Modelling the effect of temperature, water activity and carbon dioxide on the growth of Aspergillus niger and Alternaria alternata isolated from fresh date fruit.

Aims: To quantify and model the combined effects of temperature (T) (10-40 degrees C), water activity (a(w)) (0.993-0.818) and CO2 concentration (9.4-55.1%, v/v) on the growth rate of Aspergillus niger and Alternaria alternata that cause spoilage during the storage and packaging of dates. Methods and Results: The effects of environmental factors were studied using the gamma-concept. Cardinal models were used to quantify the effect of studied environmental factors on the growth rates. Firstly, the cardinal parameters were estimated independently from experiments carried out on potato dextrose agar using a monofactorial design. Secondly, model performance evaluation was conducted on pasteurized date paste. The boundary between growth and no-growth was predicted using a deterministic approach. Aspergillus niger displayed a faster growth rate and higher tolerance to low aw than Al. alternata, which in turn proved more resistant to CO2 concentration. Minimal cardinal parameters of T and a(w) were lower than those reported in the literature. Conclusions: The combination of the a(w) and CO2 effects significantly affected As. niger and Al. alternata growth. The gamma-concept model overestimated growth rates, however, it is optimistic and provides somewhat conservative predictions. Significance and Impact of the Study: The developed model provides a decision support tool for the choice of the date fruit conservation mode (refrigeration, drying, modified atmospheric packaging or their combination) using T, a(w) and CO2 as environmental factors.