Storage Bulletin - April 2012
The storage season is starting to close for many but there remains a lot to keep abreast of both for crops in store and in preparation for next season.
Crops in store now
For those crops in store at the moment, the primary issue is to move the crop to maximise its quality potential. This means keeping abreast of any issues which might influence its saleability.
Skin finish for fresh market and fry colour and/or texture for processing are important attributes and must be maintained at all costs. Take care when unloading from cold stores in warm weather to avoid condensation forming on the boxes; a coating of moisture can still be sufficient to spark an outbreak of silver scurf if there are viable spores on the crop (which is likely).
For processing, take care to ensure that crop temperatures are maintained. At this time of year, we can still get some cold mornings which could compromise fry colour if crop is being transported.
Across all crops, make sure CIPC limits are adhered to and that, where a crop is sold on, its CIPC record is supplied to the new buyer. Crops treated with more that 36 g/t must only be used for processing. See www.potato.org.uk/cipc for guidance.
The longer a potato is kept in store the more prone it is likely to be, understandably, to problems associated with tuber age. So watch for softening as this can indicate a crop which is more liable to pressure bruising or handling damage (as the cells soften then the risk of disruption of the cell structure increases and where this happens tissue can turn black). Blackheart is another factor which is not adequately understood but can be associated with older crop, especially if the inner tissues are deprived of oxygen (PCL project R456 is currently addressing this). And older processing stocks can become susceptible to irreversible ‘senescent sweetening’ as tubers start to convert starch to sugar to fuel metabolic activity.
The bottom line here is to monitor crops closely and assess them frequently for whatever attributes are key to your market so you can react to any changes.
It is important to be prepared in good time to book contractors for service work over the summer months when the store has been unloaded. All too often this is left until harvest is on the horizon and the companies already have their diaries full.
Basic cleaning should take place not only for the store but also the ventilation ducts and equipment. Checking and calibration of temperature recording systems and sensors is another key task. For refrigerated stores, it is important to get the fridge system serviced and to ensure your ‘F’ gas records are all up to date. If you are using R22 refrigerant, you may also need to start thinking about plans for dealing with its phase out in 3 years time. It is recommended that you speak with your refrigeration specialist in the first instance or visit www.f-gas.org for more information.
Ventilation rates and pressures
Adrian Cunnington writes:
Sutton Bridge has had a number of enquiries lately regarding air flow rates in stores and I thought it might be useful to provide a short explanation of some of the terms that are regularly used in quoting ventilation capacities for potato stores.
Most commonly the rate of air delivery is quoted in a volume of air per unit time per tonne, for example 0.02 m3/second/tonne. Rates are also commonly quoted in continental Europe in m3/hour/tonne, which in this example is 72 m3/h/t (0.02 x 60 x 60). These figures are equivalent to the old imperial level of 40 cubic feet per minute per ton (cfm/t) which was more or less fitted as standard to British bulk stores which were built 15 years or more ago.
Since around 2000, there has been a trend towards specifying higher airflows to boost drying capacity. This has gathered pace most recently with the advent of affordable variable speed drives (also known as variable frequency drives or inverters) which allow flows to be lowered for the holding period. Rates in the region from 0.03 to 0.05 m3/s/t (60 - 100 cfm/t) are not unusual but, historically, these higher airflow rates have been more commonly used in mainland Europe and the Netherlands, in particular. Of course, higher airflow rates require larger fan capacities and ductwork.
The other important point when considering fan capacity is to be aware of the fan’s capability in relation to the system resistance to that air flow. Energy is lost in making air move faster, in negotiating bends in the air path and in overcoming system restrictions such as louvres or fridge coils. There should also be some important resistance provided by the lateral or floor outlets as these ensure an even distribution of air across the store. The final restrictions to flow come from the crop and are related to tuber size (smaller potatoes pack together more closely), loose soil content and even sprouting. All of this resistance is measured as static pressure and typically, for a 3.5 m pile of potatoes, the total static pressure will be somewhere between 100 to 400 pascals (Pa). The old imperial unit was inches (”) of water gauge (WG) and 250 Pa is roughly equivalent to 1” WG.
Typically, a safe design figure used for a bulk processing store will be 375 Pa. The system resistance may well be lower (and the airflow higher) in seasons when the crop is clean but it is when the potatoes are wet that performance becomes most critical. In contrast, in an overhead throw box store, the system resistance will be much less, typically under 100 Pa, as the air is not forced through the potatoes.
It is therefore not adequate, when comparing ventilation systems, to purely look at airflow figures if these are not qualified with a system resistance. If there is little or no resistance, the airflow will be much higher than if energy generated by the fan has to be consumed in overcoming a high level of static pressure. Fan performance varies enormously according to the design, blade angle, motor power and torque so it is important to get specialist assistance in choosing the correct fan for your store.
A final note to industry: please quote your airflows alongside a static pressure so store owners can make an informed comparison of systems. I’ll try to do the same!
For more information on this topic, please email me at firstname.lastname@example.org.
Storage advice line: 0800 02 82 111