Lye Peeling of Fruits and Vegetables0 pages

Food & Beverage

Application Data Sheet

Lye Peeling of Fruits and Vegetables
Background
Many fruits and vegetables are prepared for peeling and canning
by exposure to caustic (NaOH). To ensure adequate and consistent
peeling, the concentration of caustic in the treatment bath must be
maintained at an effective strength. At the same time, if the caustic
bath is too strong the fruit or vegetable can be damaged.

Process
The typical process consists of an open tank of caustic heated
to 170–220 °F (76–105 °C) by live steam. The target caustic
concentration can vary from 3 to 15 %. Exposure time for the
product is usually 5 minutes or less, after which the product is placed
on moving grates and washed by high pressure nozzles to remove
entrained caustic.
A significant amount of caustic is carried out by the product, so
the bath must be continually replenished to maintain an adequate
volume of caustic solution and a caustic concentration at the target
value. This could involve addition of water, based on level control,
as well as makeup caustic, based on the conductivity measurement.
The best place to start is to look at how the bath behaves and how
concentration is maintained manually.

Since the background increase cannot be distinguished from the
conductivity due to the caustic, the conductivity set point (or
calibration curve for a concentration measurement) may need to
change as the bath ages. This effect will be smaller for higher caustic
concentrations and for shorter intervals between bath replacement.

Concentration Measurement
In general, when there is background conductivity, the conductivity
versus concentration curve will not be the same as the standard
curve for caustic (NaOH) in distilled water, due to buildup of salts
in the bath. The curve must be developed from operating data, i.e.
conductivity and titrated caustic concentrations. Data should be
taken over the life of at least three (3) baths; the age of the bath
and any additions to the bath should be noted, in addition to the
conductivity and titrated caustic concentration. Once a curve has
been developed, the on-line measurement can usually be corrected
for salt build-up by standardization following a routine laboratory
titration.

Important Considerations

The Measurement
Due to its resistance to fouling, toroidal conductivity is the best
measurement technique here. The conductivity sensor is typically
mounted in the holding tank (Figure 1). Relatively clean processes
may use the 228 sensor, but applications with potential
for heavy coating should use the large bore 226 sensor.
As the bath ages, there is a buildup of salts, which are leached
out of the peels and carried in with dirt and other suspended
contaminants. This buildup causes an increase in the background
conductivity.

Select the toroidal conductivity sensor based on how heavily
the bath is loaded with suspended solids.
When evaluating how to control bath concentration and
volume, how the bath behaves and how it is manually controlled
should be thoroughly examined.
When setting up a concentration measurement, data should
be taken over the life of three (3) baths to determine the
relationship between conductivity and concentration.