Organisation: Food and Agriculture
Organization of the United Nations, FAO, Rome,
Italy
Author: Lidia Dorantes, PhD., Lidia Parada, MSc., Alicia Ortiz, PhD.
Edited by AGST/FAO: Danilo Mejía, PhD, FAO (Technical), Emanuela Parrucci
(HTML transfer)
2 Post- Production Operations
2.1 Harvesting
2.2 Packinghouse operations
2.3. Packing and packaging materials
2.4. Cooling system
2.5. Storage of avocado
2.6. Transportation systems
2.7. Processing
In the period of time that occurs from the harvest of a vegetable product to its final destination, quality and quantity losses take place. These losses can be of 5 to 25% in developed countries, and of 20 to 50% in developing countries.
One of the objectives of post-harvest technologies is to reduce the losses, and has the following objectives:
To understand the biological and environmental factors involved in product deterioration.
To apply technologies to delay the decay of the product and best preserve its quality.
All fresh vegetable products contain a high water percentage, and therefore they are prone to dehydration effects (blight, wrinkling), bacterial and fungal attacks, as well as mechanical injuries due to inadequate handling. As decay of the fruit increases, it becomes more sensitive to infections.
When avocado reaches the physiological maturation point (harvest point), it contains almost 80% of water. It is a climacteric fruit with high respiration rates, and releases carbon dioxide and ethylene. This means that after harvest, the live tissues degrade at a high rate, together with accelerated internal and irreversible changes. Avocado can dehydrate at the relatively high environmental temperatures, and the mechanical injuries during handling will stimulate higher and faster fruit decay.
Sometimes, physiological disorders in the post-harvest stage are originated in nutritional imbalances of the trees. Orchards should have a balanced nutrition, and the fruit should be handled carefully, in order to increase post-harvest life and preserve quality (Sánchez-Pérez, 2001).
Production or pre-harvesting
Special attention should be paid to the nutrition of avocado orchards, specially when producing fruit for international markets. The trees should not have a deficiency or an excess of nutrients. Even though the soil has natural nutrient contents, they are consumed or lost during the development of the trees and need to be supplemented with minerals in the same amount as they are extracted (Figure 21 Avocado leaf showing nutrition deficiency). The main elements for a proper nutrition are: nitrogen (N), phosphorous (P), calcium (Ca), potassium (K), magnesium (Mg) and zinc (Zn).
The yield can decrease up to 70% when the soil is not properly fertilized, and the symptoms of nutrient deficiency vary according to the specific mineral. Fertilization should be done according to the age, size, and specific needs of every tree. If these factors are not properly assessed, an excess of fertilizer may contaminate the phreatic water (Eng. José Cortez personal communication, 2003; Sánchez-Pérez, 2001).
Harvesting
Harvesting of the fruit before reaching an optimal point can lead to deficient ripening and quality. On the other hand, when the harvest of the fruit is carried out after the optimal point, its post-harvest life could be diminished. In order to determine this optimal harvest point, two quantitative maturation and harvest indexes are used: oil and dry matter contents. However, other complementary indexes can be considered, such as the size of the fruit and the appearance of the seed skin.
Dry matter is measured by placing approximately 10 g of thin slices of avocado pulp into Petri dishes. The uncovered dishes are placed in a microwave oven and cooked until constant weight (from 5 to 15 min). Dry matter content should be 22.8% for "Hass" and 21% for "Fuerte". It is also important to consider that a large size of the fruit it is not always an indicator of advanced maturation. When the skin of the seed appears thin and changes its color to dark brown, the fruit tissue is probably ripe (Yahia, 2001).
Maturation indexes should be considered at the beginning of the export season. The dry matter analysis can also be performed using a domestic or microwave oven, and does not require an extensive or sophisticated training. Based on the experiences obtained in Mexico for export fruit, it can be established that avocados should have an average of 22% of dry matter, and the lowest value of a sample should not be under 20% (Sánchez-Pérez, 2001).
The harvest of the fruit destined to the Mexican domestic market is usually done by hand or with the help of hooks, which grab the fruit from the peduncle and let it fall into the ground. The fruit stays on the ground for several hours and in some cases overnight, before they are collected into boxes or sacks. The first damage to the fruit is made by the pull-off, fall, and exposure to the elements (Morales et al., 2000).
The harvest of higher-priced fruit is made with the help of a ladder and "ganchos" (hooks), which are metallic or wooden sticks of approx. 3 m long, with a collector bag and a sharp metallic hook attached at the end. The hook cuts the peduncle and the fruit falls into the bag.
In Israel, a collector machine is used. This machine has a mobile tower of approx. 5 m in height, which cuts the fruit and collects it into a bag, as directed by an operator. In Mexico, harvesting machines consist of a platform that is elevated by an operator (Figure 22 Harvesting machine to collect avocado). A worker stands on the platform, cuts, and collects the fruit.
The manual harvest should be done bearing in mind the following points:
The processing of the fruit required for its packaging, preservation, and transportation is carried out as follows:
The fruit is received, weighed, and unloaded from the trucks. It comes in boxes of different colors to identify its final destination: domestic market, export, and organic fruit (Figure 24 Weighing avocados in a packing house).
The boxes are placed in a transport band that twist them over, and takes the fruit to a classification machine. This machine separates the small avocados.
Then the fruit is cleaned with rotating brushes, and goes to the selection band (Figure 25 Rotating brushes to clean avocado). The selection is carried out manually or with the help of machines, and considering the shape, size, and sanitary characteristics of the fruit, as well as all the defects caused by insects, rodents, mechanical mishandling, and illness (viruses, bacteria and fungi). The selection criteria also depend on the final destination of the fruit. In the case of the Global Frut packinghouse in Uruapan, Michoacán, the fruit is selected by weight, which is programmed in a machine according to the marketplace of destination. The weight classification allows having a uniform package and presentation in the marketplace. The caliber is the number of fruits that can be packed in a single box. This avocado also varies according to the market of destination.
Once the fruit size is selected, and the defected pieces have been discarded, it is packed. The packaging material varies according to the market, being cardboard, plastic, or wood.
Individual boxes go to the "pallet" process, where boxes are stowed and tied together (Figure 26 Boxes with avocados are tied together, to form a packaging unit). Pallets are a packaging unit for transportation, and should comply with the measurements established by the containers they will be carried in. The number of boxes per pallet varies according to the packinghouse, but is usually 200 boxes of 4 kg each and a lower number when boxes weight 6 kg.
Pallets pass immediately into refrigerated chambers where they first undergo a pre-cooling process. Afterwards, they will enter the preservation chamber (Figure 27 Refrigeration chamber to maintain avocados at 3 to 7 ºC), where they will stay until loaded into the transportation unit. Preservation temperature ranges from 4.5 to 6.5°C (Mario Rivas, personal communication, 2003; Sánchez-Pérez, 2001; and Rodríguez-Suppo, 1992).
Avocado has a very small chance of being contaminated by pathogens, because it grows suspended on the branches and it is not in contact with chemical or biological fertilizers, and irrigation water. Avocado is also protected by a thick peel. However, packing operations present a contamination hazard. Water (if used in the process) should be disinfected, insects and pests controlled, garbage recollected, and the people working with the product should be properly trained (Salazar-Arriaga and González-Sánchez, 1999).
The purpose of packaging is to preserve the fruit in good condition. The container protects the product through all the stages of the distribution process, such as transportation, load, unload, stowing, and storage.
It has been determined that a well-designed package helps to reduce damages in food products, particularly in perishable ones such as fruits and vegetables. Bearing in mind that packaging may restrict gas diffusion; one might conclude that post-harvest losses could be increased. In this way, it is considered that 25 to 30% of food produced worldwide are lost due to deficient packaging (Kader, 1991, Rodríguez, 1997).
The use of packaging material implies an increment in the final cost of the product, nevertheless most of the times it improves the appearance and quality preservation of the fruit.
For the particular case of avocado, and exclusively for the ones destined to foreign markets, the types of packages and containers vary according to the different packinghouse specifications. The most common containers are single wall corrugated fiberboard or wooden boxes (Figure 28 Containers may be boxes of corrugated fiberboard, plastic or wood). The first ones usually have a capacity of 4 kg with one level of fruit, while the second ones contain 10 kg, and the fruit is placed in bulk. The wooden box has a lower demand than the fiberboard box. Cardboard boxes have different perforation designs, and resistance to compression: from 55 to 275 lb/sq ft, approximately (López-López, L., and Cajuste-Bontemps, J.F., 1999).
An adequate design of the packaging material should consider the following fundamental aspects:
Ventilation: should allow air circulation, particularly in the transportation container (Figure 29 Perforated boxes allow air circulation.).
Ventilation systems can be horizontal or vertical. Containers for maritime transportation are equipped with vertical air circulation, while cooling rooms and ground transportation containers have horizontal air circulation. Sometimes the box is designed for both air systems, which could make it weaker, and thus it is important to strengthen the box.
Piling force: type and quantity of the right materials should be use to avoid falling of the boxes.
Labeling: some information is legally required, such as the origin and identification of the product, net weight, and producer. It is up to the country or the buyer that will receive the fruit to require additional information. It is very convenient to inform the final consumer about ripening conditions, storage temperature, and uses of the product. For example, to inform that lemon juice prevents browning of the pulp (Yahia, 2001).
López-López and Cajuste-Bontemps (1999) studied the effect of different fiberboard box designs on the quality of avocado, and concluded that those boxes with the highest perforation surface help to preserve the firmness, general appearance of the fruit, and delay maturation.
On the other hand, the United States Department of Agriculture requires that the original boxes for Mexican Hass avocados must each have a stamp that lists the distribution restrictions. Once in the approved states, if any avocados are removed from their original shipping boxes and repackaged, the stickers required on the fruit must not be removed or obscured and the new boxes must be clearly marked with all the following information: the identity of the grower, packinghouse, and exporter, and the statement "Not for distribution in AL, AK, AZ, AR, CA, FL, GA, HI, LA, MS, NV, NM, NC, OK, OR, SC, TN, TX, WA, Puerto Rico, and all other U.S. territories."
Each industry in the Mexican Hass avocado import process is responsible for following USDA regulations. Industry employees should be aware of these regulations and the penalties if the regulations are not followed (APHIS, 2003).
Pre-cooling
A pre-cooling operation is generally carried out after packaging. Pre-cooling is of prime importance for the shelf life of avocado, because it diminishes or slows the metabolic rate, ethylene synthesis and its action on the fruit, loss of texture, fungal infections, fruit ripening, and conditions the fruit for preservation at low-temperatures. Ideally, there should not be more than six hours from harvest to pre-cooling, and when this is not possible, the harvested fruit should not be allowed to reach an internal temperature higher than 26°C in the field and during its transportation to the packinghouse.
The quantity of the field heat is usually large, and cannot be eliminated fast enough in a regular refrigeration room. The freeze-blast method is the best suited for avocado pre-cooling. It is carried out until the temperature in the fruit reaches 6-7°C for "Fuerte" and "Hass". The time that is required to achieve these temperatures varies according to the initial temperature of the fruit, temperature and velocity of the air, and the final temperature of the fruit.
However, it is important to end the pre-cooling process when the temperature of the fruit is 2°C above the ideal storage temperature. It is also of prime importance to assure that the storage temperature will not be lower than that established for the fruit, otherwise chilling injury can occur. The pre-cooling process lasts from 8 to 12 hours, with a relative humidity of 90 to 95% (Yahia, 2001, and Sánchez-Pérez, 2001).
Refrigeration
Temperature control during the post-harvest stage is the most important factor that helps maintain the quality and increase the shelf life of many fruits. Refrigeration is also useful to control illness and pests (Figure 30 Cooling chambers with avocados).Generally, the shelf life of avocado is inversely proportional to its respiration velocity.
It is of prime importance to avoid temperature fluctuations during transportation, because this can cause chilling injury, ripening, irregular softening, and rot (Yahia, 2001).
When avocado reaches the packinghouse, special care should be taken so that different batches will not mix up. The origin of the fruit, supplier, date, etc., should be registered. Before processing of the fruit, the lot shall be sampled in order to detect quarantine pests and illnesses, and determine the general appearance of the fruit and its quality (stains, discolorations, injuries, mechanical damage, etc). At the same time, the characteristics of the particular variety are verified (Sánchez-Pérez, 2001).
The response of avocado to storage temperatures varies according to temperature ranges, as follows:
10 to 25°C: the fruit softens faster as storage temperature increases.
5 to 8°C: softening is controlled, and it will only occur if the fruit is transferred to higher temperatures.
0 to 4°C, softening at these temperatures is limited by time, due to the risk of chilling injury. However recommended storage conditions may vary according to the avocado variety (Table 9)
Physiological disorders decrease when temperatures are kept at 7.5<°C at the beginning of storage, and then are lowered to 3.5°C, instead of maintaining 5.5°C the whole time (Yahia, 2001).
Table 9 Recommended storage conditions for some avocado varieties (Yahia, 2001)
Modified or controlled atmospheres can increase shelf life when combined with controlled storage temperatures, as observed in the Table 10.
| Variety | %O2 | %CO2 | Temperature °C | Remarks |
| Hass | 2-10 | 4-10 | 7 | Storage time of 7-9 weeks |
| Lula, Booth 8, Fuchs | 2 | 10 | 7.5 | Increase shelf life twofold |
| Fuerte, Edranol, Hass | 2 | 10 | --- | Reduces internal disorders |
| Non-specific | --- | 25 | --- | Reduces disorders and increases anthracnose |
| Fuerte | --- | 25 | --- | Delays maturation |
| Fuerte | 2 | 10 | 5.5 | Less dark spots in the pulp |
| Fuerte | --- | 25 | 5.5 | Less dark spots in the pulp |
| Fuerte | 3 | 0 | 24 h at 17°C | After this treatment, fruit can be stored at 2°C for 3 weeks |
| Booth 8, Lula | 2 | 10 | 4-7 | Storage time of 8 weeks |
| Fuerte, Anaheim | 6 | 10 | 7 | Storage time of 38 days |
| Waldin, Fuchs | 2 | 10 | 7 | Storage of 4 weeks, prevents anthracnose and chilling injury |
| Hass | 2 | 5 | --- | Storage time of 60 days |
Transportation of the fruit from orchards to packinghouses is done by trucks that carry one ton (Figure 31 Transportation of avocados from the orchard to the packing house). Once avocados are packed, they are distributed in trucks with a capacity of 10 tons. Export fruit travels 2 days from Uruapan to Altamira in the State of Tamaulipas (Gulf of Mexico) in a truck with refrigerated containers, and from there 15 days more to Europe in a cargo ship. It takes 17 days to the European continent when the fruit takes the route Manzanillo-Panama-Europe. Transportation to this market represents 30% of the final cost of avocado.
The storage temperature should be maintained with a maximum variation of 1°C, when transferring the fruit into refrigerated transports. This is particularly important when fruit will be stored along a relatively long period before it reaches the market. The threshold for chilling injury appears between the third and fourth weeks of the refrigeration period. This should be considered for long-distance maritime transports. When reaching the port of destination, avocados should be sent immediately to the retail distribution channels, with the aim of shortening the refrigeration time (Yahia, 2001; Sánchez-Pérez, 2001; Mario Rivas, personal communication, 2003).
Aguamich is a Mexican processor based in Uruapan, Michoacan. They produce and market avocado salsa, guacamole blend, avocado pulp, spicy guacamole, and avocado drinks (Guacamaya brand). They have their own orchards, and they also buy fruit from other farmers.
Upon receiving the fruit, it is washed in a machine with rotating brushes and chlorinated water (200 ppm) (Figure 32 Avocados are cleansed by wet or dry process)
Then its temperature is homogenized to 5°C to allow an even ripening. Avocados are stored for 3 days at 20°C at a relative humidity over 85%, adding 10 ppm of ethylene. Afterwards, the temperature is lowered again to 5°C and the fruit is kept at that point until processing (3 to 4 days). Since many clients prefer a chunky texture, the fruit must be ripe and firm.
Processing begins with a selection step, where the unsuitable fruit is discarded, the peduncle is removed, and the fruit is submerged in chlorinated water (200 ppm) for 10 min. Then it is cut, seeded, peeled and put into a mixer with other ingredients, such as onion, chili pepper, fruit concentrate, erythorbic acid (to promote color retention), and ascorbic acid. The resulting product is vacuum-packed and sealed into co-extruded five-layer bags, with a high barrier to oxygen. They usually pack in 6 pound bags, because most of their customers are restaurants, but they also have a 250 g package for retail marketing. The bags are frozen in a blast-freezer at -30<°C, and afterwards the bags are stored at -18°C (Figure 33 Guacamole bags).
80% of Aguamich's production is sold in the United States, the rest in Europe and Japan. They have only two clients in Mexico. When in high season (September to May), they produce 3 million pounds, However, in low season when the price of avocado is higher, they produce 2,000-3,000 pounds a day. They employ 150 people, 70 in pulp processing, and the rest in the packing of fresh fruit (Ing. Jorge León, personal communication, 2003).
Avocado is also used for the confection of baked products (such as cakes), to elaborate fine soup mixes, appetizers, and in the production of cosmetics (oils, skin lotions, soaps, shampoos, etc.) due to its oil content. Avocado oil is appreciated because it contains an easy to absorb sterol, and it is biodegradable.
Besides being an important cosmetic ingredient, the pharmaceutical industry considers the unsaponifiable fraction of the oil as a valuable raw material. From this fraction, the factor H is extracted, which is used in the pet food and cooking oil industries.
The avocado oil industry processes around 1000 tons of fruit per year, from which 500 tons of raw oil are obtained. From this, 5% is used in the production of edible oil (90%) and cosmetics (10%) in Mexico, and the rest is exported. The exported oil (495 tons per year) is sent to the United States, where it is refined and then sent to Japan and Europe as edible oil and cosmetic ingredient. Between 10 and 20% of the oil is lost in the refining process (SAGAR, 1999).