Studies of the Fine Structure of the Wax Layer of Sultana Grapes

Abstract

The surface waxy bloom of sultana grapes has been studied using the carbon. replica technique in combination with electron microscopy. This layer consists of a series of overlapping wax platelets, each of which is about 0·1 f' wide. The air spaces between the wax platelets become filled with liquid when sultana berries are dipped in commercial emulsions known to accelerate their drying rate. Washing in distilled water removes this layer of dipping emulsion from the surface. The appearance of dipped and washed grapes is similar to that of untreated grapes.Rubbing the grapes with lens tissue destroys the precise arrangement of wax platelets but does not remove the wax. Dipping the grapes in chloroform (four changes of 10 sec), a treatment known to remove most of the wax, leaves sufficient wax to enable replicas to be prepared. Treatment of the grapes with chloroform for longer periods and Soxhlet extraction removes all the waxy structures.These results are discussed, and a hypothesis is presented which may explain the mechanism of action whereby dipping emulsions increase the rate of water movement from grapes.

Resistance of Citrus Fruit to Mass Transport of Water Vapor and Other Gases

“…(Pi-Pa)At 0.025 g H20 fruit-' h-' RDT r 3.600sh-' (98 -70)RH 17 Therefore, r = 106 s cm-' or rounded to two significant figures = IlO0s cm-'. account for the residual gas exchange capacity.…”

“…Opinions differ widely concerning the relative contributions of the various mechanisms proposed to account for gas exchange in harvested fruits, although usually it is tacitly assumed that water and gases move by the same pathway. (17). Alternatively, it has been suggested that CO2 moves through the skin and 02 through the lenticels of fruits (23), and more recently Burg and Kosson (14) proposed that water moves preferentially through a liquid water phase, whereas gases move through airfilled pores.…”

The Surface Wax of Several Grapes in Japan

“…Since this type of surface structure is a strong barrier to the transpiration from berries, the chemical nature and fine structure of the wax have been investigated by many workers in `Thompson Seedless (Sultana)' , the most extensively grown raisin grape in the world (1,2,5,6,7,8,9,10,11). On the table grape cultivars, however, there have been hardly any detailed reports on them.…”

Combined Convective and Microwave Drying of Grapes

“…According ta their results, dipping does not remove the waxy material but only modifies the surface probably from hydrophobie ta hydrophillic, thus enabling wetting. Chambers and Possingham (1963) studied the surface waxy bloom of Sultana grapes using the carbon replica technique in combination with e1ectron microscopy. According ta their observations this layer consists ofa series oftiny wax plate1ets ofabout 0.1 J.lIIl wide.…”