TRANSPORT AND STORAGE OF FOOD 423 
itself. He found, as he expected, that the development of food preservation 
had been lop-sided—a lop-sidedness which reflected to some extent the 
difference in the rates of development of the physical and biological sciences. 
On the physical side the science of refrigeration had grown at a rapid rate, 
but on the biological side the advance had been slow. Hardy once remarked 
that the position was as if we were aware of the functioning of an internal 
combustion engine without any knowledge of internal actions and with 
little knowledge of its moving parts. How could we hope to make such 
an engine function more efficiently without some knowledge of how it 
worked ? 
Clearly it was essential to know more of the biological side of food. The 
proper order of things was for the biologist to formulate the conditions 
required for the satisfactory storage of the varied biological material which 
forms our food-supply, and for the engineer to provide the conditions. 
And so in recent years there has been a large expansion in biological research 
on foodstuffs, and to-day the biologist is beginning to frame the specifica- 
tions which the engineer must attempt to realise in practice. On the 
methods of storage of the three types of perishable food, meat, fish and fruit, 
the work of Hardy has had considerable effect. I propose to consider the 
dead foodstuffs, viz. meat and fish, first, since in some respects their storage 
presents a simpler problem. 
Meat. 
Autolysis.—Take meat. It is dead. The problem is to prevent any undesir- 
able changes. If there are agencies promoting changes, they must be resisted 
or slowed down to a point beyond which their effect becomes negligible. 
In meat, changes of two types have been discovered. First, there are the 
changes brought about by the enzymes naturally present in the tissues— 
in other words, by autolysis. Experiments show that such changes are 
dependent on the temperature. At the freezing-point of water the changes 
are slowed down so that they are negligible for a period of six months, while 
at —10° C. they appear to be completely inhibited. Cold, therefore, may be 
employed to control changes due to the enzymes. 
Micro-organisms.—The second type of change in meat is due, not to any- 
thing inherent in the meat, but to micro-organisms, chiefly moulds and 
bacteria. Withthe occurrence of death animal tissues become a rich medium 
for the growth of micro-organisms. The changes in the meat produced by 
these organisms are not only unsightly, but there is alteration of the colour of 
both lean and fat, and tainting results through the production of substances 
of unpleasant odour and taste which diffuse into the flesh. The problem is 
to prevent or reduce the magnitude of these changes. Examination of the 
flesh of animals shows it to be normally sterile, and if perfect asepsis could 
be maintained in the slaughter-house, the store and shop, micro-organisms 
would not be a cause of deterioration. 
The rate of change due to micro-organic contamination has been measured, 
and meat is found to be unsaleable when the bacterial population reaches 
a density of 30 million organisms per square centimetre. The time interval 
needed to reach this critical density depends, as would be expected, on the 
initial contamination. For instance, at a temperature of o° C. and 
100 per cent. humidity, the critical density is attained in 7 days on the cut 
surface of lean meat if the initial bacterial load is 100,000 per square centi- 
metre. If, however, the initial load is only 10 per square centimetre, the 
critical density is not reached for 18 days—in other words, the ‘ edible life ’ 
of the meat is more than doubled. Clearly it is of extreme importance to 
