed by unloading all the pallets at the front of the room. Also, only the front of the pallet is visible for inspection. In a random access room, however, there is space between the pallets, which allows the ripener the flexibility to access a smaller volume of fruit with ease (Figure 1). The wide aisle and spacious openings between pallet stacks mean the pallets are accessible for inspection from three sides, plus individual pallets can be removed at any time. Added to that, Interko equips each pallet stack with an individual fan for temperature control. This allows the operator the precision to ripen the fruit within each pallet to perfection. Interko’s random access ripening room can be built to a single-, double- or triple-tier format. They are highly energy efficient too.
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Avocado ripening technology
A random access room gives the ripening operator more control over the parameters which influence avocado ripening, as well as the ability to remove individual pallets from the room once ripened. This is extremely advantageous when ripening tropical fruits, like avocados, that must be closely monitored during the ripening process.
Figure 1. Random access room with adjustable curtain height
6.3. Conventional cold room In a conventional cold room, typically there is only a draw-through cooler to cool the room, and no air guidance. Although, due to convection and conduction, the temperatures will even out in time, this type of room is generally not ideal for avocado ripening. This is due to the larger differences in air temperature, and the concentration which can occur while the fruit is active. This can lead to uneven ripening, which requires the ripener to assess each box separately to ascertain the ripeness of the fruit. Furthermore, the boxes designed for a forced air ripening room may not be suitable in this type of room.
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6.4. Side curtain system A side curtain system stops the airflow from passing over the pallet, and forces the air to go through the pallet (Figure 2). The design relies on the air pressure generated by the fans on the cooler to force the curtain against the pallets, and, in doing so, it seals the top box. The air also flows widthway across the room, rather than lengthways down the room. This creates the shortest airflow path within the room and, therefore, less chance of air encountering an obstruction.
Figure 2. Side curtain ripening room with fixed separation baffles
With a fixed curtain, however, one of the downsides is that all the pallets should have the same height for the most effective result. Fortunately, Interko can solve this problem with our height-adjustable curtain, although it does require the ripener to adjust the curtain each time the pallet height changes.
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Avocado ripening technology
A major advantage of the side curtain system is the ability to pair this concept with reversible fans. The reversed airflow created by reversible fans provides a significant benefit in overcoming the differences in avocado box sizes, shapes and designs by harmonizing the airflow throughout the room. The side curtain system also offers better energy efficiency compared with the tarp system. 6.5. Tarp system A tarp system is a forced air style of ripening room, which relies on a tarp and suction wall to force the air through the pallets of fruit. Usually, the suction wall is placed at the back of the room under a draw-through cooler. The pallets are placed against the suction wall in lines, with a 100millimeter gap between the side wall and the pallets, and a 200 millimeter or more gap between the two lines. A trap is placed over the pallets to seal off the top of the pallets, as well as the middle space between the pallets from the suction wall to the front of the room. The air is blown above the tarp and over the pallets, down though the space between the pallets and the wall. Then the air is forced through the pallets to the middle space between the two lines of pallets, and from there the air is sucked through the hole in the suction wall back to the cooler. Unlike the side curtain system, in a tarp system the airflows lengthways down the room, meaning the air has further to travel. Due to the simplicity of its design, a tarp room is easy to use. However, it does not provide the best overall efficiency because of the high pressure drop over the cooler. Also, due to the difference in the distance that the air has to travel between the front pallet and the back pallet, a higher pressure drops over the box is needed to ensure uniform airflow, and therefore uniform ripening across the pallets. This adds to the system’s resistance and energy consumption. Another drawback is the inability to
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install reversible fans with the tarp system, which means the airflow is not consistent throughout all of the pallets within the room. 6.6. Reversible fans Reversible fans are highly recommended for ripening avocados. As discussed previously, ripening is a chemical process driven by temperature and the concentration of certain chemical species. The more uniform these conditions are across the ripening room, the more uniform the ripening cycle will be throughout the pallets of fruit. As such, the airflow is very important. Due to the design of a ripening room, the conditions at the front of the room will never be exactly the same as at the back. This is because the air temperature and the chemical concentration within the air changes as it flows from one box to the next, meaning the chemical reaction for ripening will vary slightly from box to box (Figure 3).
Figure 3. Exaggerated change of concentration and temperature over the length of a box
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Avocado ripening technology
The avocado closest to the cooler will, without reversible fans, receive colder air which also contains more oxygen and less carbon dioxide. Over the course of multiple days this will lead to a difference between avocados at the front of the pallet to those at the back. However, if reversible fans are used to reverse the airflow within the room these differences in the temperature and chemical concentration can be minimalized by levelling out the airflow (Figure 4). As a result, reversible fans also help to overcome the impact on airflow caused by different avocado box designs. Put simply, reversible fans produce more uniform conditions throughout the ripening room, and create a more uniform ripening process to produce a higher quality product.
Figure 4. REVERSO: Interko’s reversible fan technology
Interko is the only ripening solutions provider that specializes in developing fans that can reverse the airflow while also being energy efficient. Our fans consume less than 100 watts per pallet at full air flow, and they can be individually switched on or off over a network protocol. www.bibliotecahorticultura.com
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It should be noted that while reversible fans can be used within a random access room, and/or with the side curtain system, reversible fans cannot be used in a conventional room, nor with a tarp system.
7. Interko ripening solutions Interko is the global leader for the design, manufacture and installation of bespoke systems for high-quality, energy-efficient and cost-effective fresh fruit ripening. Thanks to over 50 years of experience and a team of highly-trained engineers, we can create the perfect ripening and storage environment for any fruit; from bananas and avocados, to mangoes, stone fruit, kiwifruit and even pears. Since we developed the industry’s first patented banana ripening room in the 1980s, we have continued to push the frontiers of fruit ripening with our innovative solutions. Worldwide, we have built over 7,000 custom-made ripening rooms. Every day 4 million boxes of fruit are ripened in Interko rooms.
For more information, contact us at info@interko.com, or visit our website at www.interko.com
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