| ( ING F. BONOMI )
SUMMARY
The author, with reference to the research carried out by
CRIOF of Bologna on the refrigerated storage of dessert grapes,
lists the conditions in which it is possible to obtain good
storage results. The techical solutions adopted in industrial
plants are illustrated; precisely: indirect refrigeration
of the stored product, periodic sulphuration , absorption
of SO2 in closed cycle,with brief mentions of the type of
apparatus and materials used. The Author also mentions the
use of plants for disinfestation and pre-refrigeration for
transport.
INTRODUCTION
The operators in this field agree that there is a considerable
interest in having at disposition, for sale on the market,
several varaieties of dessert grapes for a period of 2-3 months,
and above all to be able to offer the product in coincidence
with the Christmas period and even after. While in other countries
the refrigerated storage of grapes is applied with interesting
economic results, in Italy it generally represents an intervention
of the duration of 10-20 days which is resorted to so as to
lighten the markets during the harvest period. Also in this
case, the technical results are poor, given the empiricism
with which it differentiates from the other fruits normally
stored.
STORAGE TECHNIQUES
The point about the technique for cold storage of grapes has
been made in two notes published by CRIOF in 1968 and 1969
(1), (2). n these two papers the Authors, taking up the argument
already dealt with in 1962 by Prof. G.C.Pratella (3), refer
on the results of research carried out at CRIOF of Bologna
since 1963 until today and mention new constructive techniques
for grape storage plants, techniques which, after the experimental
phase referred to by the authors, have been applied in industrial
plants where hundreds of tons of fresh grapes of the most
requested cultivars in have been stored over the last years.
In the aforementioned notes the conditions for good dessert
grape storage are clearly stated , for periods of 2-3 months
and more for some varieties, precisely: temperature -1 - 0°
C; relative humidity 95% and over ; periodic sulphuration
with treatments at various concentrations. In order to obtain
these conditions certain technical problems needed to be overcome
and which only in the last few years have been resolved. It
is known that, storing grapes at 0° C, with relative humidity
of 80 -85% (good for normal refrigeration plants), there is
consistent loss of water due to transpiration - which causes
withering and the successive browning of the grape stalks
while the grapes gradually lose turgidity. It is therefore
essential to realize and maintain relative humidity at about
95% or more in the ambient in which the grapes are stored,
a condition that is very difficult to reach with the normal
industrial refrigerators built for the conservation of fruit
in general.
SULPHURATION
Periodic sulphuration treatment poses other technical problems,
such as, for example, the aggressiveness of SO2 on metals
in general but particularly the ferrous type, of which most
of the apparatus placed in the cold storage cells are made.
The susceptibility of grapes to the damage caused by sulphurous
anhydride imposes the utmost precision in dosing the fumigant
used for germicidal purposes, dosage which is variable over
time (larger doses at the beginning of storage and then reduced
during) and also depending on the sensitivity of the different
cultivar. The exposure times of the product to sulphurous
anhydride are short (20-30 mins.), the removal of the fumigant
from the ambient must also be rapid and the residues must
be quickly reduced to only a few parts per million in volume
to avoid possible damage due to the long exposure even at
low concentrations. On the other hand the necessity to keep
a constant hygrometric grade of 95% in the storage ambient
excludes the practice of sulphurous acid removal using atmospheric
air. This practice brings about sensitive variations in the
hygrometric grade of the ambient (reduction of relative humidity)
and therefore causes a lack of water for transpiration from
the fruit to the ambient to reconstruct the balance between
vapour tension of the gases contained in the intercellular
spaces of he fruit and the vapour tension of the atmosphere
in the storage ambient. This consideration leads to further
examination of the problem of SO2 absorption used for disinfestation
in closed cycle trying to realize absorption without modifying
minimally in the negative sense the hygrometric grade of the
ambient. All of these considerations, which emerged, as already
cited, during the course of a long experiment conducted by
CRIOF the university of Bologna, have been the basis of studies
which have brought about the most ideal technical solutions.
With regard to the most important condition, maintainence
of humidity at about 95%, this problem has been resolved with
the use of indirect refrigeration, keeping the grapes inside
plastic wrappers that ar impermeable to gases, placed inside
the normal refrigeration cells.
INDIRECT REFRIGERATION In normal refrigeration
plants the problem of the hygrometric grade is rarely brilliantly
solved, and the effects are the phenomen of weight loss and
depreciation of the product, as all operators know. In fact,
when a cold storage cell is in temperature regimen, when a
thermal balance has been created and which is imagined to
be existent in the mass of the stored product, each time that
the refrigeration plant starts up and therefore the air contained
in the cell circulates towards the cooler, normally a layer
of ice forms on the cold surface of the evaporator (pipes,
fins); 1 millimeter of ice on each square meter of cooling
surface represents about one litre of water that is taken
fron the ambient. This phenomen is even more evident during
the pull - down phase.
Considering that inside the cold store a balance will be
created between the vapour tension of the air surrounding
the fruit and the vapour tension existent in intercellular
spaces of the same fruits, each time that a certain quantity
of vapour condenses on the cold surface of the evaporator
the same quantity is immediately ceded by the fruits to the
ambient. In order to have an idea of the amount of water lost
by the fruits due to this phenomen, it is to be noted that
an average evaporator with a surface of 400 sq.m. is capable
of subtracting about 400 litres of water from the fruits per
mm. of ice that forms on the surface of the cooler. Obviously
any relative humidity measurement apparatus placed inside
the cold storage cell, ends up measuring the effect that derives
from this mechanism of the removal of water vapour from the
ambient (action of the evaporators) and the contemporaneous
transfer of an equal amount of water vapour from the fruits.
For this fundamental reason, when a hygrometric grade of 90%
is measured inside the cold store, a condition considered
optimal, there is still a loss of weight from the fruit.
To reduce the effects of these phenomen, particular devices
are used for the distribution of the refrigerant liquid to
the evaporators. Theoretically one of the most valid contrivances
is that of increasing the cooling surfaces so as to reduce
thermic shock, equal to the frigorie delivered, between the
circulating air temperature and that of the finned pipes,
so as to contain the condensation phenomen. This device, however,
is costly and rarely realized in a rational manner. This is
the situation existant today in the construction of refrigeration
plants, a situation which is determined not so much by a lack
of good will and capability of the builders as by certain
commercial factors which often deteriorate during negotiations
of the plant sale. The solution of indirect refrigeration
represents a new criteria for the refrigeration plants destined
for long term storage of hortofloricultural products and eliminates
the inconveniences due to the lack of the hygrometric grade.
With this technique the products are stored within the plastic
wrappers placed inside normal cold-stores and the refrigeration
of the product happens indirectly through the walls of the
wrappers and not by effect of direct ventilation onto the
fruit.
The fundamental characteristic of the system is in the fact
that a very high hygrometric grade may be maintained (over
95%) in the ambient in which the produce is stored because
the refrigeration of the fruit happens over a wide surface
(walls and ceiling of the wrapper) with a low thermic fall
(ie.temperature of cold store + 1°C and temperature stored
produce +2°C) and without loss f atmospheric water vapour
contained inside the wrapper and at contact with the products.
Using the indirect refrigeration system, with regards to the
hygrometric grade, one may prescind from the performance of
the refrigeration system, because, in fact, inside the plastic
wrapper relative humidity may be measured at 92 - 95%, whereas
outside the wrapper this normally reaches only 50 - 60% or
less. It is implicit that with such a performance it has been
possible to widen controlled atmosphere storage to species
up to now stored rarely or not at all, such as vegetables,
flowers and grapes. Another fundamental characteristic of
this system resolves one of the most important problems for
longterm storage of grapes represented by the necessity of
placing the product in optimal storage conditions the least
amount of time possible after picking. The large capacity
cold stores, that are the least expensive from the installation
point of view, as evident, are the least suited for this.
Using the indirect refrigeration system, it is possible to
take advantage of a refrigerated ambient, even of large capacity,
for the storage of grapes, dividing up the refrigerated space
in several plastic wrappers, with the advantage of storing,
for long term conservation, grapes of different cultivars
harvested in different periods, at the same thermic regimen,
and reaching in a short time after picking the optimal storage
conditions, including the sulphuration treatment, for each
lot of grapes in each wrapper. The division of the large refrigerated
space into several cells (wrappers) of small capacity also
gives the possibility of lightening the loading and unloading
operations of the cells with regard to both the harvest period
of the varieties and the market requests during merchandising
of the products, with much lower costs in respect to the probable
costs if smaller capacity cells had been built.
PLASTIC TENDS
Many materials had been examined for the realisation of indirect
refrigeration which, in the specific case of grape storage,
must meet certain requisites: gas impermeability, sufficient
mechanical resistence, imperviousness to the effects of SO2
gas, easy assembly, sufficiently high heat transmission coefficient
and low cost. The choice, after numerous experiments, fell
upon plastified polyester material. This material, used for
the realization of the wrappers, have completely answered
to all the requirements and five years of industrial plant
construction have confermed the validity of the solution.
In fact, the use of plastic wrappers, inalterable against
the action of SO2 gas, resolves the other big problem of the
corrosion of large part of the apparatus placed inside the
refrigerated cells. Using indirect refrigeration, the cooling
apparatus with their relative ventilators, thermostats etc.
are placed outside the actual staorage ambient (represented
by the inside of the tends) where the grapes are placed and
where sulphuration is carried out. The gas impermeability
of the plastic material allows precise dosage of the SO2 gas
and also helps maintain constant a constant gas concentrationduring
the sulphuration operation. These conditions are indispensable
for assuring the effectiveness of the treatment and to surely
avoid damage to the grapes caused by SO2.
EQUIPMENT
The operation of SO2 dosage, in order to give the necessary
guarantees, is achieved by measuring liquid sulphurous anhydride
with a special apparatus. The desired quantity of liquid SO2
is gassified with a hot water vaporizer, specifically studied
to guarantee the inlet into cell (tend) of the SO2 gas and
avoid any type of SO2 gas condensation forming on the stored
grapes. Groups of ventilators, opportunely protected against
the sulphurous anhydride, are provided inside the wrapper,
these make the atmospheric air, within the wrapper, circulate
in closed cycle in the grape mass so favouring the thermal
exchange between the refrigerated ambient outside the wrapper
and that within and therefore avoiding the creation of differences
in temperature in the various points of the mass of the stored
product. The action of these groups of ventilators also allows
the uniform distribution of SO2 gas in the grape mass, while
in the meantime maintaining a homogenous concentration of
the gas for the whole period of sulphuration. The problem
of removing the SO2 from the grape storage ambient has been
resolved by employment of a closed cycle absorber using water,
this omits communication between the storage ambient and the
outside. The apparatus consists of a tower connected to the
wrapper using pipes of a suitable diameter. A low power centrifugal
pump, installed at the base of the tower, provides the closed
cycle circulation in the tower of the atmosphere containing
sulphurous anhydride drawn from the cell in contro-flow with
water coming from the mains which, in turn,descends the tower
through an inert mass. The precise control of the water delivery
is possible due to the flowmeter inserted in the distribution
pipe on which a a special regulation valve is foreseen. The
whole apparatus, in the parts at contact with the sulphurous
anhydride, is protected by a special paint, the base of which
is a synthetic elastomer impervious to the action of sulphurous
anhydride. The absorption yield of this apparatus is such
that it guarantees the total removal of sulphurous anhydride
from the ambient,without the use of special chemical absorbents
which needto be substituted when reaching complete saturation.
The simplicity and functional safety are the fundamental characteristics
of this machine, given the elementary principle on which sulphurous
anhydride removal is based (physical absorption). Furthermore,
the use of water in an apparatus in closed cycle consents
enrichment of the ambient of water vapour and therefore maintains
a high hygrometric grade (tending to saturation) which is
required in order to avoid weight loss and depreciation of
the stored product. The SO2 concentration is kept under control
during both the sulphuration treatment (control of dosage)
and after the absorption of the gas (control of residues)
by a special immediate analysis apparatus (sensitivity up
to 20 p.p.m.).
DISINFECTION TREATMENTS AND PRE-REFRIGERATION
An interesting note was surveyed while using these plants
over the last four years in Italy, France, Portugal and Latin
America; (1) the cells and relative apparatus may be used
for preventive disinfection for transport, disinfection accompanied
by pre-refrigeration. In fact, for the commercialization of
grapes the problem is that of getting the product onto to
the market in the best possible conditions given the distance
from the production centres and the rate at which the product
deteriorates. The development of Botrytis during the time
that the product is on the means of transport and at the sorting
stations can compromise the results of merchandising. For
this reason normally, in this type of plant destined for long
term grape storage, one or two cells are destined for disinfection
treatments and pre-refrigeration for transport. These cells
are used up until the end of the grape harvest and then loaded
with produce from the last picking for storage. The disinfection
treatments and pre-refrieration for transport are pratically
carried out during the night by sulphuration of the produce
placed in the indirect refrigeration cells, this treatment
lasts about 20 minutes. Afterwards, de-sulphuration of the
ambient is made and this treatment happens contemporaneously
with the indirect refrigeration of the product so as to obtain
a lowering of the temperature in the mass to 7-8 °C without
loss of water from the grapes. The product, refrigerated and
disinfected, is put onto the consumer market, having avoided
the risk of developing Botrytis, at least for the period in
which sale of the produce is effectuated.
(1) - In these plants the following cultivar were stored :
Alphonse Lavallèe, Italia, Cardinal,Regina,Chasselas,
Dattier, Servant, Rosaky, Donna Maria.
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