Beth Grafton-Cardwell and her team at Lindcove - Citrus Research ...

CLEAN CITRUS
In this article, we present research occurs, the volume of base needed being
evaluating the performance of a new used to calculate % acid, or used directly
electronic handheld acidity meter that in citrus maturity ratio tables. The phe-
is both easy-to-use and portable. nolphthalein method is the prescribed
There are a number of organic acids method in the State of California matu-
present in citrus, but citric acid predomirity standard regulations.
nates, making up from 85-95% of the Both of the methods just described
Clonal
total acidity. It is for this reason that %
acid in citrus is often expressed on a %
citric acid basis.
There are currently two common
are time-consuming and tedious if done
by hand and difficult to perform under
field conditions. Use of a bench-top
instrument to do the titration greatly
methods of estimating % acid, both increases the ease of doing the measure-
using the addition (titration) of a base ments, but these units are costly, not very
Containerized
(sodium hydroxide) into the citrus juice
until the acid in the juice is neutralized.
The only difference is in how the acid
portable, and require some knowledge
and skill to maintain their accuracy.
Recently, we became aware of a new
neutralization is monitored. The first device developed by a company in South
method monitors pH change using a Korea (G-won Hitech Co., Ltd.) that
Certified
pH meter while the volume of sodium
hydroxide needed to raise the pH to 8.2
(neutralization endpoint) is recorded.
measures acidity by a totally different
method than titration. In appearance and
operation, it is quite similar to the digital
This method can be done by hand but is refractometers that are familiar to many
often automated by using a bench-top in the citrus industry (Fig 1).
instrument that handles the titration and This instrument takes advantage of
monitoring of the pH.
the fact that the electrical conductivity
In the second method, the operator of citrus juice (as well as other fruit)
monitors pH change visually by use
You have new options:
of phenolphthalein, a compound that
is influenced by the concentration of
acid present. During measurement, the
changes color from colorless to pink instrument determines the conductivity
when the juice changes from acidic to of the juice sample and then converts
• Containerized citrus is cleaner, basic. Base is added more until flexible color change and this secure reading to % acid by using a preset
calibration. The company also sells acidity
meters for other fruit types as well,
• Clonally propagated rootstocks increase uniformity but these utilize different calibrations to
and expand your options.
compensate for the different substances
present in the juice.
Operation of the instrument is illustrated
in Fig. 2. Unlike a refractometer
• Professional field service from experienced horticulturists:
that uses undiluted juice to measure
°Brix, the juice must first be diluted by
1 in 100 prior to measurement for the G-
Ed Needham
won acidity meter. This is accomplished
(559)977-7282
by first placing 30 mL of distilled water
into a dilution container using a syringe
Steve Scheuber (209)531-5065
that has been modified to dispense only
John Arellano (559)804-6949
that amount (Fig. 2A). Following this,
0.3 mL of juice is added to the dilution
container using a pipette set to dispense
only that amount, and the container
capped and the juice and water mixed
(Fig. 2B). The dilution container, syringe
and pipette are provided with the meter.
The juice/water mixture is then
poured into the measurement well of the
Clean Plants
instrument, adding enough to cover the
For electrodes Your Future on the bottom of the well, and
the measurement button pushed (Fig
2C-D). A % acid reading appears after
a few seconds. The use of distilled water
for the dilutions is important because of
the presence of interfering ions in tap
1-800-GRAFTED
www.duartenursery.com • Hughson, Ca.