Is it possible to find an optimal controlled atmosphere?

1 Postharvest Biology and Technology 27 (2003) 3 /13. Is it possible to find an optimal controlled atmosphere? Mikal E. Saltveit . Department of Vegetable Crops, Mann Laboratory, University of California, Davis, CA 95616-8631, USA. Received 25 November 2001; accepted 21 May 2002. Abstract Every controlled atmosphere (CA) conference has included recommendations for controlled atmospheres that purport to produce the optimal storage atmosphere for specific horticultural crops. However, the natural variability in the raw material and its dynamic response to processing and storage conditions may render it impossible to identify a truly optimal storage atmosphere.

2 Additional refinements in recommendations for the CA and modified atmosphere (MA) storage of fruits and vegetables will continue to accrue through empirical observations derived from traditional experiments in which the six components of the storage environment ( duration, temperature, relative humidity, O2, CO2 and ethylene levels) are varied in well-defined steps. However, the variability inherent in biological systems and the often dynamic response of the stored commodities to changes in their storage environment may not be adequately accounted for in these types of static experiments where all variables are usually held constant for the duration of the experiment.

3 Truly significant advances in the use of CA and MA may require the development of mathematical models that incorporate some measure of the commodity's dynamic response to the storage environment. These measurements should reflect the commodity's changing response to various storage parameters ( a shifting anaerobic compensation point), and should be useful in predicting future changes in quality. # 2002 Elsevier Science All rights reserved. Keywords: Broccoli; Chlorophyll; controlled atmosphere; Fresh-cut; Lettuce; Modified atmosphere packing; Modified atmosphere.

4 Phenols 1. Introduction conferences in 1969, the recommendations have been periodically revised because of additional Recommendations for the controlled atmo- information and technological innovations and sphere (CA) and modified atmosphere (MA) expanded to include new species and cultivars. storage of fruits and vegetables have been included Specific lists of CA and MA recommendations in the proceedings of the International CA and were included in the 1977 meeting (Lipton, 1977;. MA Conferences. Since the start of this series of Morris and Kader, 1977; Weichmann, 1977), and every meeting since 1985 (Saltveit, 1985, 1989, 1993, 1997a, 2002).

5 These recommendations give the temperature, ranges of O2 and CO2, and Tel.: /1-530-7521815; fax: /1-530-7524554 potential beneficial and harmful effects for the E-mail address: ( Saltveit). CA and MA storage of horticultural crops. 0925-5214/02/$ - see front matter # 2002 Elsevier Science All rights reserved. PII: S 0 9 2 5 - 5 2 1 4 ( 0 2 ) 0 0 1 8 4 - 9. 4 Saltveit / Postharvest Biology and Technology 27 (2003) 3 /13. As the years have passed, the recommendations 2. What variables can be controlled ? have become more refined and narrowed ( to individual cultivars, growing locations, and hand- The six primary environmental variables usually ling procedures).

6 Changes to the recommendations controlled in CA, MA and MAP are storage are now only made when a preponderance of new duration, temperature, relative humidity, and the data suggest a re-evaluation is warranted. The concentrations of O2, CO2, and ethylene. The advent of fresh-cut and MAP have stimulated optimum' storage environment for each commod- research in finding the optimal storage atmosphere ity is designed to maintain these variables within a for even more narrowly defined products and set of limits that produces the maximum storage storage conditions.

7 After having compiled recom- life for most of the individual members of the mendations for the CA storage of vegetables for commodity. These storage variables could be the past five conferences (Saltveit, 1985, 1989, thought of as a volume in six-dimensions. Each 1993, 1997a, 2002), I have come to question from side of the resultant polyhedron would be a plane where further substantial refinements will arise subscribed by two of the variables. By holding four (Saltveit, 1997b). The natural variability in the raw of the variables constant (usually duration of material and its response to processing and storage storage, temperature, relative humidity, and eth- conditions may render it impossible to identify a ylene concentration), the space can be collapsed to truly optimal storage atmosphere with our current a more easily envisaged two-dimensionally planar techniques and methodologies.

8 Graph that shows the recommended concentra- The vast majority of research on the CA storage tions of O2 and CO2 for optimal storage (Fig. 1). of vegetables continues to be studies of modified However, we should remember that any significant atmosphere packing (MAP). The procedure most change in the remaining four variables could often used is to enclose a small amount of generate another set of two-dimensional graphs. commodity in a film package, and periodically The space delineated for the optimal CA for one monitor the resulting atmosphere and product commodity ( iceberg lettuce or mature-green quality.)

9 If all variables are controlled , the internal tomatoes) contains within its boundaries a number atmosphere equilibrates at the recommended and of smaller spaces representing the optimal ' storage projected levels. However, this approach has a conditions for ever more homogenous groupings number of limitations in selecting an optimum MA of the commodity (Fig. 2). The importance of and in furnishing useful information on respira- defining storage conditions for small homogenous tion rates and optimum CA composition.

10 A major groups within the broader commodity recommen- problem is that the consumption and production dations can clearly be seen in the extensively of O2 and CO2 by the commodities are metaboli- studied case of apples. Fruit of the major cultivars cally interrelated, making it difficult to study the and the major growing locations have specific effect of one gas independent of the other. The recommendations that often differ significantly uncontrolled nature of the atmosphere in the from one another. Yet even within these refined package and the length of time required to reach storage conditions, fruit from different growing equilibrium further complicate interpretation of seasons and parts of the tree, and those fruit the results.