The amount of literature on the role of humidity in the subject of food is huge. This reflects both its importance and that the subject has been studied over fifty years. To give you a little bit more information, but also more than that. 
Climate change and its impacts on our worldwide and local food supply is a serious threat to our civilized progress. It has in the past and for sure will in future have random and devastating socio-economical consequences for people in all communities.
Images of failed crops and displaced people, “climate refugees” are there in front of us to see in newspaper and on TV. We also see and hear about the problems farmers have in bad weather when they cannot grow or harvest their crops. We also pay for these natural events in the form of higher food prices. Hidden from sight, and not so easy to comprehend, is the subtle and potentially lethal world of microorganisms in our food that are totally dependent on humidity and change their growth according to ERH.
Like it or not, our food is not sterile and just like us, comes naturally covered in a range of bacteria and fungi. You will know this intuitively even if you haven’t thinking about it, as you can only keep fresh food for a certain length of time before it goes off and acquires a different smell, along with cultivating various fungi.
In 1957 W.J Scott published a scientific paper “water relations of food spoilage microorganisms” reporting that he had found that it is not the absolute amount of water present the in food that resulted in microbial growth, but it is the amount of water that is available to the microorganism. This is ‘free- water’ as opposed to ‘bound-water’ and the free-water is measured as water activity. This was a highly significant finding for the food industry. It mean for first time a measurement of the food could be performed to determine if microorganism will grow on it or not. In fact it was even better than that, as Scott published a list of different species and the water activity or ERH that must be present for each to grow. It turns out that the ERH is critical for these organisms and very precise in that they simply do not grow if the ERH is one or two percent below their critical level for the growth.
Now we have a question: where does the climate change come to the story? If we are going to get wetter and warmer weather this will push up the relative humidity for longer periods. However, microorganisms do not suddenly get up and start growing madly. They have a growth lag period to overcome first and then the right conditions for growth in their surrounding environmental has to be maintained for them to continue growing. Maintaining high humidity provides these conditions for growth.
A common experience of this is fungi growing in autumn. In autumn, the temperature drops and the humidity is pushed up as longer periods. This allows growth of the fungi’s ‘mycelium’ the organic growth phase of fungi that takes place in the ground or inside decaying vegetation. The fungus then, in the appropriate conditions for its species, starts the sexual phase of growth and we see the part we call mushrooms and toadstool.
Of course fungi can grow at any time of year as long as there moisture. Carefully inspect the underside of a decaying piece of a log, where moisture has been trapped between ground and log, to see the fungi performing its natural function. Then replace the log back into the position you found it to let nature carry on as normal.
As the environment changes, different species of microorganisms will take advantage and they are very good at adapting. Generally, wetter and warmer conditions are ideal for faster growth of the microorganisms that cause concern. Some can tolerate cooler temperatures but they all need the levels of humidity. Different strains within a species may become more prevalent and the longer a microorganism is growing the greater the chance that a new strain can arise through mutation.
Let us approach this from more familiar and practical point of view. Say for the sake of argument, you do a weekly shop for some food and you place the vegetables in the very handy compartment that the manufactures provide at the bottom of your fridge.  If you have bought your vegetables packed, do you warm them or leave them warped before placing then in the vegetable compartment of your fridge.
We do not eat a lot of bread and can have a loaf sitting around for a few days. The latter type of packaging does not prevent fungal growth but it does let out moisture and lowers the humidity of the air around the bread, assuming that the humidity of the room where the loaf is kept is not too high.
Sometimes bread in supermarket’s shelf is cool and, not surprising, condensation has appeared on the inside of polythene bag. This could be due to deliberate refrigeration or through early morning delivery storage when temperature are cool.
On a very more much serious note, the problem with climate change for food are in the supply chain. For thousands of years human being have worked with and battled against nature to cultivate food crops. Plants that make up the bottom of our food chain are open to attack by weather, disease and animal pests.
Just as in the area of food spoilage organisms, plant disease are numerous and their causes multi-fold. Crops with better resistance to disease have been development by selective breeding and crop management techniques have provided us with a means of growing the vast quantities of food that are required the support of human population. Climate change shifts the balance our control and microorganisms can quickly take advantage. This threat to our food supply is constantly monitored at control centers around the world so that action can be taken where possible. One of the key factors in plant health is the amount of rain and relative humidity levels. Too much ground water and high humidity is not good for our grain crops and leads to fungal attack on growing plants and serious problems for harvesting and storage. Other key crops such as maize and potatoes are also badly affected.
It is not just a case of economics or nuisance factor, fungal attack has serious health consequence in some cases. There is a fungus that attacks coffee beans after the outside husk has been removed. Suppliers of this product should monitor the humidity levels during shipping and storage to avoid processing beans that are infected with the fungus. The fungus produces chemicals that are carcinogenic so it is best to avoid these getting info the coffee that ends up on the market.                                                                     


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