Journal of Agricultural Science Cambridge (1993) 120 331-345 copy 1993 Cambridge University Press 331

Effects of field treatments on processing quality ofRecord and Pentland Dell potatoes during storage

M C HOGGE M A STALHAM AND E J ALLEN

Cambridge University Farm Huntingdon Road Girton Cambridge CB3 OLH UK

(Revised MS received 21 September 1992)

SUMMARY

Previous research has suggested that fry colour of processed potatoes is inversely correlated withtuber reducing sugar content and agronomic practice should therefore be tailored to minimizeamounts of tuber sugars at harvest and during storage as a means of maximizing processing qualityA total of 30 experiments from three sites in England from 1985 to 1988 examined this hypothesisfor two cultivars using a wider range of husbandry than that used in practice and long periods ofstorage No tuber sugar value was sufficiently well correlated with fry colour for the relationship to beinterpreted as causal and of use predictively irrespective of whether it was measured early in growthat harvest or at the time of processing Fry colour at harvest was not correlated with fry colour afterstorage No agronomic practice (variables used were physiological age date of planting irrigation ordate of harvest) consistently caused fry colours to be darker than the acceptable colour limit whetherat harvest or after long term storage Crops producing unacceptable fry colours early in storagegenerally improved to produce acceptable fry colours after long term storage Agronomic practiceswithin the range normally found in commercial production are therefore considered to have littleeffect on long-term processing quality although harvesting handling and transport operations werenot investigated

i w T R n n i i r T i f i N (Clegg amp Chapman 1962 Sowokinos 1978 Chase elI I N I K U U U L I I U W a 1982 Nelson amp Sowokinos 1983) For all sugars

One of the major criteria for measuring quality in the amounts decrease from tuber initiation onwards andpotato processing industry is fry colour of the may reach a stable minimal amount if the growingprocessed product A continuous supply of suitable season is sufficiently long unless soil temperatures falltubers has to be available throughout the year and it below 4-5 degC when reducing sugars may increaseis advantageous to identify as soon as possible those (Smith 1959) A general improvement (lightening) incrops which are most likely to process with desirable fry colour usually occurs during growth (eg Miller elfry colours after storage In practice this usually al 1975) and it has been reported that harvestinginvolves sampling the crop during growth and at before tuber sugars have reached their minimal valuesentry to storage in order to measure tuber reducing results in unacceptably dark fry colours (Hope el alsugars and sucrose This approach is based on the 1960 Sowokinos 1973 Nelson amp Shaw 1976) Whileimplicit belief that for each cultivar a single causal there is general agreement on the changes in sugarsrelationship between fry colour and amount of tuber during growth and storage a wide range of absolutereducing sugar (or its precursor sucrose see amounts of all sugars has been reported forSowokinos 1978) exists which allows discrimination comparable harvest or sampling dates For examplebetween samples as to current and future processing Iritani amp Weller (1977) reported total sugars to bequality 1-50 of tuber fresh weight for a mid-September

Early workers (Sweetman 1930 Denny amp Thornton harvest of Kennebec while Miller el al (1975)1940) identified reducing sugars as important reagents reported a value of 0-21 for the same cultivar underin the Maillard reaction (Danehy amp Pigman 1951 similar conditions In Russet Burbank tuber reducingHodge 1953) which causes non-enzymic browning sugars ranged from 010 (Santerre el al 1986) towhen potato slices are fried to produce chips or crisps 0-92 (Mazza 1983) of fresh weight for similarMany subsequent authors have attempted to assess periods of growth Pisarczyk (1982) and Siecza ampthe potential of crops for processing by measuring Maatta (1986) observed that tuber sugar contentstuber reducing sugars or their precursor sucrose were extremely variable and large differences between

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332 M C HOGGE M A STALHAM AND E J ALLEN

treatment means of both sucrose and reducing sugarcontents in tubers were not statistically significantThe variation over cultivars is even greater Yama-guchi et al (1966) reported a tuber sucrose content inKennebec of 3 of tuber fresh weight after storage at10 degC but Sowokinos et al (1987) found only 0-2sucrose after 8 months storage of Norchip ForMajestic Burton (1965) reported only 003 sucroseafter 8 months storage at 10 degC Billington (1984)found that reducing sugars in Pentland Dell after 6months in store at c 10 degC ranged from 0-37 to 0-70 of tuber fresh weight Such variation is not matchedby changes in fry colour (eg 6-3-6-6 on the USDAcolour scale Billington 1984) and is therefore ir-reconcilable with any direct causal relationship be-tween fry colour and amount of tuber sugar Somelack of consistency in the relationship is inevitablycaused by the discrepancy in the accuracy with whichsugars and fry colour are measured The former canbe measured with great accuracy while fry colour isfrequently estimated by comparison with standardcharts using an arithmetic scale one to five or one tonine However the wide range in tuber sugar contentswhich correspond to narrow ranges of fry coloursuggest that there exists no sufficiently close re-lationship to have practical utility Roe et al (1990)showed that changes in tuber amino acid contentcaused variation in colour per unit of tuber sugarNonetheless there are many statements in theliterature that fry colours are closely and inverselyrelated to amounts of reducing sugars (eg Kirk-patrick et al 1956 Smith amp Treadway 1960 Burton1966 Carlsson 1970 Ludwig 1987 Brown et al1990) When regression analyses were carried out therelationship between fry colour and amount of tuberreducing sugar was often found to be significant forindividual treatments but when data were combinedfor a range of treatments correlation coefficients werenot sufficiently high to suggest a causal relationshipand decreased markedly where a range of data wascombined Even when data cannot be analysedtogether it is apparent that regression parametersvary considerably between treatments and betweenexperiments For example Carlsson (1970) whovaried sugar contents by delaying harvest date foundfor different experiments that the relationship betweenfry colour and tuber reducing sugar content was mostclosely fitted by positive linear negative linear andnegative quadratic functions For the nine experi-ments with Bintje giving negative linear relationshipsonly one was statistically significant and the cor-relation coefficients ranged from mdash0-55 to mdash0-96 theslopes from mdash0063 to -0-360 and the interceptsfrom 38-3 to 142-7 Hughes amp Fuller (1984) found thatthe relationship between fry colour and amount oftuber reducing sugar changed considerably with bothsoil nitrogen status and time in store Even Ludwig(1987) who reported a high positive correlation

coefficient of 0-74 for an exponential relationshipbetween fry colour and amount of tuber reducingsugar in a sample of Bintje crops seemed unconvincedof its biological validity for he wrote there isevidently a (reasonably) good relation between thecalculated colour index and the reducing sugarcontent

In order to resolve this apparent conflict betweenpractice and the published evidence a series ofexperiments was carried out with the two majorprocessing cultivars Record (crisps) and PentlandDell (chips) in which a wide range of agronomicvariables was used to produce tubers for processing(immediately or after storage) with a diverse range ofsugar contents From the data a rigorous test of thevalidity and practical utility of the relationshipbetween fry colour and tuber sugar contents has beenmade

MATERIALS AND METHODS

Field treatments

In each year from 1985 to 1988 a common seed tuberstock of each of the major processing cultivarsPentland Dell and Record was grown at three sites inthe UK Cambridge University Farm (CUF gravellyloam of the Milton Series in all years) GleadthorpeEHF (sandy soils of the Cuckney Series in all years)and Terrington EHF (silt loam of the NewchurchSeries in 1985 and 1988 and the Agney Series in 1986and 1987) The experimental treatments and basicagronomic details are given in Tables 1-3 In allexperiments there were four replicates In order tovary the growing season available to crops a widerange of dates of planting and harvesting was usedand extreme treatments such as planting in June andharvesting in early September were deliberately usedto stretch the range of crops well beyond that normallyfound in practice

The two irrigation regimes applied in Expts 20 aand 23 a using Record at Gleadthorpe were (i) earlyirrigation 15 mm at 20 mm soil moisture deficit(SMD) for 4 weeks after tuber initiation then 25 mmat 35 mm SMD and (ii) late irrigation 25 mm at35 mm SMD after tubers attained c 12 mm In Expt22a for Record at CUF the two irrigation regimeswere (i) zero and (ii) late irrigation 30 mm at 40 mmSMD

For all experiments rates of fertilizer and waterapplications (when not used as treatments) and cropprotection measures were chosen in relation to sitecharacteristics and weed pest and disease incidenceand are detailed by Hogge (1989) Colenso (1989) andStalham (1989) After 50 plant emergence per-centage ground cover was measured twice weeklyusing ground cover grids (Burstall amp Harris 1983)Full growth analyses were conducted fortnightly andleaf area indices number of tubers branches and

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Processing quality of potatoes during storage 333

Table 1 Physiological age and dates of planting defoliation and harvest of Pentland Dell in experiments atCambridge Gleadthorpe and Terrington from 1985 to 1988 (Expts 1-15)

Experiment

1-3

46

5

7-9

10-12

13

14

15

Year

1985

1986

1986

1987

1988

1986

1987

1988

Date ofplanting

29 April

6 May23 May

9 June5 May

5 May27 May

5 May25 May13 May

5 May

26 April

Field treatments

Phys age(Daydeg gt 4 degC)

c 300

c300

c 300

0300

0

300

55

10

(all combinations

Date ofharvest

4 September18 September3 September

18 September

3 September18 September3 September

18 September1 September

15 September0 and 14 days

afterdefoliation

0 and 14 daysafterdefoliation

0 and 14 daysafterdefoliation

of)

Date ofdefoliation

mdash

mdash

mdash

mdash

15 September14 October

4 September12 October

7 September10 October

Field treatments include two levels of air drying at harvest (0 and 1 hour)

above and below ground nodes tuber fresh and dryweights and haulm dry weight were recorded In 1988on 27 July 10 August and 17 August Pentland Delltubers from Terrington and Cambridge in the sizerange 20-40 mm were harvested and analysed fortuber sucrose and reducing sugar content as detailedbelow

Defoliation was mechanical (by flail mower orhand sickle) and occurred 2 weeks before harvestexcept where used as a treatment in Expts 13-15 and28-30 (Tables 1 and 2) At harvest tubers were liftedby tractor-drawn elevator digger and immediatelyhand picked off the soil surface except when airdrying treatments were imposed Tubers were mech-anically graded counted and weighed to determineyield and number of tubers then sufficient tubers of50-70 mm in Pentland Dell and 40-65 mm in Recordfor storage and processing requirements were trans-ported to the PMB Experimental Station at SuttonBridge Lincolnshire

Storage and processing

All stored samples after an initial curing phase of2 weeks at 15 degC were held in 6 kg boxes in a sealedstore set at 10 degC with relative humidity of 95Periods of storage are given in Table 4 Two weeks

after the completion of curing an application of0-5 kgt of CIPC granules was made to each boxwhich successfully inhibited sprout growth

At intake to store and at processing tubers wererumble peeled and chipped to a cross-sectional area of15-9x15-9 mm (Pentland Dell) or sliced to a thicknessof 1-25 mm (Record) and a randomly selected sub-sample was fried at 185 degC for 3 minutes or until theoil had ceased to bubble Chip and crisp colours weremeasured using an Agtron M35 Color Quality Meter(Magnusson Corporation Reno Nevada) calibratedto 0 and 100 units using black and white discsrespectively The minimum level of acceptability forboth chip and crisp colour was taken to be 18 Agtronunits There is rarely commercial benefit bonus orpremium gained by achieving Agtron ratings greaterthan 18 Each datum point was the mean of threeobservations with random reorientation of friedpotato material between readings A subsample of200 g from the remainder of the freshly chipped orsliced material was selected at random and liquidizedwith 100 ml of distilled water for 60 s After 10 min1-5 ml of the supernatant liquor was added to 0-3 ml0-25 mM zinc acetate solution to prevent further sugarsynthesis or denaturing This was diluted to 10 mlwith distilled water and after centrifuging at 2000 rpmfor 10 min the clear sap was frozen at mdash20 degC untilanalysis

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334 M C HOGGE M A STALHAM AND E J ALLEN

Table 2 Physiological age and dates of planting defoliation and harvest of Record in experiments at CambridgeGleadthorpe and Terrington from 1985 to 1988 (Expts 16-30)

Experiment

16-18

19-21

20 af

22-24

22 a

23 af

25-27

28

29

30

Year

1985

1986

1986

1987

1987

1987

1988

1986

1987

1988

Date ofplanting

29 April

6 May

26 March29 April

6 May

15 April6 May6 April5 May6 May

13 May

29 April

26 April

Field treatments

Phys age(Day0 gt 4 degC)

c300

c 300

0300

0330

0330

0330

0330350

220

190

(all combinations

Date ofharvest

18 September9 October

24 September15 October24 September

22 September8 October

22 September

22 September

20 September6 October

0 and 14 daysafterdefoliation

0 and 14 daysafterdefoliation

0 and 14 daysafterdefoliation

of)

Date ofdefoliation

mdash

mdash

mdash

mdash

mdash

mdash

19 August8 September

29 September24 October16 August14 September19 October15 August12 September17 October

Field treatments include (t) two irrigation regimes (see text) or () two levels of air drying at harvest (0 and 1 hour)

Table 3 Seed weight and spacings in all experiments at Cambridge Gleadthorpe and Terrington from 1985 to1988

Experiment

14 7 amp 102 58 amp 113 6 9 amp 12

13 amp 151416 amp 1917 20 amp 20a18amp2122 22a amp 2523 23a amp 2624amp272829amp30

Location

CambridgeGleadthorpeTerringtonCambridgeCambridgeCambridgeGleadthorpeTerringtonCambridgeGleadthorpeTerringtonCambridgeCambridge

Seed weight(g)

30-4030-4030-4040-5020-3035-4535-4535-4530-4030-4030-4040-5050-60

Within-rowspacing (cm)

26222130252622212622213030

Between-rowspacing (cm)

71869171717186917186917171

Sugar analysis of the sap extracts was performed ona Technicon Auto-analysis AA2 and spectro-photometer system (Boehringer Mannheim GmbH)Total reducing sugars were estimated by the loss of

NADP during conversion of glucose to glucose-6-phosphate and of fructose after isomerization withphosphoglucose isomerase (PGI) to glucoses-phos-phate Glucose was estimated before the introduction

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Processing quality of potatoes during storage 335

Table 4 Storage period between harvesting andprocessing of tubers from all experiments at Cambridge

Gleadthorpe and Terrington from 1985 to 1988

Experiment

1-34-67-9

10-12131415

16-1819-2122-2425-27

282930

Period of storage

1 2 34 12 20 281 24 8 12 221 4 8 322 4 8 3313 183212 18331 2 19 331 2 4 8 20 28 36

(weeks)

40

401 2 34 8 12 20 28 36402 4 8 201 2 4 8 12 16 201 3 19 3212 18351 2 20 34

2836

of PGI and fructose calculated by difference Sucrosewas cleaved into glucose and fructose with invertaseat 35 degC and the concentration of glucose determined

as above and corrected for the original amount ofglucose in the sap sample This allowed the tubersucrose content to be calculated based on the knownratio of glucose to fructose in sucrose

R E S U L T S A N D D I S C U S S I O N

Total tuber yields for crops harvested for storageranged from 160 to 63-3 tha in Record and 23-2 to64-7 tha in Pentland Dell (Tables 5-7) Thus theagronomic treatments were successful in producing amuch wider range of yields than usually found incommercial practice The short season resulting fromvery late planting andor early defoliation producedthe lowest yields and the tubers in these crops wouldnot be regarded as suitable for processing as they weredefoliated before any loss of ground cover hadoccurred and before sugars were at minimal values

In Expts 13-15 (Pentland Dell) when a wide rangeof harvesting practices was used mean yields (tha)were 45-2 (SE = 0-81) 37-5 (SE = 1-45) and 64-7 (SE= 1-52) respectively and the treatments were imposedon canopies with full ground cover through to thosewith advanced senescence

Table 5 Total tuber yields (tha) of Pentland Dell at Cambridge Gleadthorpe and Terrington from 1985 to 1988(Expt 1-12)

Year

1985

1986

1987

1988

Date ofplanting

29 AprilSE

6 May23 May9 June

SE

5 May27 May

SE

5 May25 May

SE

Cambridge

Early MidlateSeptember September

Expt 1

40-9 50-72-92

Expt 4

40-7 51-431-3 44024-2 36-9

0-67

Expt 7

390 49-628-7 37-8

1-09

Expt 10

39-4 48-728-2 37-9

0-91

Date of harvest

Gleadthorpe

Early MidlateSeptember September

Expt 2

38-9 48-71-27

Expt 5

44-6 48-7

3-60

Expt 8

29-5 36-6

0-84

Expt 11

47-0 54-236-3 44-2

0-73

Terrington

Early MidlateSeptember September

Expt 3

51-6 53-91-26

Expt 6

38-0 47-831-3 40-223-2 32-5

0-64

Expt 9

43-2 51-531-7 41-2

0-69

Expt 12

36-3 42-328-6 34-6

0-52

Means of two physiological ages

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336 M C HOGGE M A STALHAM AND E J ALLEN

Table 6 Total tuber yields tha) of Record at Cambridge Gleadthorpe and Terringtonfrom 1985 to 1988 Expts16-27)

Year

1985

1986

1986

1987

1987

1988

Date ofplanting

29 AprilSE

4 MaySE

26 March6 May

SE

6 MaySE

15 April6 May

SE

6 MaySE

Cambridge

Late MidSeptember October

Expt 16

44-8 50-6111

Expt 19

400 46-51-31

Expt 22

45-8 5210-82

Expt 22 a

56-346-2

1-03

Expt 25

53-8 62-51-63

Date of harvest

Gleadthorpe

Late MidSeptember October

Expt 17

43-9 49-81-27

Expt 20

471 51-9109

Expt 20 a

49-3430

103

Expt 23

33-9 37-6114

Expt 23 a

36-443-6

1-38

Expt 26

51-1 55-71 24

Terrington

Late MidSeptember October

Expt 18

44-2 44-61 25

Expt 21

38-6 50-3103

Expt 24

54-6 55-6111

Expt 27

45-7 50-40-82

Means of two physiological ages and two irrigation regimes

Table 7 Total tuber yields (tha) of Record in date ofharvest experiments at Cambridge from 1986 to 1988

(Expts 28-30)

Date of harvest

Mid AugustEarly SeptemberLate SeptemberMid October

SE

Experiment

28

16039-144-663-3

113

29

25-546-4

53-51-32

30

43-251-3

52-71-74

All values are means of two defoliation practices and two airdrying regimes

Tuber sugar content

There were large fluctuations in amounts of sucroseand reducing sugars in both Pentland Dell andRecord between dates of sampling during the growingseason and storage in all years when sugars weremeasured (Figs 1 and 2) although standard errors foreach sampling were small There was no such largescale fluctuation in fry colour (eg Figs 6 and 8)

The changes in amounts of sucrose total andindividual reducing sugars with time were sometimessimilar to those demonstrated by other authors Bothsucrose and reducing sugars were present in PentlandDell tubers in relatively large amounts early in thegrowing season (respectively 0-45 and 037 of freshweight of 20-40 mm tubers on 27 July data are

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Processing quality of potatoes during storage 337

040 -i

00040 80 120 160

Days in store40 80

Fig 1 Effect of date of planting on amount of (a) sucroseand (A) total reducing sugars (TRS) during storage ofPentland Dell from Cambridge in 1987 (Expt 7) Dates ofplanting 5 May (Q) and 27 May ( bull )

120 160Days in store

Fig 2 Effect of date of harvest on amount of (a) sucrose and(b) total reducing sugars (TRS) during storage of Recordfrom Cambridge in 1988 (Expt 25) Dates of harvest LateSeptember (bull) and Mid-October (bull)

means of two dates of planting at two sites in 1988)and decreased as the growing season progressed(respectively 007 and 0-23 of fresh weight of20-40 mm tubers on 17 August data as for earliersampling) Later planting in Expt 7 as for cropstermed immature by Burton (1965) caused amountsof sucrose in Pentland Dell tubers to be higher soonafter intake to store than earlier planting with tubersucrose content from both dates of planting decreasingduring prolonged storage (Fig 1) Similarly thebehaviour of tuber reducing sugar content in someexperiments followed the time courses detailed byBurton for different dates of planting with amountsof tuber reducing sugar in Expt 7 generally decreasingafter harvest before increasing after an extendedperiod of storage (Fig 1) In other experiments therewas no clear trend in amount of tuber reducing sugarwith time in store

In many instances for Record there were deviations

from the time courses demonstrated by Burton withamount of tuber sucrose in Expt 25 generallyremaining relatively low early in storage but risingdramatically after 21 weeks in store (Fig 2) wellbefore the anticipated onset of senescent sweeteningAmount of total reducing sugar followed no cleartrend with time in store (Fig 2) In Expt 30 theearliest harvest had a higher tuber total reducingsugar content than other dates of harvest althoughthis effect was not common to all experiments anddelayed harvest in some (eg in Expts 25 and 29)caused more tuber total reducing sugars than earlierharvesting

At intake to store in Expt 30 a delay of 2 weeksbetween defoliation and harvesting for late harvestedcrops resulted in more tuber total reducing sugarsthan harvesting immediately without defoliation(Table 8) The delay allowed mean daily soil temper-atures at 10 cm depth for 3 days before harvesting to

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338 M C HOGGE M A STALHAM AND E J ALLEN

Table 8 Effects of date of defoliation period between defoliation and harvest and period of air drying on amountof tuber total reducing sugars ( fresh weight) in Record at Cambridge in 1988 (Expt 30)

Period between defoliation and harvesting (days)

Time of Defoliation

15 August12 September17 October

0

0

0-39001050036

0Period of air

1

0-32500830016

14drying (hours)

0

016800190110

14

1

0-08400280120

sE for comparison between dates of harvest 00149sE for all other comparisons 00163

100 n

I 1006

80-

60

40-

20-

(b)

140 160 180 200Julian days

220 240

Fig 3 Effect of date of planting on ground cover percentageof Pentland Dell in 1986 at (a) Cambridge (Expt 4) and (b)Terrington (Expt 6) Dates of planting 6 May ( bull ) 23 May( bull ) and 9 June (A)

decrease from 11-5 to 5-8 degC Effects of date of harvestand period of defoliation were confounded in thisinstance but increase in tuber total reducing sugar

50

40

30 H

20

100 200Days in store

300

Fig 4 Effect of date of harvest on chip colour during storageof Pentland Dell in 1985 at (a) Cambridge (Expt 1) (b)Terrington (Expt 3) and (c) Gleadthorpe (Expt 2) Dates ofharvest Early September ( bull ) and Midlate September ( bull ) Horizontal line represents acceptable limit

content from exposure of tubers to cold end-of-seasontemperatures has been reported by a number ofworkers (eg Kissmeyer-Nielsen amp Weckel 1967)

Although in some years there was agreementbetween the changes in amounts of sugars with time

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Processing quality of potatoes during storage 339

20-

c

c 30260lt 20

8 40 I to30 -J

20-

10

50 100 150Days in store

200

Fig 5 Effect of date of harvest on chip colour during storageof Pentland Dell in 1986 at (a) Cambridge (Expt 4) (b)Terrington (Expt 6) and (c) Gleadthorpe (Expt 5) Dates ofharvest Early September ( bull ) and Midlate September ( bull ) Horizontal line represents acceptable limit

and previously published results there was widevariation between years in amounts of tuber sugars instore even for similar agronomic treatments In 1987(for both cultivars) tuber total reducing sugar contentincreased to c 0-30 of tuber fresh weight whilesimilar treatments in 1988 produced reducing sugarcontents less than half this value (Figs 1 and 2) Therewas no correlation between decrease in amounts ofsucrose with time and concurrent increase in amountsof tuber total reducing sugars in either year incontrast to the observations of other authors (egRichardson et al 1990)

Effects of agronomy and time in store on fry colour

In Pentland Dell dates of planting and harvest weremanipulated to produce a range of growing periods inorder to test the hypothesis that the physiologicalstate of tubers (primarily influenced by the durationof growth between tuber initiation and date ofdefoliation) was crucial in determining immediate andpost-storage processing quality Some treatmentsproduced very young tubers from crops with canopies

40 80 120 160Days in store

200 240

Fig 6 Effect of date of planting on chip colour duringstorage of Pentland Dell in 1987 at (a) Cambridge (Expt 7)and (6) Terrington (Expt 9) Dates of planting 5 May ( bull )and 27 May ( bull ) Horizontal line represents acceptable limit

retaining almost complete ground cover up todefoliation (Fig 3) Generally within a year a similartrend in processing quality of Pentland Dell with timein store was observed irrespective of site andhusbandry Between years however there was sub-stantial variation in the change in fry colour with timein store with colours improving in some years butslowly deteriorating in others When effects ofhusbandry did occur as in 1986 they were generallyearly in the storage period and after long termstorage colours from different treatments tended toconverge (Fig 5)

There was no effect of physiological age onprocessing quality of Pentland Dell at any time instore In 1985 earlier harvesting at all sites causedslightly darker chips through much of the storageperiod than later harvesting but differences were notcommercially important except after prolongedstorage when early harvesting at Terrington producedunacceptable chip colour (Fig 4) Date of plantinghad no consistent effect on processing quality ofPentland Dell Early planting produced lighter col-oured chips during the first 50 days in store than thelater planting at Terrington in 1987 while from CUFin 1987 early planting produced slightly lighter chipsthan later planting after 30 weeks in store (Fig 6)However no differences from date of planting or date

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340 M C HOGGE M A STALHAM AND E J ALLEN

40 -(fl)

50 100 150 200Days in store

250 300

Fig 7 Effect of date of harvest on crisp colour duringstorage of Record in 1986 at (a) Cambridge (Expt 19) and (b)Gleadthorpe (Expt 20) Dates of harvest Late September( bull ) and Mid-October ( bull ) Horizontal line representsacceptable limit

of harvest (except from Terrington in 1985) were ofcommercial importance

Unacceptable fry colour in Record occurred morefrequently than in Pentland Dell In 1986 and 1988the incidence of unacceptable crisp colour was shortlived (Figs 7 and 8) but in 1987 tubers from Expts22-24 produced unacceptable crisp colours for alltreatments throughout storage Storage in these threeexperiments was of limited length (20 weeks) whilethat for a similar crop of Record from Cambridge in1987 (Expt 29) was for 35 weeks In Expt 29 crispcolours became acceptable by 35 weeks in storehaving been unacceptable in the early period ofstorage (Fig 9)

In all experiments with Record in 1985 date ofharvest had no consistent effect on crisp colour duringstorage In 1986 early harvesting of Record atTerrington consistently produced darker crisps thanlater harvesting but Agtron rating did not fall below18 after an initial unacceptable value at intake Atother sites there was no consistent effect of date ofharvest on crisp colour at intake or after storage In1987 (Fig 7) effects of date of harvest were small andall crisps were of unacceptable colour early in storageOnly for the intake sample from Gleadthorpe didlater harvest produce crisps of acceptable colour

-3 40 -

HI 3 0 -onlt 20 ^

()

| 40 n

O30-

20-

10-

0 40 80 120 160 200Days in store

240 280

Fig 8 Effect of date of harvest on crisp colour duringstorage of Record in 1988 at (a) Cambridge (Expt 25) (b)Terrington (Expt 27) and (c) Gleadthorpe (Expt 26) Datesof harvest Late September (Q) and Mid-October ( bull ) Horizontal line represents acceptable limit

100 150Days in store

Fig 9 Crisp colour during storage of Record fromCambridge in Expts 28-30 1986 ( bull ) 1987 (A) and 1988(O)- Horizontal line represents acceptable limit

while earlier harvesting produced unacceptable crispcolour at intake In 1988 (Fig 8) early harvest ofRecord at Gleadthorpe produced darker crisps than

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Processing quality of potatoes during storage 341

Table 9 Regression parameters for the relationship between both amount of tuber total reducing sugars (TRS)( fresh weight) and fry colour (Agtron) after storage and amount of tuber sucrose ( fresh weight) at intake

to store for Pentland Dell

Year Site Treatments

Period instore

(weeks) VariableRegressioncoefficient SE

1987

1988

Ternngton

CambridgeGleadthorpeTerrington

Early harvest

All treatmentsAll treatmentsAll treatments

48

378

37

323232

TRSTRSTRS

ColourColour

ColourColourColour

0-250-20

-0-04- 1 6 1- 9 6

90-484-982-3

010300970-2015-44502

3311281122-13

later harvest after more than 2 weeks in store whileat other sites there was no consistent effect of date ofharvest on fry colour

Effects of defoliation in Expts 28-30 were in-termittent and confounded with date of harvestSome interesting observations however were madeDefoliation before early harvest (when ground coverwas 98 in 1986 and 96 in 1987) generally causedlighter colours than undefoliated crops althougheffects were small and not maintained throughoutstorage In 1988 defoliation of early harvest (whenground cover was 88) had the reverse effect andcaused darker colours than undefoliated crops afterlong term storage In 1986 defoliation of lateharvested crisps (when ground cover was close tozero so that the defoliation treatment was effectivelya delay in harvest of 2 weeks) caused darker crispsafter long term storage than no defoliation Mean soiltemperature in the intervening period between de-foliation and harvesting was 7-7 degC at 10 cm depthwhile in the 10 days before harvesting withoutdefoliation mean soil temperature at 10 cm depthwas 8-9 degC It is questionable whether such a differencein soil temperature is sufficient to account for theeffect of defoliation on fry colour Defoliation had noconsistent effect on crisp colours of late harvestedcrops in other years

Relationship between sugars during growth or atintake and sugars and colours after storage

In Pentland Dell in 1988 (when it was possible tomeasure early season tuber sugar contents) regressionanalyses were carried out to determine the relationshipbetween amounts of tuber sugars at sampling datesearly in the growing season and those after mid-termand prolonged storage A range of sugar contents wasgenerated early in growth on each sampling occasionthrough the use of date of planting as a treatment Nosignificant relationships could be found between

amounts of sucrose shortly after tuber initiation andat 2 week intervals in the growing season and sucroseand total and individual reducing sugars at any timein store This was contrary to the observations ofsome workers (eg Nelson amp Sowokinos 1983) whohave suggested that tuber sucrose contents early in thegrowing season can be related to and used to predictfry colours after long term storage

Other workers have suggested that amounts ofsucrose at commercial harvest time (the SucroseRating) can be used as a measure of post-storageprocessing potential (eg Sowokinos 1978 Chase etal 1982 Santerre et al 1986) In the 3 years whentuber sugar content was measured (1985 1987 and1988) in both Record and Pentland Dell amounts ofsucrose at harvest were only rarely significantlycorrelated with total or individual reducing sugarsafter both short and long-term storage Whenregressions of post-storage chip colour were madeagainst amount of sucrose at harvest there were fewsignificant relationships and these only accounted fora small percentage of the variance The closestrelationships between amount of sucrose at intake tostore and fry colour after long term storage occurredin 1988 for Pentland Dell and had positive slopeswhen all treatments were combined over sites (Table9) However this was in contrast to 1987 where thefew significant relationships were negative andaccounted for even less of the variance

Relationship between fry colours and amount of tubersugars at the time of processing

If amount of tuber reducing sugar is to be immediatelyuseful as a means of identifying potential fry coloura single unifying relationship between fry colour andamount of tuber reducing sugar at the time ofprocessing is required for each cultivar irrespective ofagronomic treatment or period in store No closelinear relationships could be found in any experiment

AGS 120

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342 M C HOGGE M A STALHAM AND E J ALLEN

50 - (a)

000 010 0-20 0-30 0-40

000 0-20 0-40 0-60TRS in tuber sap ()

080

Fig 10 (a) Relationship between crisp colour (Agtron rating)and amount of tuber total reducing sugars (TRS) in Recordin 1985 1987 and 1988 Line equation Agtron rating =23-8-28-54(TRS) (b) Relationship between chip colour(Agtron rating) and amount of tuber total reducing sugars(TRS) in Pentland Dell in 1985 1987 and 1988 Lineequation Agtron rating = 27-3- 10-36(TRS)

in either cultivar (Tables 10 and 11) although in somecomparisons a similar amount of the variance to thatpreviously reported in the literature (30-40) wasaccounted for Fry colours for some field treatments(over a range of sampling occasions from store) werefound to be correlated with total or individual sugarsat the time of processing but variation about thefitted line was large in every case and the regressionnever accounted for more than 50 of the varianceHowever in the majority of instances there was nosignificant relationship between fry colours andamounts of sugars at the time of processing Theslopes and intercepts of the relationship varied greatlyand inevitably when the data for all years werecombined in each cultivar a very weak relationshipwas found The relationships between fry colours andamounts of both TRS and sucrose for all experimentsin 1985 1987 and 1988 combined (excluding date ofharvest experiments at CUF) are presented graphi-cally for each cultivar in Fig 10

Table 10 Regression parameters for the relationshipbetween fry colour (Agtron) and amount of tuber total

reducing sugars ( fresh weight) in Pentland Dell

Year

1985

1987

1988t

Site

Cambridge

Terrington

Gleadthorpe

Cambridge

Terrington

Gleadthorpe

CambridgeTerringtonGleadthorpe

4184

184

18

3183

183

18

Date ofharvest

SeptemberSeptemberSeptemberSeptemberSeptemberSeptemberSeptemberSeptemberSeptemberSeptemberSeptemberSeptember

mdashmdashmdash

Regressioncoefficient

mdash13-8

101- 6 022-016 314 3

-15-5201

3-530

13-49-5

-29-4- 8 8

-28-4

SE

15-4024-6012-9017-73250015-62

4 915-352-602-324-375-91

9-9919-21817

Data are combined for all sampling dates from storet At harvest Data are combined for 2 harvest dates 1 and15 September

Table 11 Regression parameters for the relationshipbetween fry colour (Agtron) and amount of tuber total

reducing sugars ( fresh weight) in Record

Year

1985

1987

1988

Site

CambridgeGleadthorpeTerrington

Cambridge (Expt 22)Cambridge (Expt 22 a)Gleadthorpe (Expt 23)Gleadthorpe (Expt 23a)Terrington

CambridgeGleadthorpeTerrington

Regressioncoefficient

1-536-633-5

303-4

14-2612-4

7-78-101

SE

13-9119-201502

4-524-394-684144-92

8-296-855-37

Data are combined for all treatments and sampling datesfrom store

For fry colour against TRS in Record therelationship was found to be significant

Agtron = 23-8-28-54(TRS)

but this was because the number of observations waslarge and the variance accounted for was only 7-6In Pentland Dell the equation for the relationshipbetween fry colour and TRS was

Agtron = 27-3- 10-36(TRS)

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Processing quality of potatoes during storage 343

and this accounted for 5-3 of the variance The widerange of colours observed at a single TRS content(especially at TRS contents close to zero) in bothcultivars suggests that the Maillard reaction maysimply depend on amino acids to a greater extent thanpreviously considered and the amount and types ofamino acids may influence the effect of any sugarcontent on fry colour Here browning has occurredto some degree and has varied widely with minimalquantities of reducing sugars

From these results it is apparent that the utility ofsugars in determining or explaining processing qualityis limited Such relationships between fry colours andamounts of tuber total reducing sugars which weresignificant were generally similar to those observed inthe literature with increasing tuber reducing sugarcontent causing darker fry colour Although thissupports conclusions in the literature such relation-ships are of little practical value as there was noconsistency in regression parameters between differenttreatments whether field or storage Consequentlymeasurement of amounts of sugars cannot dis-criminate amongst samples of tubers as to theirpotential processing quality and will not be able to doso unless a greater understanding of the browningreaction and the role of other constituent reagents isobtained Reducing sugars may then be of use as onecomponent or group of components among a numberof factors combining to determine colour Ultimatelyquality is measured by colour and tuber sugar contentalone is an intermediary factor of exaggerated value

Relationship between fry colours at harvest and afterstorage

It is important for processors when managing storageand processing facilities to maintain a strategy ofallocating loads to be stored or processed immediatelyAt present those loads whose subsamples process atclose to the minimum level of acceptability aregenerally sent for processing immediately while thosewith lighter colours at intake are placed in long termstorage The results presented here suggest that thisapproach is unjustified Figs 4-9 demonstrate that theranking of samples by colour at intake was changedafter periods in store for both Record and PentlandDell A series of regressions between fry colours afterlong term storage and those at intake produced poorcorrelations in all instances for both cultivars Theclosest fitting relationship occurred for Pentland Dellat CUF in 1988 when 35-3 of variance wasaccounted for by a positive linear regression How-ever in many instances the residual variance fromeach regression analysis exceeded the variance of chipor crisp colour after long term storage Colours atintake were clearly inadequate for predicting colourafter long term storage In some instances as much as50 of the variance was accounted for by the

regression of fry colours after long term storageagainst colours after the initial 2-week curing phaseHowever this was again inconsistent and no cleartrends were apparent whether data were combined orindividual sites and years were treated separately

In 1987 for Pentland Dell tubers processed poorlyat intake (Fig 6) but in most cases processedacceptably after long term storage This contrastswith observations for Pentland Dell in 1985 whensamples processed with very light fry colours atintake but darkened rapidly with short term storagebefore the decrease in Agtron rating was reduced orarrested as storage continued (Fig 4) One canunderstand that in practice in the first case storemanagers would be unwilling to commit to storagetubers of poor fry colours as the general belief is thatthey could not improve The data suggest that this isat least as likely as tubers of light fry coloursdeteriorating during long-term storage

CONCLUSIONS

The only basis on which amounts of sugars could beof importance to processors as a means of predictingprocessing potential of potatoes would be a closerelationship between fry colours and amounts oftuber reducing sugars at the time of processing Allother observations (whether on tuber sugar contentsin the field and in store or on relationships betweenpost-storage fry colour and sugars in growth or atharvest) are of little value unless a close fitting causalrelationship can be found between fry colour andsimultaneous tuber reducing sugar content Such arelationship is not to be found in the literature Datain these experiments were wide-ranging both inamounts of tuber sugars and in fry colour eachdistributed widely around the values previouslyreported to be critical in determining processingacceptability Despite this the relationship betweenfry colour and tuber reducing sugar content was poorand patently not causal Even when curves were fittedfor individual treatments there were rarely significantrelationships between fry colours and tuber reducingsugar contents While sugars may on occasion be thelimiting factor influencing the degree of non-enzymicbrowning this does not occur with sufficient regularityor with any degree of predictability to be of use as ameasure of immediate or post-storage processingpotential Although such conclusions are contrary tomuch that has been stated in the literature in realitythere is agreement between data presented in theliterature and in these experiments It is thereforeirrelevant to the improvement of fry colours whetherthe response in tuber sugar behaviour to some storageand to a lesser extent field conditions is known andcontinued attention to refining techniques formeasurement of sugars will not help The prioritymust be to increase overall understanding of the

13-2

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344 M C HOGGE M A STALHAM AND E J ALLEN

Maillard reaction and hence allow the determinationof the relative importance of the components indetermining colour Reducing sugars clearly influencefry colours (eg low temperature sweetening in storageor after delay to harvest) but the effect of sugar is toovariable to be useful and attention must be directed tothose factors which alter the colour at any amount ofsugar

After initial disparity in fry colour between treat-ments in the period covering harvest curing and earlystorage long-term storage caused a convergence of frycolour Such developments would be expected sincecrops were accumulated from diverse sites with arange in husbandry and environmental conditionsand at intake tuber characteristics determining frycolour inevitably depended upon the scope of suchvariation Subsequently during long-term storagetubers experienced constant and similar environ-mental conditions for an extended period of time andachieved a common and stable degree of processing

quality Ultimately if harvesting transport handlingand storage conditions can be adequately controlledthese results show that agronomic parameters withinthe range found in commercial practice have littleeffect on long-term processing quality This suggeststhat store managers when relating agronomy tostorage procedure should concentrate on damageand tuber weight loss considerations since there islikely to be more impact of these on profit marginsthan deterioration in processing performance

The authors are grateful to the Ministry ofAgriculture Fisheries and Food for financial supportJ M Colenso for supplying additional data SBennett for help in preparing the manuscript and thefarm and research staff of Cambridge UniversityFarm and of Gleadthorpe and Terrington Exper-imental Husbandry Farms for assistance with experi-ments

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COLENSO J M (1989) Effects of potato harvesting practiceson skin abrasion and processing quality PhD thesisUniversity of Cambridge

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HoGGE M C (1989) Effects of site season and husbandry onyield and processing quality of the potato variety PentlandDell PhD thesis University of Cambridge

HOPE G W MACKAY D C amp TOWNSEND L R (1960)

The effect of date of harvest and rate of nitrogenfertilization on the maturity yield and chipping quality ofpotatoes American Potato Journal 37 28-33

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KIRKPATRICK M E HEINZE P H CRAFT C C MOUNT-

JOY B M amp FALATKO C E (1956) French-frying qualityof potatoes as influenced by cooking methods storageconditions and specific gravity of tubers United StatesDepartment of Agriculture Technical Bulletin 1142

KISSMEYER-NIELSEN E amp WECKEL K G (1967) The effectsof soil temperature harvest sequence and storage onFrench fry processing quality of potatoes European PotatoJournal 10 312-326

LUDWIG J W (1987) The colour index as standard forsugars in potatoes in the french fry-production InAbstracts of Conference Papers and Posters of the IOthTriennial Conference of the European Association of PotatoResearch pp 93-94

MAZZA G (1983) Correlations between quality parametersof potatoes during growth and long-term storageAmerican Potato Journal 60 145-159

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NELSON D C amp SHAW R (1976) Effect of planting and

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PISARCZYK J M(1982) Field harvest damage affects potatotuber respiration and sugar content American PotatoJournal 59 205-211

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SANTERRE C R CASH J N amp CHASE R W (1986)

Influence of cultivar harvest-date and soil nitrogen onsucrose specific gravity and storage stability of potatoesgrown in Michigan American Potato Journal 63 99-110

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at httpwwwcambridgeorgcoreterms httpdxdoiorg101017S0021859600076498Downloaded from httpwwwcambridgeorgcore Faculty of Classics University of Cambridge on 24 Nov 2016 at 160139 subject to the Cambridge Core terms of use available