FOURTH METHOD OF FOOD-PRESERVATION 169 



and the diflFerence in the amount of sugar is probably the reason why 

 shop jam so seldom goes bad in comparison with the housewife's jam, 

 and also why the latter has usually more of a fruity flavour. Con- 

 densed milk does not go bad, as would ordinary milk, because of the 

 presence of a large quantity of sugar, and in the case of many dried 

 fruits, e.g. raisins, the addition of sugar is employed as a supplement to 

 the drying process in preserving these fruits. 



Acetic acid in the form of vinegar is another substance which has a 

 preserving value, though, unlike salt and sugar, it is not beneficial to 

 bacteria, etc., even in small quantities. Its use is naturally limited on 

 account of its acid nature. Vinegar is used instead of water in the 

 preservation of vegetables by hermetical sealing. This is done in order 

 to prevent beet, gherkins, and the like from losing their natural colour 

 and to prevent decay ; for vinegar is an antiseptic. Mixed pickles, 

 for instance, are preserved in this way, and in some cases the boiling 

 previous to sealing is omitted, as for instance in the pickling of 

 gherkins. 



In addition tg these legitimate preservatives, mention must be 

 made of those, the use of which as food-preservatives is forbidden by 

 law. They usually appear in practice under concealed names. The 

 commonest of such preservatives are boracic acid, borax, salicylic 

 acid, and formalin. Boracic acid and borax are forbidden because 

 of their injurious effects on the human system when continually con- 

 sumed, whilst the other two are much more injurious. 



§6. PEESEEVATION OF EGGS. 



The method of treatment of eggs for the purpose of preservation is 

 somewhat peculiar, on account of the protecting shell with which the 

 egg is covered. It might be imagined that such a protection would be 

 suflBcient to prevent bacteria and other organisms from gaining an 

 entrance into the mass of the egg, but we know only too well that the 

 keeping power of an untreated egg is not very great, in fact eggs go 

 bad almost as quickly as if there were no protecting shell. This arises 

 from the fact that the shell is porous, and does not prevent bacteria 

 from getting in, and also from the fact that there are bacteria in the 

 egg before the shell is deposited. It is obviously impossible to kill 

 the bacteria that are already inside without at the same time ruining 

 the egg, so what is usually done is to make the shell impervious to 

 bacteria and to the oxygen of the atmosphere. This destroys the 



