KEY CONCEPTS IN POST-HARVEST MANAGEMENT OF TABLE GRAPES: as presented by Dr Luis Luchsinger Summary by Rodrigo Oliva Manchilla SATI Technical Bulletin INSERT DECEMBER 2015
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RESEARCH & TECHNICAL During the 2013/2014 table grape season, South Africa exported a volume of 50 311 million of 4.5 kg-equivalent cartons of table grapes. As a result of this export volume, South Africa is the 5th biggest exporter of table grapes in the world with a turnover of USD441.475 million. Europe and the UK remain the main export destination receiving 79% of fruit from South African producers (SATI; Statistic Booklet 2013/2014). There is tight competition between table grape exporting countries and analysis of the exports over the past few years highlights Peru as South Africa s main competitor, not only in volume but also markets. Furthermore, it is important to be aware that our competitors will continue to grow in production and export volume of fruit to markets that only a few years ago were our natural markets. In an effort to continually meet the high quality demands of export markets, South African producers have focused their efforts on becoming more competitive and cost-effective through implementation of changes to production techniques. The reality is however, that this objective cannot only rely on improvements at production level. Vitally important is to also take cognizance of post-harvest processes and logistics. South African s are no doubt excellent producers. But more emphasis must be placed on supplying target markets with good quality fruit. During the Post-Harvest Table Grape Course, Dr Luis Luchsinger, referred to several factors linked to fruit quality, which will be commented on in this summary. The four main pillars to be considered to ensure excellent quality fruit on arrival in the export market are: a) Fruit Fruit must be harvested at optimal sugar:acid ratio as fruit with lower sugar content is more sensitive to cold. Fruit must be physiologically healthy with a well-developed rachis and stem and strong, firm berries. Fruit must be handled correctly during the season and harvest. Grapes must be held and handled by the rachis. Harvested grapes should be placed in harvest crates. These crates should have a depth of at least 21 cm, a flat bottom which is properly covered by cushioning material. In addition to protecting the fruit from knocks and bumps, this material may also help in reducing dehydration by up to 50% Grapes must be handled carefully during cleaning. It is also important to ensure that stems/rachis/pedicels are cut flat to avoid puncturing of the grapes. Store grapes in a shaded area until it reaches the packing facility. The duration of this shaded storage of the fruit should be as short as possible. Perfect handling of fruit includes handling at: (1) harvest; (2) cleaning of fruit in the packhouse; (3) classification; (4) gasification (SO 2 ); (5) cooling fruit down to 18 º C; (6) fruit selection; (7) packing and forced cooling. b) Forced air-cooling It is no secret that the cooling chain will positively impact the shelf-life of fruit Figure 1. According to Dr Luis Luchsinger, the most important of the post-harvest pillars is forced air-cooling (refer to Figure 2). The fast reduction of the fruit temperature by extracting excess heat before and after fruit is packed, has a positive effect on reducing dry and brown stems, berry weight loss and soft berries. On the contrary, the delay of fruit cooling will result in dry and brown stems, berry weight loss and soft berries. Below are four graphs indicating the effect of delayed cooling on the physiological effects mentioned above. The higher the temperature, the more active the cell respiration will be, and as a consequence, fruit will be more dehydrated. Figure 1: Illustration of how proper cooling and storage will reduce the rate of post-harvest senescence and deterioration and provide a high quality product over extended periods Figure 2: Effect of delay in cooling on brown stems (B); Soft berries (C); Weight loss (W) and Dry stems (D) The cooling triangle (Figure 3), designed and developed by Dr Luchsinger, indicates that in order to have a successful cooling process which will positively impact the future of our fruit, three vital aspects must be considered: Boxes & Packaging Materials. The influence of the packaging materials on the cooling process is massive (and not necessarily directly economic as with processing costs) due to lack of ventilation of South African packing materials. Especially in the context that electricity bills are expected to double in the next 5 years. The lack of ventilation on the packing materials (e.g. cartons and liners) will impact the efficiency of the cooling process and will induce dehydration. The reason is that during cooling, we are extracting heat, and with heat also water. So, the longer we take in the cooling down process, the more water is removed, which results in more dehydration. The design of the packing materials is therefore fundamental. SATI Tegniese Technical BULLETIN 1
SATI Tegniese Technical BULLETIN avoid erroneous readings. If equipment is forced to work harder, there is an increased risk of freezing. TARGET TEMPERATURE = Freezing temperature + 1ºC. Measured at SUPPLY AIR!!! Target Temp. in ºC of air = target Temp. in ºC fruit pulp Dehydration: Table grapes consist of 80% water. Water moves from a high pressure zone to a lower pressure zone. Thus, if vapour pressure around berries is low, water will move out from the berry, producing dehydration. If we observe a psychrometric chart, we find that the higher the temperature, the higher the vapour pressure (kpa). In other words, the cooler the air, the drier it is. Air can only hold a certain volume of water vapour and the colder the air, the less water it can hold. The colder the air, the sooner condensation will form. Figure 3: The Cooling Triangle The boxes must be 100% symmetric and there should be no spaces between cartons (if there are gaps between boxes, air will not flow through our fruit, but instead will just move through the gaps) when stacked on a pallet. The carton ventilation (holes) should be 100% symmetric with uniform distribution (Figure 4). This will increase the suction capacity of cooling equipment. The holes in the base of the cartons should ideally be > 3% of the surface to allow enough vertical movement of air inside the containers. It is important that the pallets are designed in such a way that the positioning of the planks do not close these holes. The percentage of ventilation area recommended for cartons of 400 x 600 is equal to 8 10%; 400 x 500 is equal to 14-16%; and 400 x 300 is equal to 8-10%. Of course the strength of the carton should not be compromised by the ventilation holes. In the case of asymmetric pallets, air ventilation distributions must allow cross-stacking of boxes. Packing materials likes MAM sheets ideally should also be perforated, respecting the design of the bottom of the carton to facilitate vertical air movement (ascendant). When there is not enough ventilation in the box, the cooling process happens by conduction, which results in slow cooling and more dehydration (passive cooling). However, when there is sufficient ventilation and the airflow is high, there will be fast cooling and low dehydration (active cooling). Methods of Cooling. The lower the sugar content of the grape berry, the higher the chance of freezing. This explains the reason why a bunch of table grapes could have several berries with varying degrees of freezing. Normally berries close to the shoulders of the bunch have lower sugar content than the distal berries. Then also, fruit with higher Brix can resist lower temperatures than fruit with low sugar content. The thermostat of the tunnels and cooling chambers should be placed in the coolest spot (supply air) in the room in order to 2 SATI Tegniese Technical BULLETIN Figure 4: Illustration of alignment of carton ventilation on a packed pallet Condensation: Condensation of fruit takes place when warm fruit is put inside a colder ambient temperature, as well as when colder fruit is placed in a warmer ambient temperature. Vapour Pressure Deficit, or VPD, is the difference (deficit) between the amount of moisture in the air and how much moisture the air can hold when it is saturated. The psychrometric chart (Figure 5) is a tool that is used to manage moisture in terms of condensation (dew point) and moisture loss (VPD). Figure 5: Psychrometric Chart The cooling process has two vital factors that take place to cool fruit, i.e.: The cooling air temperature should be lower than fruit temperature. The cooling temperature should always be below the fruit temperature and above the minimum temperature allowed for the fruit (freezing point). Furthermore, the bigger the difference between the air temperature and the fruit pulp temperature, the faster the cooling. In order to have a more efficient cooling process, it is important to ensure that fruit that is more difficult to cool is always placed near the evaporators, and the easier cooling fruit at the far end. It is necessary, at cooling, to classify packing material according to cooling efficiency. It is known that 4.5 kg cartons are the hardest to cool, thus those pallets should go in first, and are consequently placed closer to the evaporator. It must also be borne in mind that the bigger the fruit, the more difficult the cooling thereof. All Jumbo, XXL or XL should therefore be placed in first, closer to the evaporator. It is extremely important to load the tunnels in a symmetric way. It is therefore ideal to put in the same packing materials per block, and then continue with other packing materials. In this way we are filling a forced cooling tunnel with fruit.
RESEARCH & TECHNICAL Always remember that variety, size of fruit, kind of packing material, as well as the percentage of ventilation of liners and cartons all have a direct influence in the efficiency of forced air cooling. Cooling System Design. There are two main types of cooling systems active (fast) systems such as forced air cooling (table grapes) and hydro cooling (not for table grapes), and passive (maintenance) systems, which correspond to cold storage, refrigerated trailers, vessel chambers (charters), and marine containers. The focus here is forced air cooling, because the quicker the fruit is cooled, the better the expected condition and quality of the fruit (except in varieties which are chill sensitive). A forced air-cooling system will produce enough suction to move the air produced by the fan through the fruit, using the ventilation holes in the packing materials to pass through. In this way, all the air generated by the fans will be pulled through the cartons. If the suction pressure is correct, it should be in the order of 40 to 45 mmwc (millimetres per water column). If this pressure is not correct, or if there is a leak in the pressure (i.e. a space between the pallets or between pallets and walls), it cannot be called a forced air-cooling system. Tunnels should be designed in symmetry with few, or no sharp angles. The evaporator should be installed in a way that all air will pass through the evaporator coil, and the sides of the evaporator will be sealed in order to avoid any air leak. The same will be done with the fans; it will need to be placed and enclosed in order to pull through the entire volume of air. Tunnels should have air circuit doors which allow the redistribution of air in order to change the direction of airflow and cool from both sides of the pallets. Pallets should also be covered by a tarp which covers the entire top of the pallets to avoid air from moving through any gaps other than the gap between the pallets (internal face) or the gaps on lateral sides - between pallets and walls (external face). The defrost outlet system must have a nonreturn valve and water trap in order to avoid heat coming into the cooling system through the pipes and producing condensation in the evaporator. It is a rule never to start a system when the floor under the evaporator is wet, because the water will evaporate/ condense on the evaporator and freeze faster. If water is added, it must be added in before suction in order to pass wet air through cartons. This will reduce dehydration. The temperature is set and the process started with the central door open and two lateral doors closed. The air will then move from the wall to the centre of the tunnel and through the evaporator. Figure 6a below shows a tunnel and its components before operation, and Figure 6b shows the tunnel in operation while the central door is open and airflow is from the outside to the inside. Figure 6a: Tunnel components before operation Figure 6b: The tunnel in operation Place pallets with homogenous packing materials inside the tunnel; never put half pallets in. Complete the height of incomplete pallets with empty boxes for example. These will be wrapped in film to avoid air moving through empty boxes and resulting in a short cycle. Place sensors inside and outside pallets ideally 2 sensors per pallet. Place a tarp in between pallets. Cover the base of each pallet and cover the top of the pallets with the tarp. This tarp must be wide enough to cover the top of the pallets 100% from each side, wall to wall. Finally completely cover the front to pallets (at the end of the tunnel) from top to bottom. There must not be gaps between pallets, cartons or cushioning in front of the first pallet. Make sure that every gap is covered. When the outside temperature reaches 0ºC, it starts monitoring the internal temperature reading each hour. When the inside reaches 4ºC, we proceed to invert the cycle, i.e. closing the central door and opening the lateral doors. If we do not do the inversion, the air cycle cooling process takes 25 30% longer. We do not invert earlier so as not to introduce hot air on the side of the pallet which is colder. When temperature reaches 0ºC in the internal face, we stop the tunnel and we proceed to move pallets into our cold store which should be at -0,5ºC and >95% RH. Always check the suction pressure in the pallets, which should not be higher than 45 mmwc. In case there are fewer pallets and pressure increases, the doors could be opened slightly to avoid pallets collapsing. In this case, the total pressure will be around 65 mmwc if an air circuit (10 mmwc) and evaporator (10 mmwc) are being added. It is thus very important to control the suction pressure in relation to the number of pallets inside the tunnel. The best way to solve this situation is to install fans with frequency inverters. c) SO 2 gasification and application. The use of sulphur dioxide is twofold: (1) as an oxidant - this is the reason why fruit that has been treated the rachis remains green for longer than untreated in fruit; and (2) as a fungal treatment either as a fungicide or a fungistatic. SO 2 acts as a fungicide when applied in a gasification room (right-hand picture) and pallet gasification. In order to be classified as fungicide, the concentration of SO 2 should be 100 PPM/Hr to kill spores and mycelia of Botrytis cinerea, while in to kill Penicillium spp the required concentration is 200 PPM/ Hr. The advantage of fumigation with SO 2 in the chamber is that all fruit is evenly treated. For this purpose, fumigation rooms normally have two powerful fans that work against each other in order to ensure a strong flow of air through the harvest crates. SATI Tegniese Technical BULLETIN 3
SATI Tegniese Technical BULLETIN Figure 7: Container modified and used as SO 2 gasification room. Sta. Elena Chile 2015. In instances where the development of a fungus is suppressed, SO 2 takes on a fungistatic role. There are mainly two instances of this application; the first one corresponds to application in pre-packing fruit in gasification rooms; and post-packing, as in the case of sulphur pads, bottom pads, impregnated liners and YT-Gas. There are several sulphur pads on the market; they have two releasing phases (Figure 8). The first phase is high release, which reaches the maximum concentration at 24 hours with a fungicide effect; the second phase is a fungistatic, as it is very constant in releasing, but at a very low concentration. The bottom pads only get second phase. As per general rule, 1 gram of Figure 8: Graph illustrating the SO 2 release of sulphur pads sodium metabisulphite ensures protection for about 10 days. It is very important to reduce the condensation of the fruit so as to avoid bleaching and hair line. When condensation happens and there is fast emission of metabisulphite, the combination produces sulphuric acid which will damage the berry skin. Figure 9: Container loading bill with records of pallet layout and temperature record inside the container. m3/hr of air. It must be ensured that air is always moving through the shortest distance possible. Proper air distribution through the fruit is vital to ensure successful conservation of fruit during shipment. The air distribution in the containers is a vertical ascending movement. Any existing gaps between pallets and walls or in the floor (Figure 10) must be covered to avoid air escaping through these gaps. In this way, the right distribution of air in the entire container is ensured. As soon as there is a gap between pallets and walls, or in the container floor, the air will use this route to move back to the evaporator area, reducing the air distribution of the container (short cycle). It is highly advisable to have corrugated cardboard strips of 120 x 19 and 100 x 19 mm in order to cover the pallet base and avoid air re-routing through the pallet gaps. These strips will be used to also cover parts of the container floor between the wall and the pallets in order to force air to move through the pallets. The ideal density of this cardboard is > 270 gr/m 2. As per general rule, containers should be connected to a Genset if the trip is longer than an hour. d) Loading in containers. From the moment a container is sealed pro-ducers have little or no control over what may happen to the fruit during the shipment. The only indication of is happening inside the container are temperature recordings during loading and shipment. Each container unit must be inspected before it is loaded. In the pre-load inspection, it is important to ensure that the container is in full working condition. The fluctuation of temperature should ideally be no more than 2ºC. There are two main set-points: temperature and ventilation. Both parameters need to be set properly in order to ensure proper travel conditions for the fruit. At the moment of loading, the containers should be taken, and the temperature of each pallet should be recorded. In a proper diagram (Figure 9), the loading procedure should describe the exact position of each pallet inside the container (Figure 10). Containers should be loaded only when the target temperature is reached (in case of table grapes it should be -0.5ºC to 0ºC), and ventilation should be between 0% to 25 Figure 10: Container loaded with covered gaps between pallets. Photos/diagrams supplied by Luis Luchsinger 4 SATI Tegniese Technical BULLETIN
Photos from the Post-Harvest Seminar
South African Table Grape Industry (SATI) 63 Main Street Paarl 7646 Western Cape South Africa Tel: +27 (0) 21 863 0366 Fax: +27 (0) 21 863 0339 Email: info@satgi.co.za Website: www.satgi.co.za