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Page last updated
29 Oct 2010
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Handling apples has always been a potential source of damage. Most handling damage results in one or more bruises (which range in size from barely visible to very visible and massive) on each apple. Bruising, which is objectionable to fresh market consumers, can result in a lower grade for any apple (USDA, 1978). Avoiding bruise damage has been part of the design and operating strategy for all handling operations that take place between the tree and the consumer. However, bruising has been difficult to eliminate. In 1986 the apple industry asked our cooperative research team to determine where and how bruising occurs in commercial operations and how it can be avoided.

During the 1986-87 packing season we measured the bruise damage on apples caused by: 1) the hand picking and bin hauling operations of 6 different growers for Macintosh, 2) the packingline operations of 8 different packers for Golden Delicious, and 3) the distribution operations for 2 different produce distributors for Golden Delicious.

The average number of bruises (larger than 1/4 in. dia.) caused per fruit was about 2.6 for the picking operation, 2.2 for the bin hauling operation, 5.4 for the packingline operation, 2.7 for the bagging operation, and 0.34 to 1.45 for the distribution operation, depending on the packing system and total transportation distance (up to 360 miles) in conventional spring-suspension semi-trailers (Sargent et al., 1987; Brown et al., 1989a; Schulte Pason et al., 1990). These results indicate the packingline operations caused most of the damage, the picking and hauling operations caused somewhat less, and the distribution operations caused by far the least (warehouse workers were not allowed to throw or drop any cartons). Most bruises were caused by apples impacting hard surfaces or other apples.

Since then we have continued studies aimed at identifying practical ways for minimizing apple bruising. We initially concentrated on the packinghouse operations and equipment, including the development of improved equipment for apple bagging (Brown et al., 1989a and 1989b; Marshall et al., 1992a and 1992b). We have analyzed the hand picking operations, some bulk bin structural designs, and the effects of truck suspension type and road quality on apple bruising (Sargent et al., 1987; Burton et al., 1989; Schulte et al., 1991, 1992; Armstrong et al., 1991, 1992; Timm and Brown, 1992). Two studies have also been completed comparing the relative bruise damage that can be expected during the distribution of packaged apples to retail stores (Schulte Pason et al., 1990; Singh et al., 1992).

The results of these studies show that bruising can be reduced greatly from the levels we found in the 1986-87 studies. In most cases the required changes in procedures are relatively inexpensive, but the reality of maintaining low bruising requires consistent effort to avoid bruises in each operation. If mandatory grade compliance prior to shipping were required nationwide, several changes could be made quickly and the costs would be recovered readily. Today, gradual change is occurring as management learns and chooses those alternatives that enhance their competitive advantage.

Several researchers in the United States have completed studies that show the role of compression, impact, and vibration forces in the bruising of apples. This report will not attempt to review their work but will describe the changes in handling methods and equipment, as indicated by our project results, that will help the apple industry minimize bruise damage for fresh market apples.

Apples bruise easily due to compression, impact, or vibration forces. Understanding where these forces occur can help avoid bruising.

Compression forces can be applied by the picker's body (e.g., hands, fingers, or legs), tree limbs, ladder rungs or rails, bin rails, or bin bottom. They can also be applied by other apples due to the weight of a full picking bucket, excessive bin depth or carton stack height, by equipment (fork lifts, conveyor side rails that suddenly narrow, narrow-width washers or waxers that cause a full line of rotating apples to suddenly wedge as they advance, etc.), by workers who force cartons shut or into a tight place, or by customers who like to squeeze or pinch the fruit. Most problems can be avoided if common sense is used.

Impact forces happen very quickly and many times are not obvious in mechanical handling systems, particularly on packinglines. Their effects are usually seen as bruises, permanent damage, and lower quality. The Instrumented Sphere (IS) can easily identify where these occur and when a prescribed change in any system has eliminated the problem. An IS analysis of the packingline after several months of operation will show whether the cushioning is still adequate or needs replacement.

Our laboratory tests show that large apples (about 6.5 oz) of several varieties (Macintosh, Golden Delicious, Red Delicious and Law Rome) are the most sensitive to impact bruising at picking time (Timm et al., 1989). Bruise sensitivity decreases (amount depends on variety and storage conditions) as storage time increases. The Macintosh variety seems to be most sensitive to bruise damage and remains so throughout the storage period. Consequently, our recommendations for cushioning and transfer conditions on packinglines are chosen to permit large Macintosh apples, one day after picking, to be handled without bruising.

Vibration forces usually occur during transportation, are difficult to avoid, and can be repeated at 5 to 15 times each second for many hours of the trip. Vibration forces of only 25 to 100% above the normal 1 G pull of gravity can cause severe accumulated bruise damage if the natural vibration frequency of the full bin or full carton is excited. A better understanding of apple damage as affected by the transportation system and packaging design is needed so that we can learn how to avoid the high economic losses that sometimes occur.

To help identify practical ways to avoid apple bruising, our project developed an Instrumented Sphere (IS) (Zapp et al., 1989, 1990) which can record impacts that it experiences during handling operations. The IS is a 3 1/2-in. dia. hard ball weighing 12 oz. that contains electronic components and is battery powered. The IS records each impact pulse and the accompanying real impact time in internal memory. The real time enables impacts to be related to specific locations in handling operations. The IS moves freely with the fruit during data collection. A personal computer and custom developed software are used to control the IS and analyze the data. The impact results will depend upon the cushioning on the impact surface and the velocity at which the IS hits the surface. A simple graphical technique, represented by the dashed lines in Figure 1, is used to show the impact results in relation to specific surfaces that are typically used in fruit and vegetable packinglines. Each surface has a characteristic IS response line described by the Peak G (acceleration) and velocity change values collected by the IS. The next step in analyzing any packingline, or transfer point, is to identify which impact levels correspond to apple bruising.

To relate the IS information to apple bruising, the IS and Macintosh apples were dropped (on a cheek) onto several surfaces ranging from hard to soft, from a range of drop heights (Schulte et al., 1992). Each surface was dusted with powder which marked the exact impact area on the apple. This area was later inspected (internally and externally) for the presence of bruising. The results for Macintosh apples are summarized by the solid lines in Figure 1. They show the IS impact conditions that avoid bruising most apples (0 to 1096 bruise occurrence), cause bruising in half of the apples (50% bruise occurrence), and cause bruising in all of the apples (100% bruise occurrence). Only the IS impacts which plot to the left of the 0 to 10% bruise line indicate bruise damage is unlikely on real apples. As an example, apples dropped onto a hard surface (e.g., metal, wood) remain bruise free only for very small drops (up to 1/10 in.) and low G levels. Drops greater than 3/16 in. onto a hard surface will bruise all of the apples. Similar results apply to transfer ramps that are often tilted up to 20 degrees from horizontal (Brown et al., 1991). In practical terms, cushioning is essential in all packinglines since drop heights at transfers are typically much greater than 1/10 in. Poron cushioning was chosen for these tests because it is widely used on apple packinglines and is consistent in response as sold by the manufacturer.

The picker is normally the greatest potential cause of bruise damage during hand harvesting operations (picking, bin filling, bin hauling). The picker controls bruising until the apple is in the bulk bin. In our picking tests, bruising was not reduced when pickers wore gloves but was reduced somewhat by certain types of cushioned buckets. Supervision, adequate instruction on proper apple handling techniques, and daily quality inspections of harvested fruit are key factors for minimizing fruit damage in the orchard. Don't assume that new employees know how to pick apples the way you want them picked, even if they say they do. Show them exactly how to pick apples, place them in a cushioned bucket, and empty the bucket into the bin. Check their performance daily and provide feedback and incentives. Properly trained and supervised apple pickers can pick efficiently, yet average less than 1.0 bruise per apple. However, a careless, unsupervised picker can easily triple the amount of bruising. Apples are most sensitive to bruise occurrence the day they are picked, so make sure the pickers and bin handlers understand this--and what your expectations are for quality.

Bin hauling operations will be very damaging if the bulk bins are loose, rough, dirty, or have a thin (springy) bottom. Bins made of rough-sawn hardwood boards can be especially damaging. Short forklift tines usually spring board floors upward and bruise many apples, so be sure the tines are full length. Rigid, smooth, clean, thick bottom bins cause the least bruise damage. About one-third of all apples in a bin are in contact with the bottom or sidewalls. Bruising and abrasion can be reduced by lining the bin with 1/8- to 1/4-in. thick cushioning or smooth fiberboard, but the bin must be re-used several times each season to pay for the lining, and such linings can create problems during hydro-dumping. Using good bins for high quality apples may be a more practical choice than using bin liners. A new spray-on elastic bin lining product (Modified Acrylic Bin Lining, Ponderosa Paint Co., Fresno, CA) has been praised by apple packers in California and may be practical in other producing areas.

Smooth roads without ruts and bumps should be selected when transporting full bulk bins out of the orchard. The apples in the bin can be bruised each time the bin is picked up and set down, so devise an anti-bruise strategy that minimizes handling and movement in the orchard with forklifts. Self-loading bin carriers with soft balloon tires are the most gentle on full bins of fruit. Avoid transporting bins with short forks, tractors with front-mounted forks, or stiff suspension trailers and trucks.

Highway transport of apples in bulk bins should be done on trucks equipped with air-cushion rather than steel-spring suspension. Choose the smoothest route to the destination and drive as slowly as it is practical. We have analyzed road roughness data and coded all federally funded highways in Michigan as to their quality for the bulk transport of apples. This information may help growers and packers maintain higher quality levels. The adverse effects of poor bins, rough roads, railroad and bridge crossings, travel speed, etc. will be minimized by using air suspension trucks.

In general, bruising at the packinghouse can be attributed to several handling operations found within a packinghouse: bulk bin handling, apple transfers between packingline components (e.g., washer to dryer, etc.), synchronization between components (e.g., singulator to sizer, etc.) and the operating speed of the packingline. Bulk bin unloading and handling at the cold storage and packinghouse must be done smoothly. Minimize the number of times a full bin is picked up and set down. The floors must be smooth so that fork lift travel does not transmit impact to the apples. Some single-bin, as well as continuous motion, hydro-dumpers have been observed to jar the full bin so violently that apples move or are thrown in the bin. Make sure this operation is smooth.

In theory, a packingline should be level at all transfer points. However, in practice this is not always possible, especially on lines already in place. Typically, most packinglines have 7 or more transfers that can have vertical drops exceeding 1¹/₂ in. and ramp angles that average 19 degrees. Without adequate cushioning on hard surfaces and control of fruit velocity, this sets the stage for repeated apple damage.

All hard surfaces must be cushioned. Cushioning for packinglines should be at least 1/4 in. thick, should not be so springy that apples bounce after impacting, and be durable and easy to clean. We now recommend that a 1/4 in. thick layer of Poron cellular urethane sponge be applied to all hard surfaces that apples may hit. In addition, some packers in Washington use an 1/8-in. thick Poron precision cut insert in each hard plastic cup of the weight sizer. Cushioning should not be expected to stop bruise problems where excessive height difference or fruit velocity exists. These places require redesign or the use of brushes or curtains.

Roll velocity must be kept low when apples transfer onto uncushioned sizing chains or bars, inspection rollers, or dryer rollers. Flexible ramps, curtains, overhead powered brushes, or in some cases pear brushes (which apples drop onto) can help limit velocity. High velocity is caused by excessive elevation change at a transfer point. Use only enough elevation change and ramp angle to keep the fruit moving at normal speed.

The packingline should be run at speeds that keep it nearly full (but not overfull) of apples. Some apple varieties (e.g., McIntosh) must be run at slow sizer speeds to minimize bruising. Large open areas or gaps between apples will allow them to roll through transfers at high velocity and be bruised. All pieces of equipment should be of uniform width, and a 90 or 180 degree change in direction should be avoided unless made in water or at the sizer drop-outs.

Sizer drop-outs should have cushioned lane dividers or flags to prevent high velocity apple-to-apple impacts. Conveyor belts should be suspended where apples drop to them, otherwise the apples will be bruised by the hard surface underneath the belt.

Bagging machines are a major cause of bruise damage on packinglines. Some semiautomatic baggers result in reduced bruising because the bag is filled at a low angle (apples don't drop into the bag) and the operator gently settles the apples and completes the tying operation. A commercially developed, simple, semi-automatic chute bagger, located at each drop-out on an electronic color and weight sizer, may now offer the lowest damage and the most flexible and lowest cost approach to bagging.

Another aspect of the packingline that must be addressed to maintain superior product quality packs is the lighting provided in the sorting areas. Bruises and other light-brown colored defects are often difficult for the sorters to recognize. Sorting is commonly done using cool white fluorescent light, at about 100 footcandles of intensity on the fruit. The sorting belts or rollers are often white or silver colored. Our tests have shown that 3000°K fluorescent light (GE SP-30 or equal), at 250 to 500 footcandles, and a dark or black background are much better for detecting brown defects for a wide range of apple colors (Brown, 1991). For green and yellow apples 250 footcandles is adequate, but dark red apples require 500 footcandles. This means that a switch is needed so that the low (2 bulb) or high (4 bulb) lighting level can be selected as needed. The bulbs should be about 20 in. above the belt and parallel to it. These recommendations address the needs of older workers who require twice the light levels of younger workers. Adequate lighting will help reduce the defects that are now found in some packs.

Many packaging systems have been used, and are still used, for apples in the United States. The primary purpose of packaging is to provide a unit that facilitates delivery of the desired amount of product to the consumer. A companion purpose is to economically protect the product from damage.

Although our 1986-87 studies of tray-pack and poly-bag packaging systems found that bruising during distribution was relatively low, we considered only up to 6 hours of transport. The amount and severity of bruise damage accumulated during 25 to 50 hours of cross-country transport using steel-spring suspension trucks on today's poor highways is another story. Loads can arrive in disgusting condition (most do not). Many large buyers of apples and other bruise-sensitive produce are now requiring transport on air-cushion trucks. Cooperative studies by the School of Packaging at Michigan State University show that air-cushion suspensions cause less bruise damage than standard steel-spring suspensions (Singh et al., 1992). Damage can be reduced further by using a foam plastic tray packaging system, instead of the conventional pulp tray or fiberboard cell packaging systems. Some packers have reported that the foam plastic system is not perfect either, since they have experienced carton collapse at the bottom of some pallets shipped long distances or overseas. Whether this problem can be avoided is not yet known.

We are preparing two training videos intended to help reduce bruising and help workers in the apple industry be more productive. The first is intended for new employees who will be pickers, fork lift operators, or truck drivers. The second is intended for new packingline workers and fork lift operators in the packinghouse. Hopefully, even present workers and supervisors will benefit. Both videos are scheduled to be available by June 30, 1993, from the Cooperative Extension Service, Bulletin Room, Michigan State University, East Lansing, MI 48824.

An extension bulletin (E-2290) providing recommendations based on our packingline studies is available (Guyer et al., 1991) and provides details on how to revise an existing line or install a new line to minimize bruising.

Several packingline equipment manufacturers, their dealers, and independent consultants are using the IS to analyze packinglines and make modifications which reduce impact bruising. A smaller IS (about 2 1/2 in. dia.) is also now available.

Due to space limitations, references have been omitted. They may be obtained from the editors.

G. K. Brown, Research Leader, U. S. Department of Agriculture, Agricultural Research Service, Fruit and Vegetable Harvesting; N. L. Schulte, Specialist, E. J. Timm, Specialist, P. R. Armstrong, Specialist, Agricultural Engineering Department; D. E. Marshall, Agricultural Engineer, U. S. Department of Agriculture, Agricultural Research Service, Fruit and Vegetable Harvesting; Michigan State University, East Lansing, Michigan 48824 USA.