VEGETABLE AND FRUIT DEHYDRATION 37 



blanching in dehydrated vegetables. Unfortunately, existing infor- 

 mation does not enable one to tell which enzymes may be in- 

 volved in the quality changes that occur in unblanched dehydrated 

 vegetables during storage. Accordingly, one of the most heat-stable 

 enzyme systems (peroxidase) with some exceptions has been chosen 

 as the test enzyme, on the assumption that if this system is destroyed 

 it is highly probable that the enzymes that may actually be involved 

 will likewise be destroyed by the blanching treatment. However, it is 

 known that the long blanch required to destroy the peroxidase system 

 in some products, as indicated by the interpretation of the tests em- 

 ployed at present, is in excess of that actually required for the 

 preservation of quality in the dried products. 



This is an unfortunate situation, since it may lead to the excessive 

 loss of nutritive values from the products as the result of overblanch- 

 ing. Nevertheless, since adequate information concerning the relation 

 between peroxidase activity and the keeping quality of dehydrated 

 vegetables is not available, it is necessary to adhere to the present 

 test as an indication of adequate blanching in order to be on the safe 

 side. There are cases in which judgment must be exercised, since in 

 some products, even under ideal blanching conditions, it seems impos- 

 sible to inactivate the peroxidase system, as now interpreted, within 

 a blanching time compatible with practical operating conditions. 

 Rutabagas are an example, for it has been demonstrated that inactiva- 

 tion of the peroxidase system, as indicated by the benzidine test, re- 

 quires over 30 minutes of blanching. (See section on "Determination 

 of Adequacy of Blanching," p. 147.) 



Methods of Blanching 



There are three common methods of blanching: Steam blanching, 

 water blanching, and series blanching. Steam blanching consists in 

 subjecting the prepared product in suitable equipment to the tempera- 

 ture of live steam (212° F. at sea level) or to higher tempera- 

 tures obtained with steam under pressure. Water blanching in 

 its essential features consists of dipping or passing the prepared prod- 

 uct through boiling water. Series blanching is a modification of water 

 blanching in that the soluble solids leached from the products during 

 blanching in water are allowed to accumulate to a certain concentra- 

 tion in the blanching water. This concentration (4 percent has been 

 recommended) is then maintained by the gradual introduction of 

 fresh water and removal of spent water. 



Each method offers advantages and disadvantages. With the steam 

 blanch it may be more difficult to obtain a uniformly blanched prod- 

 uct than with the water blanch. It is generally conceded that water 

 blanching results in a greater loss of water-soluble materials, such 

 as sugar, minerals, and certain vitamins, than does steam blanching. 

 This appears to be true from comparisons made on the same blanching- 

 time basis (table 6). However, the time required to blanch a given 

 quantity of material adequately in water may prove to be less, under 

 certain operating conditions, than that required to obtain the same 

 degree of blanching in steam, in which case the difference between 

 steam blanching and water blanching with respect to loss of water- 

 soluble materials might not be so great. Series blanching has been 

 recommended by British investigators on the grounds that there is 



