Introduction and Objective
The methyl alcohol content in fruit juice has been the focus of numerous discussions and publications by both national and international bodies for decades. The creation and significance of this alcohol, which does not occur in fruit in its free form (Wucherpfennig et al. 1983), was reported on as early as 1960 (Mehlitz et al. 1960). It is only released from pectins during the processing of fruit to obtain fruit juice. Pectins are, among other things, important components of vegetable cell walls and, roughly speaking, are made up of galacturonic acid where the carboxyl groups have been partially esterified with methyl alcohol. Corresponding to the degree of esterification, the amount of this alcohol in the pectin is between 3 % and 8 %.
Over the last decades intensive research work has been carried out on pectins. This has served, on the one hand, to clarify their structure and the reaction which takes place when fruit is processed, and on the other hand, to provide information about their behaviour during metabolism in the human organism, as pectins belong to the group of dietary fibres. Pectins can be broken down by enzymes, either present themselves in the fruit or created by micro-organisms. This kind of enzyme has a special significance during mashing, pressing and clarification of juices as well as during the digestion process. One important enzyme for the breaking down of pectin is pectin methyl esterase (PE), which splits the esterified carboxyl groups to release methyl alcohol. Only after this can the pectin be split by polygalacturonase, a process which is absolutely essential for the production of pure apple juice.
Read the complete manuscript in FRUIT PROCESSING 1-2011!