Evaluation of post-harvest trends of respiration rates and softening of apples and tomatoes

Physiological functions of a specimen of fruits or vegetables in storage follow trends that are either 'normal' or 'abnormal'. Normal functioning will produce changes that are known to occur under optimal storage conditions, and that result in an optimal market quality. Abnormal functioning may lead to the appearance of the typical symptoms of a storage disease. The beginning of abnormal functioning is supposed to be marked by a deviation from one or more of the normal trends.

To detect possible deviations, trends of physiological functions of stored produce were followed. A new type of conditioning machine was used, a description of which is given in chapter 3. The apparatus allows a number of test objects (single specimens) to be viewed from all sides, without disturbing their storage conditions, and without interfering with other determinations made on the test material. The storage conditions can be kept independent from the temperature, the composition and the barometric pressure of the outer atmosphere.

One of the trends that can be followed in the conditioning machine is the CO ₂ -production rate. A new method is described (chapter 4) that allows the CO ₂ -production rates of seventy apples, kept at low storage temperature, to be recorded individually, within four hours, with an experimental error not exceeding 2 %. From each apple in turn, a stream of air carrying respiratory CO ₂ is mixed with a flow of titrant, a solution of potassium methanolate in acetone and methanol, 0.001 to 0.004 N. The supply of titrant is adjusted until equivalent with the rate of CO ₂ absorbed. Adjustment is made by means of a variable air pressure, which is read on a manometer. For a number of rates of CO ₂ supply the 'equivalent manometer value' was compared with gravimetric measurement of CO ₂ ; the relation between the two was linear, so that the 'equivalent manometer values' could be used as a measure of CO ₂ production rates of individual apple specimens.

Deviations from 'normal' trends of CO ₂ production of Jonathan apples were found to start either spontaneously (i.e., apparently not induced by an environmental factor) or to be induced by a transient change in the storage temperature. In one season, spontaneous deviations were found to start several weeks before the first symptoms of decay appeared on the surface of the fruit; in another season, this sequence was reversed. This might indicate either that the two effects do not have a common cause, or that host-parasite relationships vary from one season to another (section 4.2.2.2).

A transient thermal stimulus was found to induce deviations that were either transient or apparently irreversible (section 4.2.2.3). The reversible character of a deviation would show within a few days or weeks, after which the previous trend would be resumed. Irreversible deviations sometimes brought to light differences between specimens that had followed identical courses before the stimulus was applied. Also, specific differences were found: Boskoop apples showed a deviation one day later than Jonathan apples, and less pronounced.

In an Appendix, a description is given of a new closed container method for the determination of CO ₂ -production rates of single specimens and of composite samples of apples and other objects.

New methods have also been tried to study softening of fruits in store. The 'METTLER Compressor' is a precision balance provided with a plunger (section 5.2.2). The 'Bellows Compressor' is a (plastic) bellows provided with a plunger; it operates by means of a variable hydrostatic pressure read on a liquid manometer (section 5.2.3).

Determinations with these compressors necessitate the transfer of test fruits from storage conditions to the apparatus.

In case the test fruits should be left in the same environment, the 'Hydraulic Sweller' and/or the 'Hydraulic Belt' are alternatives. The one can be fixed inside a fruit, the other around a fruit. Both operate by means of a variable hydrostatic pressure which causes a liquid meniscus in a transparent capillary tube to move; the displacement serves as a measure of the induced deformation. Deformations in the order of 0.005 to 0.04 mm/day were established, the experimental error being less than 0.001 mm. Such deformations, when found with the aid of a hydraulic belt, seem to be caused by both softening and shrinkage of the test fruits. Hydraulic swellers allowed the beginning of internal rotting in apple and pear fruits to be read from a distance.

The Bellows compressor, showed different manometer readings representing 'firmness values' on the equator of tomato fruits: over cross-walls, firmness values were 4.7 % ± 1.16 higher than over locules (section 5.3.4). The tomato blossom end was found to provide firmness values that were in close correlation with the grade of colouring of the fruits (section 5.3.5). In the course of time, blossom-end firmness values of tomato fruits (as determined with the bellows apparatus) gradually decreased, parallel with the change in colour (table 1). The plunger of the compressor gained 0.007 to 0.025 mm/day, the experimental error being 0.01 to 0.02 mm. These results are not affected by shrinkage of the fruits.

Tests have been made on objects that changed their consistence, within a few minutes, while being subjected to alternating pressures. In a jellifying mixture, the alternating time-deformation curves changed from rectilinear to asymptotic whereas the opposite tendency occurred in fruits that were subjected to an injurious electric current (section 5.3.7).

With the aid of a METTLER Compressor, different spots on the surface of an apple fruit were tested. They appeared to be identical as to their yield point. but different as to the point of rupture of their skin (section 5.3.2). In tomatoes, no yield point appeared, although the test resulted in a distinct indentation in the surface of the test fruit.