13. Contents xiii
Yellow yams, twelve months yams 367
Youngberries 368
Zapotes chupa chupa 369
Appendix: glossary of terms 371
Abbreviations 371
Carbon dioxide and oxygen in controlled atmosphere stores 371
Concentration of chemicals and ethylene in a store 371
Film thickness 371
Humidity 372
Hypobaric storage 372
References 373
Index 445
14. To
Elara, Maya, Ciaran, Caitlin and Cameron
to whom I owe much more than they will ever know
15. Preface
The technology involved in getting fresh produce from one retailer is different; the fresh produce section is
the field to the consumer has been the subject of usually the first section inside a supermarket. This
detailed research for over a century. It is enormously book therefore covers the whole range of produce from
complicated because many of the crops are highly per- the major sellers to those that are of minor importance
ishable and variable. This variability militates against in industrial countries and to those that may become
simple solutions. The fresh produce trade would pre- important in the future. The parts on the latter group
fer not to be involved with this variation and com- of produce (often referred to by names such as ‘exotic’
plexity: they would prefer to be able to look up their or ‘queer gear’ by the trade in the UK) will also give
particular crop on a chart, which will say it should be some ideas to those in the trade of what crops might
harvested, packaged and stored in a certain way. Infor- be developed for the future.
mation in this form is readily available but will rarely During the Second World War, Winston Churchill
give the best results in terms of preserving the quality concluded a long and rambling oration with the words,
of the crop. The objective of this book is to provide a ‘I am sorry to have made such a long speech, but I did
range of options from which the produce technologist not have time to write a shorter one’. During her time
can select. Additionally it puts into context our cur- as British Prime Minister, Margaret Thatcher always
rent state of knowledge on postharvest technology and insisted that briefing notes from officials should be no
thus identifies areas where research is needed. longer than half a page. There is an enormous litera-
The work is based on a selective review of the ture on postharvest science and technology of fruits
literature and my experiences since I was first and vegetables. Scientists have written much of this for
formally involved in postharvest technology in 1967. other scientists not only to contribute to the scientific
Since that time postharvest technology has taken me literature, but also to gain recognition or even pro-
all over the world doing short consultancies and motion. To extract from this literature information that
long-term assignments, of up to three years, meeting is useful to the industry in a concise form is a prohib-
particular challenges in research, training and devel- itive task. There are high losses and variable quality in
opment of the fruit and vegetable industry. Although the fruits and vegetables offered to the consumer. One
much of my time has been spent as an academic and solution to this problem is to provide those concerned
government or United Nations adviser, I have always with the technology of marketing these crops with eas-
worked closely with the horticultural industry. The ily accessible information. This, in part, means infor-
information in this book and the way that it is pre- mation that is brief, easily understood and directly to
sented are therefore largely what is required by the the point. In this book I have tried to achieve this. I
industry. Also, there is increasing pressure for univer- have searched relevant reviews and original research
sities to provide graduates who are more relevant to papers in order to extract relevant data and present it
the needs of industry and most students of posthar- in a form that should be easily accessible to all those
vest technology will eventually work in the industry or working in the industry.
in some way be associated with it; so the book will also The book is an update of one I wrote with Brian
serve their needs. Clarke, which was published by Blackwells in 1996, but
For the produce technologists in Europe and it is more focused on technology. The final chapter is
North America, the range of fruit and vegetables with based on the collected memoirs of Professor C.W.
which they come into contact is constantly increasing. Wardlaw, published in 1938, when he and his col-
One of the reasons for this is that retailers are com- leagues did so much research on the postharvest tech-
peting for customers and therefore they need con- nology of fruit and vegetables and the work of Dr J.M.
stantly to find an edge to attract new customers. Fresh Lutz and Dr R.E. Hardenburg published in the
fruit and vegetables are a major factor in showing that United States Department of Agriculture Bulletin 66.
16. Acknowledgements
To Mr Allen Hilton, Dr Wei Yuqing, Dr Dick Sharples, Dr Nick Smith, Mr Derek Plilchar, Mr Gary Bradbury
Professor Don Tindall, Dr Sulafa Musa, Dr Bob Booth, and Mr Graham Clampin for technical help and
Dr Andy Medlicott, Dr Robin Tillet, Dr James advice.
Ssemwanga, Mr David Bishop, Mr Devon Zagory, Mr. I wish to express my deep appreciation to Dr Chris
Tim Bach, Silsoe Research Institute, FAO Rome, Bishop who proof-read the book due to difficulties of
WIBDECO St. Lucia and Positive Ventilation Limited communication while I was working in a village in the
for use of photographs and other illustrative material. Central Lowlands of Eritrea.
To Dr Graham Seymour, Dr John Stow, Mr John Love,
18. 2 Fruit and vegetables: harvesting, handling and storage
Table 1.2 Summary of the most consistent significant correlations between mineral composition (fruits and leaves) and
storage attributes in a three-year survey (1967, 1968 and 1969) of Cox’s Orange Pippin commercial orchards (source:
Sharples 1980)
Positive correlation years Negative correlation years
Fruit firmness Fruit P (68, 69) –
Gloeosporium rot susceptibility Fruit K/Ca:Mg Ca (67, 68, 69) Fruit Ca (67, 68, 69)
Bitter pit Fruit K/Ca: Mg Ca (67, 68, 69) (67, 68, 69)
Senescent breakdown – Fruit Ca (67, 68, 69)
Core flush Leaf K (67, 69) (August) Leaf N (68, 69) (July)
Low-temperature breakdown Fruitlet Ca P (67, 68, 69) (July)
minimum storage in 2% oxygen and <1% carbon (1980) showed that high rates of application of nitro-
dioxide at 4°C until March (Sharples 1980): gen fertilizer to apple trees could adversely affect the
flavour of the fruit. High nitrogen increased the sus-
• 50–70% nitrogen
ceptibility of Braeburn apple fruit to flesh and core
• 11% minimum phosphorus
browning during storage (Rabus and Streif 2000). In
• 130–160% potassium
fertilizer trials on avocados, Kohne et al. (1992)
• 5% magnesium
showed that the application of nitrogen could reduce
• 5% calcium.
the percentage of ‘clean’ fruit, but where it was com-
The physiological disorder that results in the pro- bined with magnesium and potassium there was no
duction of colourless fruit in strawberries is called effect. Bunches of Italia grapes from vines treated with
‘albinism’. The fruit, which were suffering from this 35% nitrogen as urea and 65% as Ca(NO3)2 through
physiological disorder, were also found to be softer. The fertigation had less water loss and less decay after 56
potassium:calcium and nitrogen:calcium ratios were days of storage at 2–4°C and 90–95% r.h. than
found to be greater in fruit suffering from albinism bunches from treatments that had higher levels of
than in red fruit (Lieten and Marcelle 1993). nitrogen (Choudhury et al. 1999). Alternaria alternata,
Albinism was associated with the cultivar Elsanta and Cladosporium herbarum, Penicillium spp., Rhizopus
some American cultivars and the recommendation for spp. and Aspergillus niger caused storage decay in those
control was either to grow only resistant cultivars or trials.
decrease the application of nitrogen and potassium High nitrogen content in bulbs was associated
fertilizers (Lieten and Marcelle 1993). with short keeping quality of shallots in Thailand
The application of fertilizers to crops has been (Ruaysoongnern and Midmore 1994). Pertot and Perin
shown to influence their postharvest respiration rate. (1999) showed that excessive nitrogen fertilization
This has been reported for a variety of fertilizers on significantly increases the incidence of rot in kiwifruit
several crops including potassium on tomatoes, in cold storage, both in the year of application and in
nitrogen on oranges and organic fertilizers on man- the following year. In contrast, Ystaas (1980) showed
goes. An example of this is that an imbalance of fer- that the application of nitrogen fertilizer to pear trees
tilizers can result in the physiological disorder of did not affect the soluble solids content, firmness,
watermelon called blossom end rot (Cirulli and Cic- ground colour or keeping quality of the fruit. In a field
carese 1981). experiment in the Netherlands there were variable
results to field application of nitrogen fertilizer. How-
Nitrogen ever, during storage at 12°C and 90% r.h., 10 days after
Generally, crops that contain high levels of nitrogen the first harvest, nitrogen had no effect on the yellow-
typically have poorer keeping qualities than those with ing of small Brussels sprouts, but the application of
lower levels. Application of nitrogen fertilizer to 31 kg N hectare–1 as calcium nitrate resulted in
pome fruits and stone fruits has been shown to increase increased yellowing of large sprouts. At the second har-
their susceptibility to physiological disorders and vest, no effect of nitrogen was observed (Everaarts
decrease fruit colour (Shear and Faust 1980). Link 2000).
19. Preharvest factors on postharvest life 3
Phosphorus Calcium
There is little information in the literature on the The physiological disorder of stored apples called
effects of phosphate fertilizers on crop storage. Singh ‘bitterpit’ (see Figure 12.5, in the colour plates) is
et al. (1998) found that the application of 100 kg principally associated with calcium deficiency during
hectare–1 of phosphorus minimized the weight loss, the period of fruit growth and may be detectable at
sprouting and rotting in onions compared with lesser harvest or sometimes only after protracted periods of
applications during 160 days of storage. storage (Atkinson et al. 1980). The incidence and sever-
Phosphorus nutrition can alter the postharvest phys- ity of bitterpit is influenced also by the dynamic bal-
iology of cucumber fruits by affecting membrane lipid ance of minerals in different parts of the fruit as well
chemistry, membrane integrity and respiratory as the storage temperature and levels of oxygen and
metabolism. Cucumbers were grown in a greenhouse carbon dioxide in the store atmosphere (Sharples and
under low and high phosphorus fertilizer regimes by Johnson 1987). Also, low calcium levels in fruit
Knowles et al. (2001). Tissue phosphorus concentra- increased the susceptibility of Braeburn apples to flesh
tion of the low-phosphorus fruits was 45% of that of and core browning (Rabus and Streif 2000).
fruits from high-phosphorus plants. The respiration Dipping certain fruit and vegetables in calcium after
rate of low-phosphorus fruits was 21% higher than that harvest has been shown to have beneficial effects (Wills
of high-phosphorus fruits during 16 days of storage at and Tirmazi 1979, 1981, 1982; Yuen et al. 1993) (see
23°C and the low-phosphorus fruits began the cli- Chapter 6). There is some evidence in the literature
macteric rise about 40 hours after harvest, reached a that preharvest sprays can also be beneficial. The
maximum at 72 hours and declined to preclimacteric treatment of tomatoes with a foliar spray and a
levels by 90 hours. The difference in respiration rate postharvest dip in calcium was the most effective at
between low- and high-phosphorus fruits was as high increasing cell wall calcium contents, which is asso-
as 57% during the climacteric. The respiratory cli- ciated with fruit texture. Niitaka pear fruits from trees
macteric was different to the low-phosphorus fruits that had been supplied with liquid calcium fertilizer
and was not associated with an increase in fruit eth- were firmer after storage than fruit from untreated
ylene concentration or ripening. trees. Fruit weight loss was also reduced following liq-
uid calcium fertilizer treatment, but there was no effect
on soluble solids contents (Moon et al. 2000). Gypsum,
Potassium
applied to sapodilla trees at up to 4 kg per tree once
The application of potassium fertilizer to watermelons every week for the 6 weeks prior to harvest, improved
was shown to decrease the respiration rate of the fruit the appearance of fruit, pulp colour, taste, firmness,
after harvest (Cirulli and Ciccarese 1981). In tomato aroma and texture after storage in ambient conditions
fruits, dry matter and soluble solids content increased in India (Lakshmana and Reddy 1995). High calcium
as the potassium rate increased, but there were no sig- fertilizer levels reduced the acidity of strawberries and
nificant differences in titratable acidity at different played a part in loss of visual fruit quality after har-
potassium rates (Chiesa et al. 1998). Spraying vest (Lacroix and Carmentran 2001).
Shamouti orange trees with 9% Bonus 13-2-44, a
potassium fertilizer from Haifa Chemicals Ltd,
increased leaf potassium concentration in the fruit and Organic production
reduced the incidence of the physiological fruit stor-
age disorder called superficial rind pitting (Tamim et The market for organically produced food is increas-
al. 2000). Hofman and Smith (1993) found that the ing. There is conflicting information on the effects of
application of potassium to citrus trees could affect the organic production of fruit and vegetables on their
shape of their fruits and increase their acidity, postharvest characteristics. Organic production has
although this effect was not observed when potassium been shown to result in crops having higher levels of
was applied to banana plants. High potassium gener- postharvest diseases. Massignan et al. (1999) grew Italia
ally increased acidity in strawberries, but this effect var- grapes both conventionally and organically and after
ied between cultivars (Lacroix and Carmentran storage at 0°C and 90–95% r.h. for 30 days they found
2001). that organic grapes were more prone to storage decay
20. 4 Fruit and vegetables: harvesting, handling and storage
than those grown conventionally. In another case there Rootstock
was evidence that organic production reduced disease
level. In samples from organically cultivated Bintje and For various reasons, fruit trees are grafted on to root-
Ukama potato tubers, the gangrene disease (Phoma stocks and the rootstock can have a profound effect on
foveata) levels were lower compared with convention- the performance of the crop, including its postharvest
ally cultivated ones. However, there was no such dif- life. Considerable work has been done, particularly at
ference in King Edward and Ulama tested 4 months Horticultural Research International at East Malling in
later. The dry rot (Fusarium solani var. coeruleum) lev- the UK, on the use of different rootstocks to control
els were generally lower in organically cultivated tree vigour and cropping. Tomala et al. (1999) found
potatoes compared with tubers from the conventional that the rootstock had a considerable effect on matu-
system (Povolny 1995). ration and storage of Jonagold apples. Fruits from trees
Producing cops organically can have other effects. on the rootstock B146 had the lowest respiration rates
Although harvested on the same day, conventionally and ethylene production after 2 and 4 months of stor-
produced kiwifruits were generally more mature, as age at 0°C but not after 6 months. Fruits from trees on
indicated by soluble solids concentrations, but their P60 and 62-396 started their climacteric rise in respi-
average firmness did not differ significantly from those ration rate 5–7 days earlier than fruits from PB-4.
produced organically. Despite the differences in matu- Fruits were yellower at harvest from trees on P60, 62-
rity, whole fruit softening during storage at 0°C did not 396 and M.26; fruit colour was weak on PB-4 and fruits
differ significantly with production system. However, from these trees coloured most slowly during storage.
organic fruits nearly always developed less soft patches Rootstocks also affect other fruit crops. In some
on the fruit surface than conventionally produced fruits work in South Africa on avocados (Kohne et al. 1992),
(Benge et al. 2000). The effect of organic compost fer- it was shown that the cultivar Fuerte grown on seedling
tilization on the storage of Baba lettuce was evaluated rootstocks showed a large variation in both yield and
by Santos et al. (2001). The organic compost was quality of fruit. There was also some indication that
applied at 0, 22.8, 45.6, 68.4 and 91.2 tonnes per hectare rootstocks, which gave a low yield generally, produced
on a dry matter basis, with and without mineral fertil- a higher proportion of low-quality fruit. Kohne et al.
izer. During storage at 4°C, lettuce grown in increasing (1992) also showed similar results for the avocado
rates of organic compost had reduced levels of fresh cultivar Hass on different clonal rootstocks (Table 1.3).
weight loss by up to 7%. The chlorophyll content Rootstock studies conducted in Australia on Hass avo-
decreased during storage when plants were grown with cado by Willingham et al. (2001) found that the root-
45.6 and 91.2 tonnes per hectare of organic compost stock had a significant impact on postharvest
with mineral fertilizer. The fertilization with organic anthracnose disease susceptibility. Differences in
compost and mineral fertilizer altogether resulted in anthracnose susceptibility were related to significant
plants with early senescence during cold storage. differences in concentrations of antifungal dienes in
In a survey in Japan of fruit quality of Philippine leaves, and mineral nutrients in leaves and fruits, of
bananas from non-chemical production, the problems trees grafted to different rootstocks.
highlighted all related to management practices and Fruits of Ruby Red grapefruit, which had been
none to the effects of organic production on posthar- budded on Citrus amblycarpa or rough lemon (C.
vest aspects (Alvindia et al. 2000). However, in Britain jambhiri) rootstocks, were stored at 4 or 12°C for 6
Nyanjage et al. (2000) found that imported organically
grown Robusta bananas ripened faster at 22–25°C than
Table 1.3 Effect of clonal rootstock on the yield and quality of Hass
non-organically grown bananas as measured by peel avocados (source: Kohne et al. 1992)
colour change, but ripe fruit had similar total soluble
solids levels from both production systems. The peel Rootstock Yield in kg per tree % fruit internally clean
of non-organic fruits had higher nitrogen and lower Thomas 92.7 96
phosphorus contents than organic fruits. Differences Duke 7 62.1 100
in mineral content between the pulp of organic and G 755 12.1 100
non-organic fruits were much lower than those D9 7.4 64
Barr Duke 3.1 70
between the pulp and the peel.
21. Preharvest factors on postharvest life 5
weeks by Reynaldo (1999). Losses due to decay and length is shorter and less variable during the matura-
chilling injury were generally lower in fruits from tion period. In such cases the onion bulbs have very poor
trees budded on rough lemon than on C. amblycarpa storage characteristics (Thompson 1985).
rootstock and there was an indication that rootstocks
affected the metabolic activity of fruits during subse-
quent storage at 4°C. Temperature
The temperature in which a crop is grown can affect
Light its quality and postharvest life. An example of this is
pineapple grown in Australia. Where the night time
Fruits on the parts of trees that are constantly
temperature fell below 21°C, internal browning of the
exposed to the sun may be of different quality and have
fruit could be detected postharvest (Smith and
different postharvest characteristics than those on the
Glennie 1987). The recommended storage temperature
shady side of the tree or those shaded by leaves. Cit-
for Valencia oranges grown in California is 3–9°C with
rus and mango fruits produced in full sun generally
a storage life of up to 8 weeks. The same cultivar grown
had a thinner skin, a lower average weight, a lower juice
in Florida can be successfully stored at 0°C for up to
content, a lower level of acidity but a higher total and
12 weeks. Oranges grown in the tropics tend to have
soluble solids content (Sites and Reitz 1949, 1950a, b).
a higher sugar and total solids content than those
Woolf et al. (2000) showed that during ripening of
grown in the sub-tropics. However, tropical grown
avocados at 20°C, fruit that had been exposed to the
oranges tend to be less orange in colour and peel less
sun showed a delay of 2–5 days in their ethylene peak
easily. These two factors seem to be related more to the
compared with shade fruits. The side of the fruit that
lower diurnal temperature variation that occurs in the
had been exposed to the sun was generally firmer than
tropics rather than to the actual temperature difference
the none exposed side, and the average firmness was
between the tropics and subtropics.
higher than that of shade fruits. After inoculation with
The apple cultivar Cox’s Orange Pippin grown in the
Colletotrichum gloeosporioides the appearance of
UK can suffer from chilling injury when stored below
lesions on sun fruits occurred 2–3 days after shade
3°C whereas those grown in New Zealand can be suc-
fruits.
cessfully stored at 0°C. However, this may be a clonal
There is also some evidence that citrus fruits
effect since there are considerable differences in many
grown in the shade may be less susceptible to chilling
quality factors, including taste and colour, between
injury when subsequent kept in cold storage. Specific
clones of Cox’s Orange Pippin grown in the UK and
disorders such as water core in apples and chilling
those grown in New Zealand (John Love, personal
injury in avocado can also be related to fruit exposure
communication 1994). In Braeburn apples, the grow-
to sunlight (Ferguson et al. 1999).
ing conditions were shown to influence scald, brown-
ing disorder and internal cavities during storage. Hence
Day length following a cool growing season it was recommended
that they should be stored in air at 0°C to avoid the
Day length is related to the number of hours of light in risks of those disorders, but they may be stored in con-
each 24-hour cycle, which varies little near the equator trolled atmospheres after warm seasons because this
but varies between summer and winter in increasing retains texture and acidity better (Lau 1990). Fergu-
amounts further from equator. Certain crop species and son et al. (1999) found that in both apples and avoca-
varieties have evolved or been bred for certain day dos, exposure of fruits to high temperatures on the tree
lengths. If this requirement is not met by using an could influence the response of those fruits to low and
unsuitable variety then the crop may still be immature high postharvest temperatures. Specific disorders
at harvest. An example of this is the onion, where such as water core in apples and chilling injury in
cultivars that have been bred to grow in temperate coun- avocado can also be related to fruit exposure to high
tries, where the day length is long and becomes pro- temperatures, and disorders such as scald in apples may
gressively shorter during the maturation phase, will not be related to the frequency of low-temperature expo-
mature correctly when grown in the tropics, where day sure over the season. Oosthuyse (1998) found that cool,
22. 6 Fruit and vegetables: harvesting, handling and storage
humid or wet conditions on the date of harvest quickly, then maximum yields can be achieved by keep-
strongly favour the postharvest development of lenti- ing the soil at 80–90% of field capacity. However, when
cel damage in mangoes. Conversely, dry, hot conditions they were stored for 7 months at 0–1°C and 75–80%
discouraged the postharvest development of lenticel r.h. the best irrigation regime was 70% of field capac-
damage. ity throughout the growing season.
Water relations Tree age
Generally crops that have higher moisture content have Not much information could be found on the effects
poorer storage characteristics. For example, hybrid of tree age on the postharvest characteristics of fruit,
onion cultivars that tend to give high yield of bulbs with but fruit from young Braeburn apple trees were more
low dry matter content but only a short storage life susceptible to flesh and core browning than those from
(Thompson et al. 1972; Thompson 1985). If bananas are older trees (Rabus and Streif 2000).
allowed to mature fully before harvest and harvesting
occurs shortly after rainfall or irrigation, the fruit can
easily split during handling operations, allowing Flowering time
microorganism infection and postharvest rotting
(Thompson and Burden 1995). If oranges are too turgid In the tropics, the flowering time of fruit trees can
at harvest the oil glands in the skin can be ruptured, affect the postharvest life of fruits. Mayne et al. (1993)
releasing phenolic compounds and causing oleocellosis showed that jelly-seed, a physiological disorder of man-
(Wardlaw 1938). Some growers harvest crops in late goes, is associated with flowering time in Tommy
morning or early afternoon. In the case of leaf vegeta- Atkins. They showed that delaying flowering by
bles such as lettuce they may be too turgid in the early removing all the inflorescences from the tree greatly
morning and the leaves are soft and more susceptible reduced jelly-seed in fruit, which developed from the
to bruising (John Love, personal communication). Also, subsequent flowering. These fruit were larger than
too much rain or irrigation can result in the leaves those produced from trees where the inflorescences had
becoming brittle with the same effect. Irrigating crops not been removed but the number of fruit per tree was
can have other effects on their postharvest life. In car- reduced.
rots, heavy irrigation during the first 90 days after
drilling resulted in up to 20% growth splitting, while
minimal irrigation for the first 120 days followed by Harvest time
heavy irrigation resulted in virtually split-free carrots
with a better skin colour and finish and only a small Late-harvested Braeburn apples were more suscepti-
reduction in yield (McGarry 1993). Shibairo et al. ble to flesh and core browning (Rabus and Streif 2000).
(1998) grew carrots with different irrigation levels and Harvey et al. (1997) found that Cucurbita maxima cul-
found that preharvest water stress lowered membrane tivar Delica harvested at 7 kg force, which occurred
integrity of carrot roots, which may enhance moisture between 240 and 300 growing degree days (base tem-
loss during storage. The effects of water stress, applied perature 8°C) from flowering, required a postharvest
for 45 or 30 days before flowering on Haden mangoes, ripening period to enhance sweetness and texture and
which were stored at 13°C for 21 days after harvest, were to optimize sensory quality that was not necessary for
studied by Vega-Pina et al. (2000). They found that the fruits of later harvests. Ahmed et al. (2001) found very
45-day fruits exhibited a higher incidence and severity strong evidence that for Robusta bananas the fruit had
of internal darkening, were firmer, contained a higher much better organoleptic properties the more mature
content of titratable acidity and had redder skins than they were when harvested. Medlicott et al. (1987a)
30-day fruits. showed that early maturing mangoes tended to have
In a study of the storage of onions grown in Tajik- better quality and postharvest characteristics than
istan by Pirov (2001) under various irrigation those that matured later. See also Chapter 2 for more
regimes, it was found that if onions are to be used fairly details.
23. Preharvest factors on postharvest life 7
Preharvest infection the soil. They may also attack tubers postharvest, and
it is therefore important to protect the stored tubers
Crop hygiene can be important in reducing field infec- to prevent access to them by the moths. Mealy bugs on
tions and infestations that may be carried into storage pineapples occur in the marketing chain from field
or the marketing chain. This usually involves removal infestations (Figure 1.1). Their presence may affect
of rotting material from the field, especially fruit wind- their acceptance on the market or the damage they
falls or tree prunings. It can also involve efficient weed cause may allow infection by microorganisms that can
control of species that might be alternative hosts for cause the fruit to rot.
disease-causing organisms. Aspergillus niger infection in onions occurs during
Frequently, crops are infected with microorganisms production but will only develop on the bulbs during
or infested with invertebrate pests during production. storage where the conditions are conducive. Infection
They may well be on or in the crop at harvest and taken with bacteria such as Erwinia carotovorum can occur
into storage or through the marketing chain. Almost in the field on vegetables, especially where they have
all postharvest pests originate from field infestations, been damaged and cause postharvest soft rots
and if the storage conditions are conducive they can (Thompson et al. 1972).
multiply on or in the crop. Field infestation of yam
tubers with parasitic nematode were shown to
increase when the tubers were stored in tropical ambi- Chemical treatments
ent conditions resulting in areas of necrotic tissues.
However, when the tubers were stored at 13°C there The control of pests and diseases is commonly
was no increase in nematode population in the tubers achieved by spraying chemicals directly on to the crop,
and no increase in necrosis (Thompson et al. 1973a). although this is becoming less prevalent with increas-
The potato tuber moth (Phthorimaea operculella) may ing use of techniques such as integrated pest manage-
infest tubers during growth if they are exposed above ment and integrated crop management. The control
of field infection can have considerable effect on the
postharvest life of the crop. An example of this is
anthracnose disease that is caused by field infection by
the fungus Colletotrichium gloeosporioides [Glom-
erella cingulata], which if not controlled it can cause
rapid postharvest losses (Thompson 1987). The fruits
look perfectly healthy at harvest and the disease symp-
toms develop postharvest. The time between infection
and the symptoms of the disease developing may be
lengthy, e.g. anthracnose (Colletotrichium musae) in
bananas can take over five months (Simmonds 1941).
Generally if a crop has suffered an infection during
development its storage or marketable life may be
adversely affected. Bananas may ripen prematurely or
abnormally after harvest because of leaf infections by
fungi during growth, which cause stress and therefore
shorten their storage life. This can be manifest on the
crop before harvesting or it may only be observed as
a ‘physiological disorder’ postharvest. Fungicide
applications in the field to control Sigatoka leaf spot
(Micosphaerella musicola) were shown to reduce pre-
mature ripening (Thompson and Burden 1995).
Chemicals may also be applied to certain crops in
the field to prevent them sprouting during storage and
Figure 1.1 Pineapple infested with mealy bug in a field in Sri Lanka. thus to extend their storage period. An example of this
24. 8 Fruit and vegetables: harvesting, handling and storage
is the application of maleic hydrazide to onions. fruits were slower to mature since daminozide tended
Because it is necessary for the chemical to be trans- to retard the climacteric rise in respiration. In a com-
located to the apex of the growing point towards the parison between preharvest and postharvest application
centre of the bulb, it has to be applied to the leaves of of daminozide to Cox’s Orange Pippin apples, immer-
the growing crop. sion of fruits in a solution containing 4.25 g litre–1 for
Growth-regulating chemicals have been applied to 5 minutes delayed the rise in ethylene production at
trees to increase fruit quality and yield. One such chem- 15°C by about 2 days, whereas orchard application of
ical, which has been the subject of considerable debate 0.85 g litre–1 caused delays of about 3 days (Knee and
in the news media, is daminozide (N-dimethy- Looney 1990). Both modes of application depressed the
laminosuccinamic acid), also called Alar, B9 or B995. maximal rate of ethylene production attained by ripe
When applied to Cox’s Orange Pippin apples at 2500 μg apples by about 30%. Daminozide-treated fruit were also
litre–1 in late June and mid August, they developed more shown to be less sensitive to ethylene in the storage
red colour in the skin and were firmer than unsprayed atmosphere than untreated fruit, but this response var-
fruits (Sharples 1967). Sprayed fruit were less suscepti- ied between cultivars (Knee and Looney 1990).
ble to Gloeosporium rots but had more core flush dur- Daminozole has been withdrawn from the market in
ing storage. There was some indication that sprayed several countries (John Love, personal communication).
26. 10 Fruit and vegetables: harvesting, handling and storage
Table 2.1 Effects of harvest maturity, as measured by fruit diameter,
on weight, price and income from the fruit where 100 is a compara-
tive base (source: Blumenfeld 1993)
Fruit diameter (mm)
60 65 70
Weight 100 120 140
Price 100 115 130
Income 100 138 182
it was shown that the size of the fruit at harvest could
have a major effect on its profitability during market-
ing (Table 2.1). However, the longer the fruits were left
to mature on the tree, the higher were the postharvest
Figure 2.1 An on-line colour sorting machine being used on potatoes. losses, but even if 70 mm diameter fruit were harvested
and had postharvest losses, it may still be economic
the shoulders slope away from the fruit stalk, on more (Blumenfeld 1993a). In longan fruit, size and weight
mature fruit the shoulders become level with the point were consistently shown to have a high correlation with
on attachment and on even more mature fruit the eating quality (Onnap et al. 1993).
shoulders may be raised above the point of attachment Several devices have been developed to aid size grad-
(see Figure 12.74). Using this method of determining ing, including hand-held templates (Figure 2.2) and
mango fruit maturity, Thompson (1971) showed that large-size grading machines used in packhouses.
the percentage of fruit still unripe after storage at 7°C
for 28 days was 68% for fruit with sloping shoulders, Aroma
57% for fruit with level shoulders and 41% for fruit Most fruits synthesize volatile chemicals as they
with raised shoulders. ripen. These may give the fruit its characteristic odour
and can be used to determine whether a fruit is ripe
Size
The changes in size of a crop as it is growing are fre-
quently used to determine when it should be harvested.
In fruits this may simply be related to the market
requirement and the fruit may not be physiologically
mature, e.g. example in capsicums and aubergine. Par-
tially mature cobs of Zea mays saccharata are marketed
as sweetcorn while even more immature, and thus
smaller, cobs are marketed as babycorn. In some crops
fibres develop as they mature and it is important that
they are harvested before this occurs. In crops such as
green beans, okra and asparagus this relationship may
be related to its size. In bananas the width of individ-
ual fingers can be used for determining their harvest
maturity. Usually a predetermined finger from the
bunch is used and its maximum width is measured
with callipers, hence it is referred to as the calliper
grade. The length of the same finger may also be mea-
sured for the same purpose. Both of these measure-
ments are often used as quality criteria during
marketing of fruit. Fruit size can also be used for deter-
mining the harvest maturity of litchi. In South Africa Figure 2.2 Templates used for size grading limes in Colombia.
27. Assessment of crop maturity 11
or not. These odours may only be detectable to human Leaf changes
senses when a fruit is completely ripe and therefore This is a characteristic that is used in both fruit and
have limited use in commercial situations. This vegetables to determine when they should be har-
applies to several types of fruit, but in practice they are vested. In many root crops the condition of the leaves
used in association with other changes. Equipment can indicate the condition of the crop below ground.
fitted with aroma sensors has been developed for If potatoes are to be stored then the optimum harvest
postharvest measurement of fruit ripeness. time is after the leaves and stems have died down. If
they are harvested earlier the skins are less resistant to
harvesting and handling damage and are more prone
Computation (Ribbon tagging)
to storage diseases. Bulb onions that are to be stored
The time between flowering and fruit being ready for should be allowed to mature fully before harvest, which
harvesting may be fairly constant. For many fruit crops is judged to when the leaves bend just above the top
grown in temperate climates, such as apples, the annual of the bulb and fall over. When the leaf dies in whose
optimum harvest date may vary little from year to axis a fruit is borne in melons, then that fruit is judged
year, even though the weather conditions may differ to be ready for harvesting.
considerably. In tropical fruit, flowering may occur at
various times of the year, but the time between
flowering and maturity may vary very little. With most Abscission
fruit it is difficult to utilize this consistency in practice. As part of the natural development of fruit, an abscis-
In mangoes, for example, if flowers or young fruit are sion layer is formed in the pedicel. This can be judged
marked or tagged so as to identify their flowering or by gently pulling the fruit. However, fruit harvested at
fruit-set time, they almost invariably shed that fruit this maturity will be well advanced and have only a
before it is fully developed. In bananas it is different. short marketable life.
At anthesis a plastic cover is placed over the bunch to
protect the fruit as it is developing. In order to identify
exactly when anthesis occurred, a coloured plastic Firmness
ribbon is attached to the bunch (see Figure 12.18). The Fruit may change in texture during maturation and
same colour is used for one week and changed to especially during ripening, when they may rapidly
another colour the following week and so on. This become softer. Excessive loss of moisture may also
means at the harvest time the age of is bunch affect the texture of crops. These textural changes may
is precisely known. Jayatilake et al. (1993) showed be detected by touch, and the harvester may simply be
that the Ambul variety of banana grown in Sri able to squeeze the fruit gently and judge whether to
Lanka reached physiological maturity 8–9 weeks harvest it. A non-destructive firmness test was inves-
after the flowers had opened. Fruit growth and tigated at Cranfield University, which simulated the
development continued until the thirteenth week practice of customers who may test a fruit’s ripeness
but changes in other physical and chemical parameters by feeling it. A narrow metal cylindrical probe was
were minimal after 11 weeks. In Ecuador the pressed on to the skin of the fruit (approximately 1
maximum time from anthesis to harvest is usually newton was sufficient) and the amount of the depres-
12 weeks and in the Windward Islands it is 13 sion of the skin was measured very accurately on an
weeks. Instron Universal Tester (Curd 1988; Allsop 1991). This
In apples the time of petal fall may be recorded. This was found to correlate well with maturation and ripen-
gives an approximate guide to when fruit should be ing of the fruit and also caused no detectable damage.
harvested. Harvest maturity for rambutans may be Similar studies had previously been carried out by
judged on the time after full flowering. In Thailand this Mehlschau et al. (1981), who used steel balls, one each
is 90–120 days, in Indonesia 90–100 days and in on opposite sides of the fruit, to apply a fixed force.
Malaysia 100–130 days (Kosiyachinda 1968). In They then measured the deformation that was caused
New Zealand, optimum harvest maturity of kiwifruit to the surface of the fruit. Perry (1977) described a
is some 23 weeks after flowering (Pratt and Reid device which applied low pressure air to opposite sides
1974). of fruit and then measured the surface deformation.
28. 12 Fruit and vegetables: harvesting, handling and storage
Firmness, or what is usually called ‘solidity,’ can be
used for assessing harvest maturity in many leafy veg-
etables. The harvester who slightly presses vegetables
such as cabbages and hearting lettuce with the thumb
and fingers can do this by hand. Harvest maturity is
assessed on the basis of how much the vegetable yields
to this pressure. Normally the back of the hand is used
for testing the firmness of lettuce in order to avoid
damage (John Love, ,personal communication).
Postharvest methods
Firmness
In some cases, a representative sample of fruit may be
taken from the orchard and tested in a device which Figure 2.3 Testing the firmness of a banana with a pressure tester.
Source: Mr A.J. Hilton.
will give a numerical value for texture; when that value
reaches a predetermined critical level, then all the fruit
in that orchard are harvested. These so-called ‘pressure 1991). Pressure testers used for fruits and tenderom-
testers’ were first developed for apples (Magness and eters are destructive tests which assume the sample
Taylor 1925) and are currently available in various taken is representative of the crop.
forms (see Figure 12.85). Hand-held pressure testers
could give variable results because the basis on which Juice
they are used to measure the firmness of the crop is The juice content of many fruit increases as they
affected by the angle at which the force is applied. An mature on the tree. By taking representative samples
experienced operator may be able to achieve consis- of the fruit, extracting the juice in a standard and spec-
tent and reliable results, but greater reproducibility can ified way and then relating the juice volume to the orig-
be achieved if the gauge for measuring firmness is held inal mass of the fruit, it is possible to specify its
in a stand so that the angle of force applied to the crop maturity. In some countries legislation exists which
is always constant. The speed with which the probe specifies the minimum juice content before fruit can
presses against the fruit can also affect the measure- be harvested (Table 2.2).
ment of firmness, so instruments have been developed
which can control it (Figure 2.3). The performance of Oil
a firmness penetrometer developed by DeLong et al.
This is probably only applicable to avocados, where the
(2000) was evaluated over two growing seasons with
oil level increases as the fruit matures on the tree. Also,
post-storage apples against the Effegi, Magness–Tay-
it is only applicable to those grown in the subtropics.
lor and electronic pressure tester. Highly significant
This is because it is based on a sampling technique
instrument–operator interactions indicated that the
influence of operators on instrument performance was
Table 2.2 The minimum juice content levels for citrus fruits harvested
not consistent, but overall the newly developed pen-
in the USA
etrometer performed as well as or better than the other
instruments tested. In a comparison between a Type of citrus fruit Minimum juice content (%)
penetrometer (puncture) test and a flat plate com- Navel oranges 30
pression test, Sirisomboon et al. (2000) found that the Other oranges 35
penetrometer was superior for analysing the texture of Grapefruit 35
Japanese pears. Lemons 25
In crops such as peas, a shear cell is used to mea- Mandarins 33
Clementines 40
sure texture and is called a ‘tenderometer’ (Knight
29. Assessment of crop maturity 13
where it is assumed that the sample of fruit on which practice in England, samples would be taken from pears
the oil analysis has been taken is representative of the from mid August, when the whole fruit surface should
whole field. In the subtropics there are distinct seasons contain starch and harvesting should be carried out
and flowering of avocados occurs after a cold season when samples show about 65–70% of the cut surface
and the trees tend to flower and thus set fruit over a which has turned blue–black (Cockburn and Sharples
short period of time. Trees of the same variety in one 1979). Studies using this technique on apples gave
orchard will have fruit that therefore mature at about inconsistent results in England, but it was very effective
the same time and so a representative sample can be on several cultivars in Turkey.
taken. In the tropics the flowering period, even on the
same tree, is over a much more protracted period and Acidity
so there is a wide range of fruit maturities. It is rarely
The acidity of many types of fruit changes during
possible, therefore, to obtain a representative sample.
maturation and ripening. In many fruit acidity
progressively reduces as the fruit matures on the tree.
Sugars Taking samples of these fruit, extracting the juice and
titrating it against a standard alkaline solution gives a
In climacteric fruit, carbohydrates are accumulated dur-
measure that can be related to optimum time of
ing maturation in the form of starch. As the fruit ripens
harvest. It is important to measure acidity by titration
starch is broken down to sugars. In non-climacteric
and not by measuring the pH of the fruit because of
fruits it is sugars not starch that are accumulated dur-
the considerable buffering capacity in fruit juices.
ing maturation. In both cases it follows that measure-
Normally acidity is not taken as a measurement of
ment of sugars in the fruit can provide an indication
fruit maturity by itself. It is usually related to soluble
of the stage of ripeness or maturity of that fruit. In prac-
solids, giving what is termed the oBrix:acid ratio.
tice the soluble solids, also called oBrix, is measured in
the juice of samples of fruit because it is much easier
to measure. Usually sugars are the soluble solids that are Specific gravity
in the largest quantity in fruit, so measuring the solu- The specific gravity of solids or liquids is the relative
ble material in samples of the juice can give a reliable gravity or weight compared with pure distilled water
measure of its sugar content. This is done either with at 62°F (16.7°C), which is reckoned to be unity. By
a suitable Brix hydrometer or in a refractometer (see comparing the weights of equal bulks of other bodies
Figure 12.85). This factor is used in certain parts of the with the weight of water their specific gravity is
world to specify maturity of, for example, kiwifruit, hon- obtained. In practice, the fruit or vegetable is weighed
eydew melons, peaches and longan. in air and then in pure water and its weight in air is
divided by the loss in weight in water, thus giving its
specific gravity. As fruit mature their specific gravity
Starch
increases. This parameter is rarely used in practice to
In apple and pears, carbohydrates are accumulated dur- determine when to harvest a crop, but it could be
ing maturation in the form of starch. The measurement where it is possible to develop a suitable sampling
of starch content in the developing fruit can provide a technique. It is used, however, to grade crops into
reliable method for assessing its harvest maturity, but different maturities postharvest. To do this the fruit or
it does not work for all cultivars. The method involves vegetable is placed in a tank of water and those which
taking a representative sample of fruit from the float will be less mature than those which sink. To give
orchard as the harvest approaches. These fruit are cut greater flexibility to the test and make it more precise,
into two and the cut surface dipped in a solution con- a salt or sugar solution can be used in place of water
taining 4% potassium iodide and 1% iodine. The cut in the tank. This changes the density of the liquid,
surface will be stained a blue–black colour in the places resulting in fruits or vegetables that would have sunk
where starch is present. It is possible, often with the use in water floating in the salt or sugar solution. Lizada
of Perspex templates marked with concentric rings, to (1993) showed that a 1% sodium chloride solution
determine the percentage starch (see Figure 12.2). Starch was suitable for grading Carabao mangoes in the
is converted to sugar as harvest time is approached. In Philippines.
