The Potato Quality Guide

The Economics of Damage and Bruising Prevention

Potato bruising is a serious problem in potato production worldwide. Damaged potatoes going into storage can cause deterioration of the entire crop. Potatoes bruised during handling operations will be rejected by processors looking for perfect material to make perfect potato products. Estimates indicate that more than 60% of some crops are damaged in some way which leads to financial losses running into Billions and a massive waste of crop. The effects of such losses to any potato-related business cannot be over-estimated.

Martin Lishman potato harvester mister

Reducing Damage and Bruising

Achieving the highest quality in all types of harvesting conditions can be difficult, but mist spraying has been shown to reduce bruising, particularly in dry conditions when dolmen rollers tend to cause potatoes to bounce.

The fine mist spray from the nozzles dampens both the potatoes and the rollers in the harvester. This lubricates the movement of the potatoes over the rollers and helps to prevent them being pinched and damaged.

Even when lifting conditions are relatively easy, other factors such as dry matter, handling temperature and nutrient levels can influence bruising susceptibility, so a Harvester Mister should be standard equipment fitted to all harvesters in all conditions.

Monitoring Damage and Bruising

TuberLog is an economically priced incentive for growers and processors to monitor bruising, take preventive action and reduce their losses. It should be used regularly so that bruising problems caused by poor machine maintenance or incorrect settings can be detected quickly. Machine settings are often changed, but the effects on bruising may not be realised. A quick check with TuberLog can make sure.

Different varieties or the same variety at different temperatures can bruise at different impact levels. In conjunction with bruise testing using a Hot Box, TuberLog can help to ensure bruise-free working and provide peace of mind that quality standards are being maintained.

TuberLog has been devised by a team of researchers from Germany, together with electronics specialists and a panel of potato industry experts. Building on the experience of the Martin Lishman potato quality equipment team, going back some 25 years, TuberLog takes previous electronic potato designs several steps further by incorporating Bluetooth and the latest Android App technologies.

Assessing Potato Bruising

When potatoes are bruised, the physical effects are not immediately visible. A biochemical reaction takes place which gradually causes a colour change at the point of impact. In cool dry conditions this effect can take several days to appear.

The Hot Box speeds up the reaction time by creating the optimum warm, damp conditions for bruise development to occur more quickly. This allows action to be taken sooner, resulting in fewer bruised and damaged potatoes. 

Quality Control for Stored Potatoes

Before storing potatoes, it is important to know the eventual market and the specifications required of that market.

Specifications will depend on a range of quality measures such as skin finish, dry matter, fry colour and sugar content. Crops should be assessed in relation to these parameters, both before and during storage. Inevitably, the presence of rots and diseases or damage caused by bruising will have a significant effect on both the storage outcome and the ability to meet customer specifications.

Quality Control is an investment. Initial inspection of loads should include use of a Martin Lishman Hot Box to check for the bruising damage and the presence of rots. This will determine if the loads can be safely taken into storage. Once in the store, monthly monitoring of factors that are likely to change, such as sprouting, rots, fry colour and certain blemish diseases should be carried out.

potato temperature monitoring

Store and Tuber Temperature Monitoring

Accurate temperature monitoring of stored potatoes is critical for early warning of quality issues, preventing deterioration of product and maintaining correct storage temperatures for different markets.

Equipment available from Martin Lishman includes simple T-Bar thermometers for measuring the internal temperature of individual tubers, permanently located rigid or cable temperature sensors designed for bulk or box stores and the Barn Owl Wireless internet-based remote monitoring and automatic ventilation control system. In addition to monitoring and logging temperatures and presenting the data in both graphical and tabular format, Barn Owl Wireless provides the facility to link measurements to automatic equipment controls.

Post-storage Quality Assessment

To avoid the problem of rejection on delivery, it is important to know the quality of product before it leaves the store. Tests may include dry matter, glucose and fry colour tests. Martin Lishman supplies a choice of dry matter testing equipment, the Glucose testing kit, French fry sample fryer and fry colour reference chart.

glucolis blue potato sugar testing kit with press
dry matter measurement with the weltech digital hydrometer

The Importance of Dry Matter

Dry matter content directly influences the yield of processed potatoes, the oil absorption rate in fried products and the texture of cooked potatoes. Dry matter is also used as an indicator of bruising risk, with high levels associated with more damage.

Dry matter varies between varieties, but dry matter of the same variety may also vary between seasons in the same locality. This can be the result of differences in the time of planting, soil moisture and ambient temperature.

We offer 3 types of Dry Matter testing kits

Typical dry matter ranges for various potato products*:

French Fries – 19.7 to 24.1%

Potato chips – 21.7 to 25.1%

Dehydrated products – 20.7 to 24.1%

* guidance only

The Principles of Measuring the Dry Matter of Potatoes

The dry matter of potatoes can be calculated because extensive research work was carried out many years ago using oven tests which related the density or specific gravity of potatoes to their dry matter.

The Zeal Hydrometer and Weltech Weigher methods use this relationship to display dry matter either on a paper scale or a display screen. These methods can be used to measure the specific gravity of potatoes because they both rely on the potatoes having negative buoyancy (they sink in water). This creates a contrast between the downward force of the potatoes in water and the upward force of the air in the hydrometer float or the weight in air of the potatoes in the case of the weigher.

The Dry Matter Field Kit method records the weight of a volume of material (specific gravity). The material is sampled from a potato using a corer which has been scientifically measured to record its volume. The conversion of specific gravity of potato material to potato dry matter is only possible because of the oven test research.

dry matter measurement with the weltech digital hydrometer

Measuring the Dry Matter of other produce

The Field Kit method can be used to measure the specific gravity of any other produce (such as beetroot, parsnip, carrot etc) that can have a core of material taken from it. However, unless there is existing conversion data for the other produce being tested the only way to convert the specific gravity reading to dry matter is to carry out some oven tests and calculate a specific gravity to dry matter relationship for that produce.

The hydrometer and weigher methods cannot be used to measure the specific gravity of other produce because virtually all other types do not have negative buoyancy and tend to float.

Measuring Glucose Levels in Potatoes

Sugar content plays an essential part in the culinary and technological quality of potato tubers. It also depends on variety, maturity at harvesting and storage conditions. Measuring glucose levels (such as with the Glucolis kit) can help to manage harvest times and storage temperatures to ensure precise colour control of crisping potatoes and French fries and determine the best time to move potatoes out of storage.

glucolis blue potato sugar testing kit with press
potato temperature monitoring

Potato Temperature Monitoring

Good potato store management should ensure crop temperatures are as uniform as possible to minimise the risk of condensation. Sensors need to be sufficiently accurate to measure differences as small as 0.5°C. In both box and bulk stores, sensors should be preferably located in the top and bottom of the stack, with most located in the top of the crop and some at the base. Alkathene tube, eg water pipe, makes a good sleeve in which to fit sensors at low level in a bulk store; fit it on the face of the stack, rather than vertically.

Store temperatures should be checked daily and a log of key probe readings kept manually or using a remote cloud-based monitoring system such as Barn Owl Wireless which can also provide automatic ventilation control.

This information is a guide only and is given more comprehensively in the AHDB Potato Store Managers’ Guide. If you do not have a copy it can be downloaded using the following link: https://ahdb.org.uk/knowledge-library/potato-store-managers-guide

Temperature Monitoring in Bulk Stores and Boxes

In bulk stores, it is recommended to take temperature readings using an imaginary 6m x 6m grid over the bulk surface and take a reading in the centre of each grid square at 70mm and 300mm below the surface. This means readings are always in the same place and show actual changes rather than location differences. 

In boxes, place sensors in stacks at the edge and centre and in the top and bottom box 70mm and 300mm from the surface. The Potato Store Managers’ Guide provides more details.

Crop temperature readings should be recorded at least once per week until the potatoes have been cooled to the target storage temperature; and then every two weeks thereafter. The more comprehensive and the more frequent that temperature monitoring takes place, the better the chances of good store management. Manual readings should ideally be taken daily; automatic readings should be 3-hourly in the crop and continuous for ambient temperature.

Martin Lishman Barn owl Wireless automatic crop monitoring and control system used to monitor potatoes

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