Painless and Efficient Maturity Testing
J. Clements, AgriculturalAgent, Michigan State University
Win Cowgill Agricultural Agent

Our observation has been that few growers utilize the Starch Index (SI) method of
determining harvest maturity. Perhaps SI testing is perceived as time consuming and
difficult to properly judge.

We contend that SI testing, however, is the best and easiest indicator of apple maturity that
a grower can use to plan their harvest and storage regimes.

Why is it important to perform SI testing? First, as mentioned, the SI method is probably
the best way to judge fruit maturity without expensive equipment. The Sl technique,
wherein the starch to sugar ratio is measured, is correlated with ethylene evolution. In fact,
ethylene synthesis occurs as fruit ripens. Therefore, the SI index is an inexpensive way to
assess the degree to which fruit has converted starch to sugar, and is indicative of the onset
and progress of ethylene production.

Secondly, because SI is a reliable indicator of relative fruit maturity, SI testing can help you
determine if harvested fruit should be placed in early CA, late CA, or regular cold storage.
Remember that, as a rule, fruit with SI readings of 3-4 are suitable for late CA, apples
measuring 4-6 on the SI scale are best for early CA, and any fruit reading 6 or above
should be placed in regular cold storage or marketed immediately. Of course, reliability in
using the SI method for determining apple maturity is predicated on good sampling
techniques, i.e., looking at fruit that has sufficient size and color. Or, in other words,
sample apples that you expect are approaching harvest readiness.

Note: Apples going into late CA (available in April-June, etc.) should not average less than
15 Ibs. firmness

Having tested tens of thousands of apples over the past few years, per numerous
experimental protocols, we can now suggest a simple, quick and efficient method for
evaluating orchard by orchard or block by block SI apple samples. Here is
our quick and simple testing technique:

Equipment consists of a one quart hand-operated spray bottle filled with SI solution, a
pocket-knife, and a Starch Index chart. The most commonly used chart on the east coast is
the generic starch chart developed by Cornell University. Specific starch charts have also
been developed for Gala, Empire, Liberty. On the west coast they have also been
developed for Fuji. The most thing is to just use the chart and begin sampling and testing
the fruit two weeks before anticipated harvest to get a baseline on the maturity.

It is important to keep good records on your maturity determinations by cultivar and block.
You will start to build a good database of harvest maturity information for your orchard.

The procedure is simple--pick a sample of apples that appear ready to harvest, based on
size, color, days after full bloom, and taste. Spray the SI solution on transversely halved
fruit, wait one to one and one-half minutes and make your readings based on the SI chart.
The whole process is portable, quick, simple, and saves Sl solution compared to dipping
individual apple in a solution filled pan.

No doubt you (hopefully!) have an orchard filled with apples waiting to be harvested.