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Rot Fragments
A method for the estimation of Rot Fragments in Tomato Products (Method M13D) published
by the National Canners Association of U.S.A. in 1943, was introduced into the A.O.A.C. in
1945, omitted in 1950, reintroduced in 1955, and appears as an "Official, First Action" method
No. 36.072 in the current 10th Edition, 1965. No British papers have come to light on the test
and although some experimental work on the method has been published in U.S.A. no recommended
limits for rot fragments in tomato products have been discovered. H.R. Smith has attempted to
show the relationship between the percentage of visible rot and the rot fragment count of the
puree, but the results varied too much to form a basis for quality assessment. The samples tested
were, however, drawn from the trimmed tomato input and the cyclone juice output at six factories
over several days under ordinary working conditions where accurate sampling was impossible; and
further tests under laboratory controlled conditions would be desirable to show a truer relationship
and the natural variation in the rot fragment count for a known percentage of rot.
There are unfortunately two ambiguities in the A.O.A.C. directions which may affect the
results considerably. A rot fragment is defined as "tomato tissue to which mould filaments are
attached"; but in several of the photomicrographs of rot fragments illustrated in the A.O.A.C. no
tomato tissue can be recognised, and the question arises whether mould colonies without obvious
tomato tissue should be included. In McCormack's definition11 a rot fragment is a clump of opaque
or semi-opaque material with a periphery of mould filaments, — which would apparently include
colonies if they were dense but perhaps not if they were light and open. Since about half the
clumps of fungus one finds do not show identifiable tomato tissue the question cannot be ignored
and it is necessary to decide which to count.
The second point is that the minimum size of rot fragment to be counted is not specified. This
may be because the area or length of a fragment with a fluffy edge defies definition, or it may
have been assumed that the size would be controlled by the method, all fragments of less than
0.25 mm. in width passing through the No.60 sieve. In practice however much fine material fails
to pass through the sieve, being retained by the coarser material, and it becomes necessary to
decide upon a minimum, however rough the definition may be.
Conversely, Olsen and Stoner12 have found that a considerable number of rot fragments pass
through the sieve, indeed, many more than are retained by it; the implication being that to obtain
a correct count the sample should be diluted, stained and examined without being screened. To
count unscreened aliquots, however, would take a great deal of time owing to the high dilution
necessary (ten times as great as in the A.O.A.C. method), and it is suggested that their findings
reflect the importance of defining the size of the fragments to be included.
In the writer's laboratory, McCormack's definition of a rot fragment, which seems to accord
best with the illustrations in the A.O.A.C. has been adopted; and all fragments exceeding 0.3 mm.
in longest measurement, including about a third of the sparse peripheral filaments in the measurement
in the case of dense colonies, have been counted. This is an arbitrary procedure, but in the
absence of special instruction or more detailed directions in the method it is necessary for each
analyst to decide upon his technique and to acquire his own experience before drawing any inference
from a rot fragment count.
For counting the fragments a stereo microscope has been found best, scanning the aliquot
with the aid of a moving stage and cyclometer counter at a magnification of x 40. Ordinary cleaned
slides are used, taking 0.15 to 0.2 ml. of the suspension, measured from an inverted straight
sided graduated 1 ml. pipette, spread with a needle over an area of about 3x 1.5 cm. on the slide.
As the method has only been used up to the present time as a general confirmation of the presence
of rot, only two slides have been counted (about 0.3 ml. ex 10 ml. prepared from 2 g. puree, with
glycerin as stabiliser), unless they showed a wide difference when four slides would have been
counted. (The quantities taken were based on an earlier edition of the A.O.A.C., now modified).
The method provides a very practical confirmation of the presence of rot in a puree or other
tomato product. The appearance presented by pieces of tomato tissue with a periphery of mould is
quite convincing and makes an effective subject for photography if permanent evidence is required.
Slime from a tomato factory, and fungal development in a puree after the container has
been opened, usually seem to consist of Oospora sp., a feathery growth with tapering filaments,
which will be observed in the liquid phase, in-between particles of tomato tissue rather than
emanating from them. In the case of an infected product due to a faulty closure or delay after
sampling a copious surface growth develops first and is sufficiently obvious to render any further
examination unnecessary.
In case they might be of interest to other workers of limited experience the results obtained
in this laboratory over the past five years, mostly on purees of high mould counts, have been
collated and are shown against the respective mould counts in the accompanying figure.
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