Year

1998

Degree Name

Doctor of Philosophy

Department

Department of Chemistry

Abstract

Prune is an important product in the dried fruit industry. The production of prunes involves drying of high-moisture fresh plums. Drying is an energy intensive food processing operation, which plays a significant role on the quality of the product. Increased competition in today's global market means that avenues to improve the process efficiency and product quality are desirable. In order to do this it is important to understand the physical and chemical changes occurring during drying.

Comprehensive physico-chemical investigations of the drying process of plums were conducted. This would help to establish a scientific basis for suggesting improvements to the commercial dehydration of plums with a view of maintaining or enhancing the quality of the dried product. A computer-based laboratory drier was designed and built to study the kinetics of drying plums under controlled conditions.

The effects of the process parameters (i.e., temperature, relative humidity and velocity) on the kinetics of moisture loss were studied to ascertain the major factors controlling the rate of drying plums. Results of the study showed a remarkable influence of the process parameters demonstrating the need for better control of the conditions of the drying air. These together with the ancillary experiments revealed that there may be different rate-controlling mechanisms involved during drying implicating an important role of the skin layer to moisture transfer. Notable differences between the drying curves of plums dried without skin and those with skin intact corroborated the importance of the skin layer to the drying process.

Modelling the drying process has also provided more insight into the mechanisms of mass transfer during drying under different conditions. The proposed models predicted well the drying curves and could serve as descriptive tools in predicting the drying process of plums. This is important for the optimisation process. In addition, the study of using chemical pretreatment based on fatty acid esters further disclosed the significant effects of the skin layer to moisture loss during drying. It showed that dipping pretreatment can be employed to enhance the drying process of plums depending upon the composition of the drying emulsions and the drying conditions.

The approach of monitoring the kinetics of changes in the carbohydrate constituents of plums using HPLC has shown dramatic alterations in the amounts of the major carbohydrates present in plums (i.e., fructose, sorbitol, glucose and sucrose) during drying and storage. It unveiled the major carbohydrate degradations occurring during drying including the hydrolysis of sucrose, Maillard reactions and caramelisation. Drying conditions and the pH of the fruit were found to affect the rate and onset of these reactions. The study shows the usefulness of the approach in detecting the progress of the chemical degradation reactions and further manifests the importance of controlling the conditions during drying of plums.

It is known that the decomposition of carbohydrates would lead to the generation of wide spectrum of volatile flavours depending upon the type of reaction. A solid-phase microextraction (SPME) technique in conjunction with GC-MS was used to characterise and follow the changes of volatile flavours during drying of plums. The study of aroma profiling showed significant changes to the volatile constituents of plums during drying identifying relevant routes leading to the formation of the volatile flavours associated with the carbohydrate degradations. The result demonstrates the importance of aroma profiling in diagnosing further the progress of the chemical degradation reactions, which has important implications for the control of the quality of the product.

The impetus of controlling the drying process manifests the need for better knowledge of the conditions in which the fruits are experiencing during commercial dehydration. This information is also important for simulating the commercial drying conditions in laboratory studies. In this respect, a computer-based data acquisition system was developed to monitor the conditions during commercial dehydration of prunes. The monitoring work was found to be useful in identifying anomalies in drying conditions in the tunnel exemplifying the utility of the device in assessing the performance of the tunnel.

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