Monitoring peach and nectarine ripening

Picking peaches and nectarines at the right stage can be tricky. Here are some tips to help make the eating experience great.

Ripeness guidelines are presented here to help Michigan growers, packers, shippers and marketers deliver high quality peaches and nectarines. The key factors are fruit firmness, fruit size, shape and skin color. Generally, the ripening sequence of a typical peach fruit on the tree is

  1. Rounding or filling out of peach,
  2. Change of background color from green to yellow along with a drop of firmness, and
  3. Increase of peach aroma.

Fruit left on the tree will generally continue to develop sugar and reduce acidity. Fruit picked late will have poorer storage characteristics. Once a peach or nectarine is picked, the sugar content does not increase significantly, but the acidity will decrease. Sugar content is very important to eating satisfaction – peaches testing below 11° brix usually are not satisfying to consumers. Some varieties will reach 14° brix or more.

Judging the correct time to harvest peaches is complicated. A trained picker will use their hand to feel for a rounded suture region, indicating that the peach has filled out (Photo 1). Once the fruit has filled out and developed proper skin color, decisions on whether or not to harvest depend on judgments of fruit firmness. Fruit firmness can be estimated by gently squeezing of the fruit with the sides of the fingers.

A penetrometer is a handy tool for monitoring firmness peach ripening (Photo 2). Although experience is needed to learn the ripening characteristics of each peach type, firmness monitoring provides an objective tool to help determine the extent and speed of ripening, and to communicate with pickers, wholesale buyers and sellers. A penetrometer (also called a pressure tester), commonly used by the apple industry to test apple firmness, can be used to plan peach harvesting and storage. Common brands in use by growers are the Wagner FT Fruit Tester, Cropmeter, McCormick, Effegi, and Magness-Taylor pressure instruments. Penetrometers can be purchased via the internet and from local orchard supply businesses.

Test peach fruit using a penetrometer with an 8-millimeter tip (5/16-inch pear tip). This is the smaller of the two tips provided with several standard penetrometers. The part usually sampled is the cheek of the fruit, however, the first region to become soft may be the shoulder, tip, or suture, depending on the variety and year. Fruit should be at about room temperature when tested because cold fruit can give higher readings. Remove a dime to quarter-sized slice of peel from the fruit (Photo 3) and then use sufficient uniform force to submerge the tip to the guide mark in approximately two seconds. Better readings are obtained if the fruit is held against a non-moving surface – a drill press stand works very well (Photo 4). The drill press stand can be mounted sideways so that the juice can be collected easily for sugar measurement.

With typical Michigan peaches, peach background color changes from green to light yellow at approximately 11 to 13 pounds firmness, although some older varieties retain a green background even when ripe. Proper picking time depends on the marketing strategy. Fruit destined for local markets can be picked less firm (Table 1).

Peaches picked for medium distance shipping or local sale can be harvested in the 6- to 12-pound firmness range. Some peach varieties, especially late-season varieties, have a tendency to develop flesh mealiness or lack of juiciness in storage. This mealiness may occur when very firm (9 pounds or more) fruit are stored in the so-called “killing zone” of 36 to 45°F. Unfortunately, many farm market coolers operate in this temperature range to accommodate a wide assortment of fruits and vegetables. Peaches testing 8 pounds or over can be stored in the 36 to 45°F range without enhancing mealiness. Peaches picked below this firmness will require more care to avoid bruising. Peaches below 3 pounds require extra care, quick sales and cautioning the buyer that the fruit should be used soon. Careful attention to these strategies and indicators of fruit ripening will help insure the best quality fruit.

Table 1. Ideal peach and nectarine firmness ranges for steps in supply chain for
three marketing destinations.

Long distance chain store
16 to 12 lbs.* Picking, packing, transport to regional warehouse
12 to 8 lbs. Soft enough for transport to retain stores
8 to 6 lbs. Transfer to retail store and put out on display
3 to 2 lbs. Purchased by end consumer
Medium distance chain store
12 to 8 lbs. Picking, packing
8 to 6 lbs. Transfer to retail store and put out on display
3 to 2 lbs. Purchased by end consumer
Local farm market
8 to 6 lbs. Picking and display
3 to 2 lbs. Purchased by end consumer

*This summary is based in part on work and summaries developed by Carlos Crisosto, Kearney Agricultural Center of the University of California.

Uneven halves
Photo 1. Uneven halves and prominent ridge indicate this peach is not quite
ready for harvesting.

Firmness test
Photo 2. Use of penetrometer to test peach fruit for flesh firmness. Brace hands
against a table or other firm surface to make readings more reliable.

Fruit peeler
Photo 3. Fruit peeler used to remove dime- to quarter-sized area of skin before
testing flesh firmness. Some penetrometer kits include peelers similar to the
one shown.

Pressuer tester
Photo 4. Pressure tester on drill press stand is used sideways so juice can be
collected for brix test.

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