Further studies on the removal of spray residues from eastern-grown apples. Page: 22

                                
22   TECHNICAL BULLETIN 828, U. S. DEPT. OF AGRICULTURE
On the other hand, when no oil was used the omission of bordeaux
(lime) (spray 3) from the late cover sprays did not significantly increase
the lead residues at harvest, but as a lower percentage of the original
residue was removed, the residues on such lots were appreciably higher
after washing than they were on comparable lots (spray 1) in which
bordeaux was used throughout. The omission of bordeaux from late
cover sprays of lead arsenate and oil (spray 4) not only increased the
residues at harvest but also reduced the percentage removal when
washed, particularly on the York Imperial apples, so that residues after
washing were much higher with this type of spray treatment than they
were with any of the others. This type of spray treatment is more
common in the Pacific Northwest. The higher residues with the same
number of sprays in the Pacific Northwest and the greater difficulty
in removal compared with the central and eastern regions have been
attributed to greater weathering because of rainfall in the central and
eastern regions. These results indicate that the differences may be due
rather to the difference in the type of spray program generally used in
the different regions, since rainfall apparently has very little weathering
effect, whereas type of spray treatment may have a great effect.
RELATION OF WASHING TREATMENTS TO LEAD-RESIDUE REMOVAL
INFLUENCE OF ACID CONCENTRATION
Lots from the different spray treatments on Stayman Winesap in
1936 were washed with 0.5 percent HC1 at room temperature in a
flotation machine (fig. 2 and table 3, washing treatment B-1).
After this relatively simple washing treatment an average of 27.2
percent of the original residue remained. The washing treatment
reduced the residue on the apples from all of the spray treatments to
appreciably below the present lead tolerance of 0.05 of a grain per pound
of fruit. This percentage of residue remaining is in fairly close agreement
with the average of 31.5 percent obtained in 1934 but considerably
lower than the 46 percent average reported for 1935 (6, p. 29).
When the apples were washed with 1.5 percent acid (B-2) instead of
0.5 percent acid (B-1) there was 28.2 percent less residue remaining
than when they were washed with the weaker solution. Similar
benefits of 37.5 and 30.5 were reported (6, p. 29) for the 1934 and 1935
seasons, respectively.
INFLUENCE OF TEMPERATURE OF WASHING SOLUTION
Various lots of apples were washed with 1.5 percent HC1 with the
washing solution at room temperature (60 to 70 F.) and at 100, in
a flotation washer for 60 seconds (treatments B-2 and B-3), in a flood
washer for 35 seconds (treatments C-1 and C-2), in a flood-brush
washer for 35 seconds (treatments D-1 and D-2), and with an acid
wetting-agent solution in a flotation machine for 60 seconds (treatments
B-4 and B-5). In 34 comparisons with acid alone in a flotation
machine there was an average of 23.4 percent of the original residue
remaining after washing at room temperature (B-2) and 20 percent
after washing at 100 (B-3). Thus the percentage residue was 14.5
percent lower when the apples were washed with the heated solution.
The benefit from heating was considerably greater with an acid wetting-agent
solution in a flotation machine (B-4 versus B-5) than with
acid alone. This comparison was possible only with Stayman Winesap