Bruising has been a problem since apples were first harvested and is still the most serious disorder on retail shelves. This season I have received a number of calls from packers who are seeing bruising on apples before packing, and even more after packing.
There are two factors that influence bruising: impacts and compression. Impact bruising, the most common type, occurs when fruit are dropped onto a hard surface. The damage is influenced by the surface onto which the fruit fall and the speed of the fruit at time of impact. The other type of bruising is compression damage, which occurs when fruit are pushed into a bin or bag.
Some cultivars are more susceptible to bruising than others, but no variety is immune. Red Delicious has a reputation that it will resist bruising. This is not true, as we found out when we peeled several thousand fruit after packing. Numerous small bruises were found in the flesh that could not be seen through the skin.
Dr. Gary Hyde at Washington State University determined that Red Delicious was actually more susceptible than Golden Delicious. The belief that Golden Delicious is more susceptible stems from the fact that it discolors more easily. A study in the United Kingdom found Gala and Jonagold apples more susceptible to bruising than Golden Delicious. The portion of the apple with green skin bruised more easily than the portion with red color.
Harvest maturity alone does not influence bruising, but large fruit bruise more easily than smaller fruit. The method of harvest, the dumping of fruit from picking bags, and the swamping of the bins in the orchard play huge roles in the amount of bruising. Fruit harvested wet will show numerous finger bruises, and more bruising will occur when pickers are wearing gloves.
Picking into padded buckets reduces harvest bruising compared to picking into a soft-sided bag or unpadded bucket. Scientists in Michigan demonstrated that bin pads reduced bruising by 25 to 50%, and most bruising occurred when the bin was carried on the back of the tractor rather than the front. The most serious bruising occurred on fruit at the sides of the bin where it is in contact with the wooden surface.
Bruising was most severe in the bin furthest from the tractor. Moving fruit from the orchard to the packing house can be another source of bruising. Trucks equipped with air suspension had fewer bruised fruit than those equipped with spring suspension. These studies suggest that 35% of bruising occurs during harvesting and hauling.
A study on Delicious and Granny Smith determined that the longer they are stored, the more susceptible to bruising they become. Packers notice that the fruit from certain rooms are more susceptible to bruising than others. This can be traced back to the humidity within the room.
Newer rooms are airtight and are run with a minimum number of defrost cycles to minimize moisture loss to prevent shrivel. Fruit from these rooms develop tremendous internal turgor pressure and are very susceptible to bruising. Packers have developed various methods of “conditioning” this fruit to allow for a controlled amount of moisture to be lost by the fruit to reduce the susceptibility to bruising.
Some methods of conditioning include increasing the number of defrost cycles, opening the doors and increasing the temperature, or placing the bins in a warm room before packing. It is easy to remove a small amount of moisture, but difficult to ensure that all fruit in a bin are affected equally. There is no uniform method, and most packers use a number of techniques.
Temperature and humidity
Hyde, working with Red and Golden Delicious, found that the colder the apple, the higher the bruise susceptibility. However, manipulating fruit moisture was more powerful than temperature. He determined that by slightly dehydrating the fruit (2-3%), the bruise threshold will double.
Firmness is not a good reflection of bruise susceptibility since over the course of five weeks in storage the fruit had lower firmness levels, but bruise susceptibility did not change. The effect of temperature and fruit moisture was less than the effect of the impact force hitting the fruit; thus it is more effective to reduce bruise impact points than to change the fruit. After injury, the bruise will be larger if the fruit remains at a higher temperature—another reason to cool fruit rapidly after packing.
Packing line factors
Simple things like using pads to cushion drops, reducing elevation changes, and minimizing turns in the line can help reduce bruising. An instrumented pseudo-apple was developed that can precisely measure the impact forces on apples. This Impact Recording Device, or Instrumented Sphere, is available from Techmark, Inc. (www.techmark-inc.com), and has been used by packers to survey their lines to minimize bruising.
The use of this device enabled Hyde to survey a number of Washington apple lines. He found that, in general, damage was less when the equipment was full of fruit. The brush section can cause small bruises, and high brush speeds used for Red Delicious provided more impact than the low speeds for Goldens. Transfer points can provide impact damage, and analysis of every drop is necessary. Drops of 12 inches or greater cause damage to about 75% of the fruit.
This has been cited in several reports as the most dangerous packing operation, as far as bruising is concerned. This is no surprise to anyone who has looked a hand bagging operation. Automatic bagging machines should have a cushion on top of the plate supporting the bag; even a shag rug will help.
Bruising increases the susceptibility of fruit to blue mold decay, which is found in most packing houses. Even a bruise not visible to the naked eye can assist the fungus in gaining entry into the fruit. Reprints of a number of studies on bruising can be obtained by contacting me at: firstname.lastname@example.org.
Much of the information cited here comes from studies done at Michigan State University and USDA-ARS, Michigan, as well by Dr. Gary Hyde (retired) at WSU. Information is also available on the Internet at www.postharvest.tfrec.wsu.edu. m