Waxing Red Delicious
Over the last several months, I visited warehouses to discuss waxing and waxes in an effort to understand the process. It very soon became obvious that programs used in the industry are based on the experience of each operator. Each shed, regardless of similarity of equipment and wax, follows a different program of fruit preparation and waxing. The shine quality is very different even when the same conditions are imposed for a number of reasons. In some cases, the finish on the fruit is different. Conversations with many individuals have convinced me that the quality of the wax job, like so many things in the warehouse, starts in the orchard.
Researchers have not made detailed studies of how production practices affect fruit finish, other than looking at russetting. Many of those I spoke with in the warehouses are convinced that orchard factors affect the continuity of the outermost layers of the fruit. Of course only scientists have access to equipment needed to examine whether these practices are affecting the epidermal cells, the pectin layer or the cuticle (See Figure 1 of Dr. Kolattukudy's article). However, if you examine fruit with a 20x hand lens from a lot which waxes well, and compare it to fruit which is tough to wax, you often can detect a difference.
Orchard factors which have been noticed to affect finish include:
- Nitrogen level
- Specific chemicals (growth regulators, pesticides, etc.)
- Side bloom vs. king bloom
- Frost-crop load
- Maturity at harvest
Although not orchard-related, storage length has been mentioned as a factor. It is obvious that research is needed in this area.
A second factor affecting the shine obtained on a specific lot of fruit is the quality of the preparation before waxing. I was told repeatedly that a good shine with any wax was dependent upon the cleanliness of the fruit and the ability of the line to remove water from the surface of the fruit (dewatering). Some said that 70% of the reason for a good shine was based on fruit cleanliness and dewatering.
Although the equipment in each packingline varies, there are several steps which must be undertaken. A review of these steps might serve useful in improving the quality of the shine. These are not recommendations, rather they are offered as observations of industry practices. Comments are made only about Red Delicious.
Removal of Residue
Overhead irrigation with hard water generally leaves a residue on fruit which must be removed. Some houses attempt to remove this with fruit cleaners on the line. In this case an acid type cleaner would be used. More information can be found in the section on fruit cleaning.
One alternative is to use an acid (acetic acid or hydrochloric acid) in the dump tank, a drencher, or a special tank. The length of time the fruit is in the tank and the agitation of the fruit will affect the cleaning. Be aware that acids are corrosive and chains and tanks must be protected or they will wear quickly. Painting tanks or using stainless steel will increase longevity. Plastic chains are available. Metal chains will last longer if oiled constantly with a food grade oil. The application of too much oil to the chains may leave a residue on the apple skin and impair the coating characteristics of the wax. If an acid is used in the dump tank Reds should not be left in the tank more than 1/2 hour or they will be damaged.
Fruit skin must be warmed before a wax is applied, otherwise application is very difficult. Raising the skin of the fruit 3-4°F will help to break the "sweat" of the fruit. Technically, the skin of the fruit must be raised above the dew point. The dew point is the temperature at which vapor begins to condense and be deposited as liquid. This temperature change must occur before wax is applied. The point at which this is done on the packingline varies with the operation.
Many packinghouses use the dump tank to raise the skin temperature above the dew point. Water is kept at 80-85°F often by using a swimming pool heater. All too often, the water in the dump tank falls well below the desired temperature. The temperature setting should not be increased, rather the water heater should be checked to make sure that it can handle the large volume of water and the temperature drop caused by cold fruit.
I have seen fruit which has been damaged by very hot water in the dump tank. Darkening of the calyx lobes is often a first sign of too much heat on the line. Fruit temperature is a function of the water temperature and the length of time in the tank. Many systems are designed so that the fruit stays in the tank a minimum of 90 seconds. The water in the dump tank should be changed regularly.
If any of the compounds containing Sodium Orthophenylphenate (SOPP, etc.) are used, the water in the dump tank should be kept below 65°F or calyx and skin injury may appear.
Dump Tank Rinse
In some cases, a spray is used on the fruit as it is elevated out of the dump tank. The spray is either dump tank water or fresh water. This rinse may be warm water.
Products and application techniques vary greatly. In some houses hot water is used for the application of the cleaner (under 140°F). In others the water is unheated.
Several houses change their cleaner on the basis of the type of deposit on the fruit. Alkaline cleaners do a better job of removing dirt while acid cleaners seem to be better on calcium deposits. This change is facilitated by the use of an injection pump system.
This step is a problem for houses running a larger volume of fruit than they were designed to run. Cleaning is another step which requires time and agitation. In addition to removing dirt and hard water deposits, the cleaner removes some of the natural wax and prepares the surface of the fruit for waxing. Some sheds have foaming nozzles or systems, while others have found that modifying the type, numbers or speed of the brushes helps improve cleaning. This is a critical step.
Removal of the soap is important. Sheds have successfully used hot water (under 140°F) with varying types of brushes. Consideration could be given to 2 rinse sprays. Fresh water should be used; volume is important.
The goal of dewatering is to remove water on the surface of the fruit. Success can be measured by making sure that the calyx is free of water. At this point in the line the fruit must not sweat--try breathing on the apple.
Most sheds dewater by using fans and brushes. Some also use heat lamps. The close proximity of the rinse nozzles to the dewatering area increases the relative humidity and makes the job more difficult. Lowering the relative humidity by using a dehumidifier on the dryer with the inlet here will help. Blocking off the rinse area can improve performance. Having alternate brushes running at different speeds can help. Some sheds use sponges then brushes. Modifying the air velocity under the fans is another possibility. Fans remove fine droplets while the brushes break up larger drops. Approximately 4-5 feet under fans seem to be necessary.
Check the adjustment of the beater bars (flipper bar, dewatering bars or angle bars) so that the bar touches the brushes about 1/8 inch down the bristle. If the bars are placed too deep into the brush, the drops will not be removed and it will increase the wear on the brush, and be more expensive. If the bars are placed too shallow, then not enough water will be removed. The angle of the bar is important. They should wipe the brushes on the downstroke of the brush slightly past the horizontal. If sponges are used, the roller bars should be equipped with wiper plates. The equipment manufacturer should be consulted for more precise directions.
If you have skipped to this section for a recommendation on a wax to use I'm sorry to disappoint you. The depth and quality of shine seems to depend more on the other factors described here than on the specific wax. I will try to give some methods of evaluating waxes later in the article.
In sheds using waxes with low amounts of solids, no special type of nozzle is used. Waxes are dribbled on and applied with the brushes. Others believe that it is necessary to atomize the wax as it comes out of the nozzle. Too much atomization of the wax will make an unpleasant working environment. An air nozzle system is recommended when applying low volumes of wax with high solids. A few houses use heated nozzles with heating coils in the nozzle body. They claim that this works better than other nozzles, especially in cold weather.
If the fruit is not dry, the wax formulation will be diluted by the water remaining on the fruit and the shine will be poor. The wax is dispersed by the nozzle but applied by the brushes. In a well performing system only three drops of wax per fruit are necessary. The system should stop the wax applicator immediately if the line stops.
The practice of applying waxes to fruit and vegetables after harvest is actually quite ancient. In China, as early as the twelfth or thirteenth century, oranges and lemons were dipped in molten waxes in order to ferment the fruit. In the United States, wax coatings have been used commercially for over 50 years on many types of perishable produce.
As early as the 1930's hot-melt paraffin waxes were being applied to oranges by misting the melted wax onto brushes. In the late 1940's, solvent type waxes replaced the paraffin waxes. Solvent waxes are so named because they include one or more resins dissolved in a blend of petroleum solvents with plasticizers and other film forming agents added to improve shine, leveling and film flexibility.
Carnauba wax emulsions were introduced in the late 1950's. Waxes containing shellac and alkali soluble resins dissolved in water, supplemented by film forming adjuvants, were introduced between 1962 and 1964. Just a few years ago, "concentrate" waxes became commercially available. These high solids waxes, generally applied with air nozzles, produce an extremely high shine and have the advantage of having less water to remove during the drying process.
Types of Waxes Used Today
Carnauba wax is excreted by the leaves of the palm Copernica prunifera (Mill.), a native of northern Brazil. It is a fast growing palm, reaching heights of 45 feet, grown in the dark fertile soil of the Brazilian river banks and valleys. The leaves are harvested, sun dried, and then beaten to remove the wax. The powdery wax is then melted and filtered. The cooled wax is then broken into pieces and sent to market.
Shellac is a refined form of raw lac, a secretion of the lac bug, Laccifera lacca Kerry, found in India, Sri Lanka, Thailand and Indonesia. The secretion is formed on trees where colonies of thousands of the lac bug feed. The lac is harvested and purified by washing and filtering. For use in fruit coating, a soda solution of shellac is bleached with sodium hypochloride.
There are a number of food coatings approved by the Food and Drug Administration for application to various kinds of produce but most are not approved for apples. For one reason or another these have not been found suitable for use in the fruit industry.The waxes that we are using are either a carnauba based wax or a shellac based wax or a combination of both, all with various additives to make the wax flow on better, improve drying time, and decrease drying temperature. It is obvious that wax chemistry is extremely complex.
It is often not possible to get a better shine by simply increasing the temperature in the dryer. Increased dryer temperature will be very detrimental to fruit condition. Modify other steps before increasing the dryer temperature. The purpose of the dryer is to drive off the solvents in the wax formulations and set the wax.
Dryer temperatures can be lower on days with low atmospheric relative humidity and raised on rainy days. Using a dehumidifier on the line can help reduce temperature and save energy.
Raising the temperature of the fruit is detrimental to its condition (eating quality). Hot water heats more rapidly than hot air -- just the opposite of hydrocooling. Keep the temperature of the water as cool as possible while still doing the job. The rate of deterioration depends upon both time and temperature. The cooler the line can be run and still get a good shine, and the faster the fruit can be cooled down, hence less deterioration.
I was pleased to see that many sheds figure that it took 5-10 minutes from bin to packed box for a total of 15-30 minutes out of storage. Unfortunately, cooling packed fruit in storage is a relatively slow process. (See the PPN Vol. 1 No. 1 pages 14-15 for an article on this subject by Gene Yost, USDA).
Immediate shipment of freshly waxed fruit should be avoided since refrigerated trucks do not have the capacity to cool fruit. In addition, many waxes perform better if cooled down before the fruit is jostled.
Judging Wax Quality
I came across several methods of judging wax quality which I thought I would pass along.
At the waxer check for foaming and evenness of application. After the dryer look for evenness of shine and tackiness. Look also for scuffing from the dryer rollers. Check for wax globules on the fruit especially at the calyx end. Look for the amount of wax in both the skin and calyx cavities. Reduce the amount being applied if bubbles appear on the calyx end. In addition, bubbles may be the result of uneven fruit flow or high rush speed. Check for wax shattering.
In the packed box look for additional scuffing, evenness of shine after cooling. By placing packed fruit alternately into warm and cold places several times, the potential for whiting can be gauged. Other tests are to dip fruit in warm water, wrap fruit in a hot, wet towel, or jostle warm fruit.
Waxes which contain morpholine or ammonia in the formulation need time to allow the volatiles to evaporate from the fruit. Some waxes may take as much as a week to "cure". However, when cured they show vastly different whiting characteristics than fruit right off the line.
Attention to detail is critical. Consider assigning a person working at the sorting table to monitor wax application constantly.
Some sheds have found it valuable to clean brushes every night with hot water and soap and use high pressure water on the flipper bars. Wax residue on the dryer rollers will dry hard and puncture or injure fruit. It may mean excess wax on the fruit. Steam cleaning dryer roller bars regularly will reduce this problem, or use ammonia and mechanical scraping. Sorting table rollers should also be cleaned regularly.
Research on Waxes
The Tree Fruit Research Commission, USDA and Washington State University researchers have begun a series of experiments to test procedures for apple waxing that will maximize shine, shine durability, and minimize wax discoloration, whiting or chalking. We are currently looking for pictures of apples sent to the orient (or southern markets) which have developed whiting. Please send photographs of these fruits to:
Wenatchee, WA 98801
Phone (509) 662-0981
In order to assist the research project we need information about wax formulation used, market and date. Thank you.
Obtaining a good luster is extremely complex but seems to be essential for marketing the volume of fruit produced in Washington. Paying attention to detail is a critical step in the process and does pay off. As a physiologist concerned with the edible quality I urge sheds to apply only the minimum amount of heat necessary on the line and to cool the fruit rapidly.
I would like to thank those sheds who have shared information with me for the betterment of the entire industry. I have also received excellent cooperation from many of the wax formulators in both written responses to my questions, and in discussions. Thanks especially to Dr. Hesh Kaplan, Pennwalt; Tom Watson, Fruit Packers Supply; and Dick Lutz of Van Doren Sales.
Dr. Eugene Kupferman, Postharvest Specialist
WSU Tree Fruit Research and Extension Center
1100 N. Western Ave., Wenatchee, WA 98801
Post Harvest Pomology Newsletter, 2(2):8-15