Sydney Postharvest Laboratory Information Sheet.

www.postharvest.com.au spl@postharvest.com.au

Potatoes: Handle with care.

By Jenny Jobling

Potatoes are one of the world’s most importantfood crops. In Australia potatoes are thelargest volume and highest value horticulturalcrop (excluding wine grapes) produced.Potatoes, compared to most horticultural cropshave a long storage life. This difference is dueto many physical attributes of potatoes. Theyare storage organs and so have many storedfood reserves and also “nature” has designedthem to last at least until the following season,to sprout again and begin the next generation. Potatoes, as a result are often regarded astough or robust vegetables. This is amisconception and it does not mean thatpotatoes shouldn’t be handled with care. It hasbeen estimated that losses due to physicalwounding, such as cuts and bruises can be ashigh as 40%. This article aims to highlight theimportance of handling potatoes carefully toavoid physical damage and as a result, minimisepostharvest losses.

A recent survey of potato quality in the marketplace carried out by the Agriculture Victoriaand the Victorian Farmers Federation showedthat mechanical damage of potatoes waswidespread with up to 49% of potatoessampled having significant damage. Thedamage occurred both on the farm and postfarm gate. This very high level of damagehighlights the fact that handlers of potatoes allalong the marketing chain need to take morecare.

If potatoes are harvested when the stem andfoliage is still green and vigorous or has just

recently been cut, the tubers are immature andhave very delicate skins. These potatoes aresometimes sold as ‘new potatoes’ and fetch apremium price. However they are difficult tohandle as their natural skin is easily removedand is no defence against microbial infectionand rots. Most potatoes are harvested laterwhen the foliage dies and when the skin of thepotato is more developed. At this stage theskin is much tougher but rough handling duringharvesting and grading can easily damage thepotato skin. This damage can detract fromtuber quality, increases moisture loss andprovide easy entry points for postharvest rots.

A recent HRDC project carried out by SydneyPostharvest Laboratory and Food ScienceAustralia showed that wound type has a biginfluence on the development of postharvestrots. The severity of the disease infectionincreased as the level of damage increased. Scuffs, where the skin is broken and somebruising occurs, splits and skin grazes were allshown to be entry points for rots. Uninjuredpotatoes did not develop postharvest rots.

Postharvest rots.There two main postharvest rots of potatoes inAustralia are bacterial soft rot and Fusariumrot.

Bacterial soft rot (Erwinia sp.) - Soft rottedtissue is characteristically creamy at first andlater black. Under favourable conditionsrotting can be very rapid and if there isdevelopment of a secondary infection(Clostridium sp.) Then a foul odour can alsodevelop.

Sydney Postharvest Laboratory Information Sheet.

www.postharvest.com.au spl@postharvest.com.au

Bacterial soft rot of potatoes.

The bacteria is usually present in the soil or canbe present in the seed tuber. The diseasenormally enters the potatoes via wounds in theskin. The disease is therefore most effectivelycontrolled by minimising physical damageduring harvest and handling. Bacterial diseasescan also be easily spread to other healthypotatoes so it is important to frequently cleanand wash harvesting and grading equipmentwith chlorine. The chlorine wash water shouldalso be regularly checked as chlorine is lesseffective in dirty dip tanks. Correct packagingand storage practices are also essential.Condensation on potatoes should be avoided atall costs. If the potatoes have been washedthen they should be allowed to dry beforepackaging in plastic bags. The plastic bagsshould also have perforations to permitadequate air movement around the potatoes.Stacks of potatoes transported or exported tomarket should be shipped with adequate airflow to avoid around the stow so thatcondensation is avoided. The incidence ofbacterial soft rot is also reduced by controllingthe level of Fusarium rot as the presence of onedisease can encourage the development of theother.

Fusarium rot (Fusarium spp.) - Fusarium rotis commonly called dry rot. The symptoms ofthis rot are that the rot forms cavities in theflesh which are lined with a pale covered mouldand the skin tends to wrinkle as it loses water. The surface dries out over the infection andbecomes a brown or black colour.

Fusarium rot of potatoes.

The fusarium fungi is present in the soil and thefungi enters the potato via physical wounds.mature tubers can resist decay for a shortperiod but often will begin to decay afterseveral weeks in storage if there is any fungipresent in the wounds of the potatoes. Themain method of control is to avoid or minimisephysical damage during harvesting andhandling. Storage at 5oC also slows thedevelopment of this rot but it quickly developson return to warmer temperatures. Thepotatoes should also be treated with aregistered fungicide directly after harvest. Fungicide treatment will be ineffective if it isdelayed as the fungus becomes established andresistant to the treatment.

Sydney Postharvest Laboratory Information Sheet.

www.postharvest.com.au spl@postharvest.com.au

Curing

Curing is a process that is encourages thepotato to naturally heal its own wounds. Curing potatoes before storage and transporthas been shown to significantly reduce theincidence of postharvest rots. Potatoes can becured by holding them for 5 days at 15 - 20oCand 90 - 95% RH. This can be done in a in ahigh humidity coolroom or conventional coolroom could be used if the humidity wasincreased with an atomising nozzle and relativehumidity controller. Curing is a very effective,non-chemical control method for bothFusarium and bacterial soft rot. if potatoes areto be stored for a length of time or exported bysea then it is essential that the potatoes arecured. Curing will ensure a quality out turn.

Harvest and handling

At all stages of harvest and handling physicaldamage must be avoided. Recent research hasshown that most damage occurs in the packingshed rather than in the field. Potatoes are notas indestructible as many people imagine. Somepractical measures that can be taken to preventdamage include minimising the height of dropsand any sharp edges on trailers or conveyorsshould be smoothed. It is important thatconveyors fill bins or trucks from the bottom tothe top rather than staying at a constant height,this means that the potatoes are not dropped togreat a distance into the bin. The height of thestack in the bin should not be allowed to gettoo high or too steep. A high, steep pyramid ofpotatoes means that the tubers can roll downthe sides of the stack and be bruised. Highstacks can also promote a cascading effectwhere many potatoes are dislodged andsubsequently damaged and bruised.

The effect of the bruising and cutting potatoesduring harvest and handling does not becomeevident until several weeks after harvest. This

means that many growers may not be aware ofthe damage that a certain handling methodcauses. The Victorian survey showed thatmost damaged potatoes don’t get graded outduring packing as nearly 50% of potatoessampled at the retail level showed signs ofsignificant damage. This damage is not aproblem if the potatoes are consumed soonafter harvest. However it does become a veryimportant issue if potatoes are transported orexported over long distances. In these cases itis critical that the damaged potatoes must begraded out to ensure postharvest rots don’truin the load.

Physical damage must be avoided as itencourages postharvest rots.

The type of packaging and the environmentaround the potatoes significantly influences thedevelopment of rots. Foe example, plasticpackaging in comparison to hessian sacks hasbeen shown to increase the level of rots. Thehigh humidity inside plastic bags may minimiseweight loss but it also encourages condensationwhich in turn promotes the development ofrots. Even wounds containing minor tissuedamage such as grazes and scratches have beenshown to develop more rots in plasticpackaging compared to hessian bags.

Sydney Postharvest Laboratory Information Sheet.

www.postharvest.com.au spl@postharvest.com.au

Storage

The optimum storage temperature for potatoesdepends on their final use. It is recommendedthat fresh market potatoes are stored between5 to 6oC. Potatoes that are used for makingchips are stored between 7 and 10oC. Thedifference in temperature is due to the fact thatpotatoes for chips need to have low levels ofsugars. Potatoes are mainly starch. At lowtemperature the starch is converted into sugars. This change detracts from the cookingattributes and flavour of the potatoes. Highsugar levels result in chips with a dark browncolour rather than the golden colour consumersprefer.

There is enormous potential for the quality andvolume of Australian fresh potato exports toincrease. However this is only possible if thequality of the potatoes on arrival can beassured. The practical steps out lined in thisarticle will help all members of the marketingchain handle potatoes with the care that isneeded to ensure the final quality of thisimportant product.

References

• Harris, P. (Editor)(1992). The PotatoCro p. Chapman and Hall, London.

• Henderson, A., Freeman, K., de Jong,R. and Franz, P. (1995). Quality in themarket place. measuring fresh potatoquality from paddock to retail store.Institute for Horticultural Development,Victoria, Australia.

• Morris, S., Stephens, B. Bokshi, A. andSharp, A. (1999). Potatoes for exportneed a soft touch. Potato Australia,September. p 51 - 53.

• Snowden, A.L.(1991). A Colour Atlasof Postharvest Disease and Disordersof Fruits and Vegetables. Volume 2:Vegetables. Wolfe Scientific Ltd.,London.

This article originally appeared in Good Fruitand Vegetables magazine 11(4): 34 - 35,September 2000 (Melbourne, Australia).

Jenny Jobling is research manager at SydneyPostharvest Laboratory. Sydney PostharvestLaboratory provides independent, expertpostharvest horticultural research and advice.The laboratory is located at Food ScienceAustralia, North Ryde, NSW Australia.

Sydney Postharvest LaboratoryPO Box 52 North Ryde NSW 2113 Ph: 02 9490 8333 Fax: 02 9490 8499Email: spl@postharvest.com.