Caballero B. (ed.) Encyclopaedia of Food Science, Food Technology and Nutrition. Ten-Volume Set

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insulin. Both hormones are produced by the pancre-

atic islets of Langerhans which respond to the fluctu-

ations in circulating nutrient concentrations by

adjusting the secretion of glucagon (from the pancre-

atic A cells) or insulin (from the pancreatic B cells)

into the circulation. The plasma ratio of these two

hormones plays a central role in the regulation of

glucose metabolism by the liver and ketogenesis

(Figure 4). In the fed state, the ratio of insulin to

glucagon is high and glucose is stored as glycogen

and oxidized via glycolysis and the tricarboxylic

acid cycle, producing a range of biosynthetic inter-

mediates and adenosine triphosphate. The presence

of the biosynthetic intermediates, in particular mal-

onyl CoA (used for fatty acid biosynthesis), inhibits

the transport of fatty acids into the mitochondria,

thereby preventing fatty acid oxidation. As such,

ketogenesis is prevented when there is a plentiful

supply of plasma glucose for oxidation by the tissues

(Figure 4).

0005During periods of food deprivation the ratio of

circulating insulin to glucagon falls, causing acti-

vation of glycogenolysis, a reduction in the activity

of glycolytic enzymes, and reduced activities of

acetyl CoA carboxylase. The decreasing concentra-

tion of malonyl CoA results in activation of the

Acetone

COOH

Acetoacetate

COOH

D-3-Hydroxybutyrate

fig0001 Figure 1 Chemical structures of the ketone bodies. Repro-

duced from Ketone Bodies, Encyclopaedia of Food Science, Food

Technology and Nutrition, Macrae R, Robinson RK and Sadler MJ

(eds), 1993, Academic Press.

Acetyl-CoA

CoA-SH

Acetoacetyl-coA

thiolase

Acetoacetyl-CoA

3-Hydroxy-3-methylglutaryl-

CoA synthase

3-Hydroxy-3-methylglutaryl-CoA

3-Hydroxy-3-methylglutaryl-

CoA lyase

Acetyl-CoA

Acetoacetate

Spontaneous

Acetone

NADH

NAD

3-Hydroxybutyrate dehydrogenase

3-Hydroxybutyrate

fig0002 Figure 2 Major pathway for the formation of ketone bodies.

NAD+/NADH, Nicotinamide dinucleotide (oxidized and reduced

respectively); SH, Sulphydryl Group. Reproduced from Ketone

Bodies, Encyclopaedia of Food Science, Food Technology and Nutri-

tion, Macrae R, Robinson RK and Sadler MJ (eds), 1993, Aca-

demic Press.

CHOHCH

COOH

D-3-Hydroxybutyrate

COCH

COOH

Acetoacetate

COCH

CO-S-CoA

Acetoacetyl-CoA

D-3-Hydroxybutyrate

dehydrogenase

NAD

NADH

3-Oxoacid-CoA transferase

CoA-SH

Acetyl-CoA

Acetoacetyl-CoA thiolase

Acetyl CoA

Oxidation via the

tricarboxylic acid

cycle to produce ATP

COOH

Succinate

COOH

CO-S-CoA

Succinyl-CoA

fig0003Figure 3 Major pathways of ketone body utilization by extra-

hepatic tissues. CoA, coenzyme A; ATP, adenosine triphosphate.

NAD+/NADH, Nicotinamide dinucleotide (oxidized and reduced

respectively); SH, Sulphydryl Group. Reproduced from Ketone

Bodies, Encyclopaedia of Food Science, Food Technology and Nutri-

tion, Macrae R, Robinson RK and Sadler MJ (eds), 1993, Aca-

demic Press.

3422 KETONE BODIES

mitochondrial transport system, allowing free fatty

acids to be transported into the mitochondria for

oxidation. As the concentration of acetyl CoA in the

mitochondria of the liver rises, ketogenesis is stimu-

lated. In the adipose tissue, glucagon stimulates ade-

nylate cyclase which in turn activates lipases that

convert the stored triglyceride into free fatty acids.

The increased delivery of these fatty acids to the liver

results in increased ketogenesis, the rate of which is

primarily governed by the delivery of the fatty acids

(Figure 4).

0006 When a fast is terminated, the increased concen-

trations of nutrients in the plasma cause insulin

release from the endocrine pancreas. The rising

insulin-to-glucagon ratio causes cessation of adipose

tissue lipolysis, and plasma free fatty acid concentra-

tions rapidly fall. Insulin also acts on the tissues,

causing a reversion to glucose utilization. In the

liver, glucose output is reduced, glycogen stores are

repleted, lipogenesis is reinstated, and ketogenesis

inhibited.

Physiological Ketosis

0007A normal balanced diet provides sufficient nutrient

content to satisfy the glucose requirements of the

brain and other tissues. Under these conditions keto-

genesis by the liver is negligible, with plasma ketone

Glycogen

Glycogen synthesis

Glucose

Glucagon

Insulin

Glycolysis

Acetyl CoA

−

Acetyl-CoA

Carboxylase

Malonyl CoA

inhibits

Lipogenesis

Triacylglycerol in

adipose tissue

Free fatty acid

transported to the

liver

Hormone-sensitive lipase

Insulin

Glucagon

Exported from liver

Lipoproteins

Esterification

Acylglycerols

Acyl CoA

Carnitine

acyltransferase

Inner mitochondrial

membrane

Pyruvate

Acyl CoA

β-Oxidation

Acetyl CoA

Ketone bodies

Ketogenesis

Tricarboxylic acid

cycle

During prolonged fast, starvation or

diabetes mellitus (low insulin:glucagon ratio)

During adequate food supply

(high insulin:glucagon ratio)

Effect on enzyme activity

increase, +, decrease, −

fig0004 Figure 4 Regulation of ketogenesis in the liver. CoA, coenzyme A. Reproduced from Ketone Bodies, Encyclopaedia of Food Science,

Food Technology and Nutrition, Macrae R, Robinson RK and Sadler MJ (eds), 1993, Academic Press.

KETONE BODIES 3423

body concentrations generally less than 0.5 mmol l

1

However, the ability of an individual to survive

periods of food deprivation normally relies on a com-

plex hormonal and biochemical interaction that

insures the appropriate interorgan redirection and

utilization of major fuels.

0008 In the early stages of a fast there is a shift in liver

metabolism from glucose storage to glucose produc-

tion, coupled with other metabolic alterations that

conserve the glucose released for the maintenance of

brain and central nervous system function. Following

an overnight fast, most of the glucose produced by the

liver is utilized by the tissues with an obligatory re-

quirement for glucose as the major fuel. At this stage,

tissues such as skeletal muscle are primarily using

nonesterified fatty acids (NEFA) as their major fuel

supply. Muscle NEFA oxidation causes a decrease in

glucose uptake and subsequent glucose metabolism

by muscle cells and glucose transport across the

muscle cell membrane is decreased.

0009 Reduction in the plasma glucose concentration as

a result of a prolonged fast or food deprivation will

result in the production of glucose from noncarbohy-

drate precursors. Fatty acids are mobilized from the

adipose tissue as the fast takes place and gluconeo-

genic and ketogenic pathways are activated in the

liver. The plasma glucose concentration may fall sig-

nificantly with the concomitant increase in plasma

fatty acids and ketone bodies to about five fold and

20-fold, respectively. As more than 99% of plasma

fatty acid is complexed with albumin, very little is

free to diffuse into the interstitial spaces, limiting

their utilization by tissues. However, ketone bodies

are soluble in aqueous medium and the concentration

in the interstitial fluid will be comparable with that in

the plasma (2–3 mmol l

1

) during prolonged fasting.

As the concentration of glucose in the interstitial fluid

under these conditions may be around 4–5 mmol l

1

the ketone bodies are oxidized in preference to glu-

cose and free fatty acids and sparing glucose for use

by the brain and central nervous system.

0010 The pancreatic B cells remain responsive to the

plasma nutrient concentration and facilitate the

maintenance of plasma fatty acid concentrations

such that the plasma ketone body concentration

does not usually rise to such an extent that ketone

bodies are excreted in the urine.

Pathological Ketosis

0011 IDDM is probably the best-known disease associated

with life-threatening ketosis if not treated by insulin

therapy. IDDM is associated with autoimmune

destruction of the pancreatic islets of Langerhans.

This results in very low or undetectable levels of

circulating insulin in the plasma and is often also

associated with increased plasma concentrations of

glucagon. As described previously, the alteration of

the insulin-to-glucagon ratio has significant effects

on the metabolism of the liver and other tissues. The

increased relative or absolute levels of glucagon in-

crease the rate of lipolysis in the adipose tissue,

resulting in increased circulating levels of free fatty

acids. The liver responds to this alteration in the insu-

lin-to-glucagon ratio by increasing glucose synthesis

and release. As insulin-stimulated glucose uptake by

extrahepatic tissues is decreased, the circulating glu-

cose concentration increases and hyperglycemia

occurs. Lipid and glycogen synthesis are inhibited

and there is increased oxidation of the free fatty acids

being delivered to the liver. In physiological ketosis

there is a regulated release of fatty acids from the

adipose tissue allowing tight regulation of the plasma

ketone body concentrations. However, in IDDM this

regulation is nonfunctional due to the autoimmune

destruction of the pancreatic B cells. As a result, hyper-

lipidemia and hyperketonemia occur and, as the con-

centration of ketone bodies continues to increase

above a threshold level of around 7 mmol l

1

, the

kidneys begin to excrete the excess ketone bodies

into the urine.

Metabolic Acidosis

0012Metabolic acidosis is caused by a decrease in the

concentration of bicarbonate ion in the plasma with

little or no change in the carbonic acid concentration.

The condition is frequently associated with uncon-

trolled IDDM where it is caused by the presence of

high plasma concentrations of ketone bodies. Acet-

oacetate and 3-hydroxybutyrate are relatively strong

acids and dissociate in the plasma, giving rise to H

and the corresponding anion. The H

ions are effect-

ively buffered by plasma bicarbonate, giving rise to

, which breaks down to yield CO

and H

The increase in plasma CO

concentration stimulates

the respiratory center, producing rapid and deep

breathing, thereby excreting the excess CO

by way

of the lungs. This is an attempt by the body to rees-

tablish the normal CO

tension in the blood. As the

kidney is the major organ responsible for the main-

tenance of the acid–base balance, it is also employed

to help correct the imbalance caused by the presence

of the ketone bodies. The anionic components of the

dissociated ketone bodies are filtered in the kidney,

each with a cation (principally a sodium ion), thus

maintaining electrical neutrality. The renal tubular

cells secrete H

into the filtrate while reabsorbing

one sodium ion and one bicarbonate ion for each

secreted. The energy-dependent H

carriers in

3424 KETONE BODIES

the tubular cells are capable of secreting H

against a

concentration gradient until the urine becomes ap-

proximately 800 times more acidic than the plasma

(approximately pH 4.5). At this point the concentra-

tion gradient becomes too great for the secretory

process to continue. For further H

secretion to pro-

ceed, the majority of the secreted H

must be buf-

fered in the tubular fluid. This is achieved in the first

instance by phosphate. However, phosphate is pre-

sent in the urine as a result of dietary excess and the

buffering capacity of urinary phosphates is quickly

exceeded in severe acidosis. Under these circum-

stances the tubular cells secrete ammonia (NH

which combines with the urinary H

, giving rise to

. In this way larger amounts of H

can be ex-

creted into the urine before the concentration gradi-

ent becomes too large for the secretory mechanism to

operate. However, when the acid load is very large, as

in diabetic acidosis, this occurs and the filtered

cations are lost with the ketone bodies. The concomi-

tant electrolyte and water loses lead to dehydration,

hypovolemia, hypotension, and subsequent death, if

not treated.

Diabetes and Drug Metabolism

0013 Numerous studies have now demonstrated that

IDDM in animals and humans is associated with

the alteration of drug metabolism by the liver.

The cytochrome P450-dependent mixed-function

oxidase system is the most important enzyme system

involved in the metabolism of plethora of drugs

and chemicals. IDDM is associated with alterations

in the levels and activities of some of these

cytochrome P450 enzymes and ketone bodies have

been shown, at least in part, for the alterations

observed. Ketone bodies appear selectively to induce

some of the cytochrome P450 enzymes, and this

has been shown to result in alteration of metabolism

of some drugs in humans. Whether these alterations

in clinically controlled IDDM individuals over

prolonged periods have any significant therapeutic

consequences has yet to be determined.

See also: Diabetes Mellitus: Etiology; Liver: Structure

and Function

Further Reading

Ioannides C, Barnett CR and Flatt PR (1999) The effect

of experimental diabetes on the cytochrome P450

system and other metabolic pathways. In: McNeill JH

(ed.) Experimental Models of Diabetes, pp. 81–117.

Boca Raton: CRC Press.

Sherwood L (1993) Human Physiology: From Cells to

Systems, 2nd edn. New York: West.

Kidney See Renal Function and Disorders: Kidney: Structure and Function; Nutritional Management of

Renal Disorders

KIWIFRUIT

A R Ferguson and R Stanley, HortResearch,

Auckland, New Zealand

Introduction

0001 Kiwifruit are enjoyable to eat. They are also a good

source of minerals, of dietary fiber, and they contain a

most effective laxative. Their outstanding nutritional

quality is their very high content of vitamin C. There

is very little, if any, loss of nutritional quality during

long-term storage or ripening even if the overall

appeal and quality of the fruit deteriorates. In

particular, kiwifruit that are still fit to eat are likely

to be an excellent source of vitamin C. Their main

disadvantage is that they can cause allergic or inflam-

matory responses in some consumers. Kiwifruit ac-

count for perhaps only 1% of the total world

production of fresh fruit. Nevertheless they are an

important fruit crop. They have rapidly become

popular amongst consumers, they are novel and new

cultivars are now coming on the market, they

are available in a range of colors and flavors,

and they can mostly be stored for remarkably long

periods under refrigerated and controlled-atmosphere

(CA) conditions. (See Allergens; Antioxidants: Nat-

ural Antioxidants; Ascorbic Acid: Properties and

KIWIFRUIT 3425

Determination; Controlled-atmosphere Storage:

Effects on Fruit and Vegetables; Dietary Fiber: Prop-

erties and Sources; Food Intolerance: Food Allergies;

Fruits of Temperate Climates: Commercial and Diet-

ary Importance.)

What are Kiwifruit?

0002 Kiwifruit are amongst the most recently domesti-

cated of all fruits. Although the term ‘kiwifruit’ has

traditionally been restricted to cultivars of

Actinidia deliciosa (A. Chev.) C.F. Liang et A.R.

Ferguson, the name is now being extended to

include cultivars of the closely related species,

A. chinensis Planch. Fruit of this latter species

have recently appeared on the international

market: they are still recognizably kiwifruit, even

though they are much less hairy, the fruit shape

can be different, the skin may be a different color,

and the internal flesh can be yellow instead

of green. Fruit of other Actinidia species, such as

A. arguta (Sieb. et Zucc.) Planch. ex Miq. are also

being commercialized. Although they are very dif-

ferent in appearance to the usual kiwifruit, being

much smaller and with smooth green skins, it is

likely that they too will eventually be commonly

referred to as kiwifruit. We have therefore included

any species within the genus Actinidia under the

term ‘kiwifruit.’

0003 The genus is widespread throughout Asia, with its

center of origin in China. Plants of Actinidia species

were first grown outside Asia during the last decades

of the nineteenth century. Commercial orchards of

A. deliciosa were first established in New Zealand

about 1930 and the very first commercial plantings

of A. chinensis outside China have only been estab-

lished during the past five years.

World Production

0004 The principal areas of kiwifruit production are

located approximately between the latitudes 25



and 45



. Kiwifruit are deciduous, temperate plants

and they require a period of winter chilling for

adequate budbreak and flowering. They are suscep-

tible to damage from spring and autumn frosts

and, depending on the species and cultivar, require a

long frost-free growing period of at least 7 or 8

months (about 220 days). They require shelter from

strong winds and have a high demand for water:

irrigation must be sufficient to supplement natural

rainfall to between 800 and 1200 mm of water

throughout the growing season.

0005 There are extensive natural resources of kiwifruit

in China and appreciable quantities of fruit (perhaps

100 000 tonnes) are collected from the wild

each year. China also has the greatest commercial

plantings of kiwifruit – about 50 000 ha. Outside

China, there are about 70 000 ha planted – approxi-

mately 20 000 ha in Italy, 12 000 ha in New Zea-

land and 8000 ha in Chile. Many of these plantings,

particularly in China, are still not mature and

yields are therefore increasing. Production in the

various countries also varies with climatic conditions

each year. In 2001, Italy was producing around

350 000 t of kiwifruit, New Zealand 250 000 t,

and China and Chile each about 150 000 t per

annum.

0006The kiwifruit industries of New Zealand,

Chile, and Italy are dependent on exports and

these industries expanded largely to meet external

demand. Thus, 85–90% of the kiwifruit produced

in New Zealand are exported each year and Chile

and Italy each export about 75% of the fruit

they produce. China is quite different and nearly all

the kiwifruit it produces are consumed within the

country: exports are negligible. However, it is

predicted that Chinese exports of kiwifruit may

soon be significant in world trade. (See Fruits of

Temperate Climates: Commercial and Dietary

Importance.)

Kiwifruit Cultivars

0007In most countries with commercial kiwifruit plant-

ings, only A. deliciosa has been grown, and ‘Hay-

ward’ is the only fruiting cultivar planted along

with its corresponding males. (All Actinidia species

are dioecious, with both male and female vines

required for fruit production.) ‘Hayward’ has

therefore become the standard cultivar of inter-

national commerce in kiwifruit, accounting for

more than 95% of the kiwifruit traded. This reliance

on a single cultivar in most parts of the world

is most unusual. Such reliance has advantages in

that it facilitates standardization amongst suppliers

from different countries. There are, however, also

disadvantages in that standardization can lead to

greater competition in individual markets and

makes branding more difficult. ‘Hayward’ is such

a good cultivar, and responds so well to storage,

that it is likely to remain important for many

years yet.

0008Kiwifruit plantings in China are much more diverse

– approximately 75% in cultivars of A. deliciosa and

25% in A. chinensis. The cultivar ‘Qinmei’ (A. deli-

ciosa) accounts for about 40% of the total plantings

and is therefore the second most widely planted

kiwifruit cultivar in the world after ‘Hayward’. Six

cultivars of A. deliciosa and seven of A. chinensis

3426 KIWIFRUIT

together make up about 85% of all Chinese kiwifruit

plantings.

0009 One cultivar of A. chinensis, ‘Hort16A’, bred in

New Zealand from germplasm introduced from

China, is now being planted in New Zealand

(about 2000 ha in 2001) and in California, Italy,

and Japan. It is the first cultivar of A. chinensis to

have been grown commercially outside China but

it is likely that other cultivars, selected in China

itself, will soon be grown commercially in other

countries.

The Structure of Kiwifruit

0010 The kiwifruit is a berry: that is, it has a large number

of seeds embedded in fleshy, edible tissue. The fruit

develops after pollination from the lower parts of

numerous carpels (usually more than 30) which

have fused to form a syncarpous, superior ovary.

Each carpel has two rows of 10–20 ovules attached

to the central axis. If pollination is adequate, a fruit

can thus contain 1000 or more seeds.

0011 Fruit of large-fruited selections of A. chinensis and

A. deliciosa typically weigh on average about 100 g

but fruit of many of the other species are much

smaller, e.g., those of A. arguta average between 10

and 20 g. The exterior surface of the fruit differs

according to the species and the cultivar. Fruit of

A. deliciosa (e.g., ‘Hayward’) are brown and

densely covered with persistent, long, stiff hairs, al-

though these are largely lost during commercial

grading and packing. Fruit of A. chinensis (e.g.,

‘Hort16A’) are light-brown or greenish and are

covered with sparse, much softer hairs, rather like

the down of a peach. Fruit of some species, e.g.,

A. arguta, lack obvious hairs and their edible skins

are smooth and polished.

0012 Internally, the fruit are very attractive with dark

seed in a green to dark-green flesh (A. deliciosa)or

flesh which ranges from green to lime-green to a clear

golden-yellow (A. chinensis). In both species there

may also be attractive red pigmentation in the inner

pericarp. The external appearance and the fruit flesh

of some species change color during ripening.

0013 The bulk of the outer pericarp consists of thin-

walled parenchyma cells, variable in size. The cells

are loosely organized and there can be large intercel-

lular spaces. Nevertheless, fruit are reasonably solid

(‘Hayward’ density, c. 1.03 Kg m

3

). There are also

many mucilage-containing cells with crystals of cal-

cium oxalate. Chloroplasts are generally similar to

those found in leaves. In the inner pericarp, the

seeds are suspended in a mucilaginous matrix in the

locules. Chloroplasts of the inner pericarp are very

different in appearance and there are many raphide

cells containing calcium oxalate. The central core of

the mature fruit is formed of homogeneous, large

parenchyma cells, although there is often a hard,

woody spike at the stalk end.

0014Following pollination, there is a phase of rapid

growth for the first 8 or 9 weeks, during which the

fruit reaches half to two-thirds final volume, then

about 3 weeks of much slower growth followed by

another period of growth, which is initially rapid but

then decreases. During the first stage of fruit growth,

there is rapid cell division in the outer and inner

pericarp and the central core. Cell division in the

pericarp stops after 3–4 weeks and most of the subse-

quent increase in fruit size is due to cell enlargement

as the parenchyma cells of the fruit are 10–15 times

the size of those in the ovary wall. Cell division may

continue for much longer in the central core, although

at a much reduced rate.

The Chemical Composition of Kiwifruit

0015Only fruit of A. deliciosa have been systematically

analyzed in any detail and most results are for the

cultivar ‘Hayward’. There is considerable variation in

the published values and those given in Table 1 should

be considered only as indicative. Individual fruit from

a single vine can vary greatly in composition and

many factors such as growing conditions or posthar-

vest treatment can affect some fruit constituents. The

ripeness of fruit will obviously also affect the propor-

tions of the various carbohydrates. (See Fruits of

Temperate Climates: Factors Affecting Quality;

Ripening of Fruit.)

tbl0001Table 1 Approximate chemical composition of ‘Hayward’

(Actinidia deliciosa) kiwifruit

Proximates g 100 g

1

fresh weight edible portion

Total solids 15–20

Protein 1

Lipids 0.5

Carbohydrate 15

Energy 60 kcal

250 kJ

Minerals mg 100 g

1

freshweight

Calcium 40

Chloride 35

Iron 0.4

Magnesium 25

Phosphorus 30

Potassium 300

Sodium 5

Vitamin C 80 mg 100 g

1

fresh weight

KIWIFRUIT 3427

Proteins, Lipids, and Amino Acids

0016 Kiwifruit, like most other fruit, contain insignificant

amounts of proteins, lipids, and amino acids.

Carbohydrates

0017 Ripe kiwifruit contain almost no starch as it has

largely been converted during ripening into soluble

sugars, mainly glucose, smaller quantities of fructose,

and only minor amounts of sucrose. Carbohydrates in

kiwifruit would normally meet only a very small part

of the daily requirements. (See Starch: Structure,

Properties, and Determination.)

Minerals

0018 Kiwifruit are typical fruit, being a good source of

potassium, with a high potassium-to-sodium ratio.

They are also a useful source of magnesium, although

a 100-g serving supplies less than 10% of the recom-

mended dietary allowance (RDA) for an adult male.

Other minerals are not sufficient to make a significant

contribution to the diet. Refer to individual minerals.

Vitamin C

0019 Vitamin C is currently recognized as the single most

important nutrient in kiwifruit. ‘Hayward’ kiwifruit

typically contain about 85 mg ascorbate 100 g

1

fresh

weight of edible portion. The vitamin C content can

vary with fruit size, position on the vine, the season,

and growing location. Many kiwifruit are eaten after

having remained in cool store for long periods and it is

therefore a great advantage that comparatively little

vitamin C is lost from kiwifruit during storage or

ripening. A ‘Hayward’ kiwifruit stored for 6 months

at 0



C and then ripened will still contain at least 90%

of the vitamin C present in the fruit at harvest. Most

other selections of A. chinensis or A. deliciosa like-

wise show little or no loss of vitamin C during storage

over extended periods. It is therefore very likely that

when consumers buy or actually eat kiwifruit, the

fruit contain about as much vitamin C as they did at

harvest. Assuming usual storage conditions and sens-

ible handling, kiwifruit that are acceptable eating are

also likely to be an excellent source of vitamin C.

0020 Kiwifruit contain more vitamin C than almost all

other fruit: on a fresh-weight basis they typically

contain 50% more vitamin C than an orange, five

or six times as much as a banana, and 10 times as

much as an apple. Only a few readily available fruit,

such as blackcurrants, are richer in vitamin C.

0021 Recommended daily requirements (USA) for vita-

min C range from 30 mg for a child to 90 mg for

a lactating mother. These requirements can easily

be met by two medium-sized peeled ‘Hayward’

kiwifruit. Any new selections of A. chinensis and

A. deliciosa are likely to contain as much or even

more, as ‘Hayward’ contains only relatively modest

amounts of vitamin C compared to the rest of the

genus Actinidia. ‘Hort16A’, at present the only com-

mercially available fruit of A. chinensis, outside of

China, usually contains 30–40% more vitamin C

than ‘Hayward’ and many of the commonly grown

cultivars in China contain at least twice as much.

Even these levels are an order of magnitude less than

those in fruit of A. eriantha Benth. or A. latifolia

(Gardn. et Champ.) Merr. which can contain an

astonishing 1 or 2% fresh weight of vitamin C. (See

Ascorbic Acid: Properties and Determination;

Dietary Requirements of Adults.)

Other Vitamins

0022One kiwifruit could provide 10–20% of the daily re-

quirements for folic acid. Folic acid is readily available

in a wide variety of foods and is especially rich in offal

and raw leafy vegetables but easily destroyed by

cooking. Fresh fruit are therefore a useful source.

(See Folic Acid: Properties and Determination.)

0023Most foods contain vitamin E but it is seldom found

in fruit or vegetables without significant fat content. It

is likely, although not yet confirmed, that the vitamin

E in kiwifruit is present in the seed and that these seeds

will survive passage through the gut, thus making any

vitamin E present unavailable. (See Tocopherols:

Properties and Determination.)

0024The amounts of other vitamins are small in terms of

human daily requirements and are similar to those

found in many other fruit and vegetables. These

vitamins are also available in a wide range of foods.

Pigments

0025Green-fleshed kiwifruit (e.g., A. deliciosa ‘Hayward’)

are amongst the very few fruit that are still green

when ripe. This color is due to the presence of chlor-

ophyl and, although many other common fruit are

green during the early stages of development, they

lose their chlorophyl during subsequent maturation

and ripening. The color of green kiwifruit is one

of their most appealing characteristics. Chlorophyls,

however, have very little direct nutritional value.

(See Chlorophyl.)

0026Fruit of other kiwifruit cultivars or species may be

yellow, orange, or red when ripe, and these colors are

due to their content of carotenoids and anthocyanins.

Carotenoids could contribute usefully to vitamin A

requirements. (See Colorants (Colourants): Proper-

ties and Determination of Natural Pigments.)

Oxalate

0027Kiwifruit contain appreciable but not exceptional

amounts of oxalate. At least half of this oxalate is

3428 KIWIFRUIT

complexed as highly insoluble calcium oxalate,

mostly in the form of raphides (needle-like crystals),

and this would render much of the calcium present in

the fruit unavailable. The levels of oxalate in kiwifruit

do not constitute a nutritional problem, assuming

normal consumption, as some other common foods,

e.g., spinach, contain much higher levels. Eating some

processed ‘Hayward’ kiwifruit products such as

nectars, dried slices, or fruit leathers, can cause irrita-

tion of the mucous membranes of the mouth. This is

due, at least in part, to the mechanical irritation of the

membranes by the oxalate raphides. In the fresh fruit,

the raphides are embedded in mucilage and therefore

do not cause any appreciable irritation. The shape of

the raphides varies with Actinidia species and this

could affect the amount of irritation caused.

Actinidin

0028 Kiwifruit contain large amounts of the highly active

proteolytic enzyme actinidin (EC 3.4.22.14) similar

to the proteases found in other fruits such as pine-

apple, figs, or papaya. Actinidin is ideal as a meat

tenderizer but can cause problems if fresh fruit are

incorporated into gelatine-based jellies (which then

do not set) or are mixed with dairy products.

0029 Actinidin, at the levels found in ‘Hayward’ kiwi-

fruit, does not seem to be a major health hazard for

most people, but has been associated with allergic

responses to kiwifruit. Actinidin can cause damage

to the lips (especially at the corners of the mouth) but

only if very large quantities of fruit are eaten. Peeling

large numbers of fresh fruit can result in skin loss if

hands are not protected. Other Actinidia cultivars

need to be checked to insure that they do not contain

appreciably higher levels of actinidin, capable of

causing problems. There is no published evidence on

the postulated beneficial role of actinidin in gut

health.

Dietary Fiber

0030 Kiwifruit contain about 2–3% dietary fiber due to

pectins and other oligosaccharides and polysacchar-

ides that are not broken down and absorbed in the

small intestine. A 100-g serving of kiwifruit will

therefore supply about 10% of the recommended

daily requirement. The relative contributions of the

various polysaccharide fractions in kiwifruit to diet-

ary fiber effects such as stool-bulking and laxative

action are not yet defined. (See Dietary Fiber: Proper-

ties and Sources.)

Laxatives

0031 Kiwifruit are notorious for being laxative and indi-

viduals vary in their response. Consumption of large

numbers of fresh fruit could therefore have an exces-

sively vigorous purgative effect and the laxative effect

can limit the number of kiwifruit some people are

able to eat in any one day. Fresh kiwifruit or dried

products are often used to maintain regularity in

bowel movement, especially for older and sedentary

people such as hospital patients. Although the laxa-

tive action is not in doubt, there is as yet no definitive

information on the mode of action or the compounds

responsible. (See Elderly: Nutritional Management of

Geriatric Patients.)

Allergens

0032Kiwifruit contain allergens that can cause allergic

responses in susceptible consumers. The response

can include swelling and itching of the lips and

tongue, swelling of the tongue and throat which can

make breathing difficult, rashes, stomach pains and

vomiting, itchy nose and eyes, or, at worst, anaphyl-

actic shock requiring urgent medical attention. Fortu-

nately, extreme allergic responses to kiwifruit are not

frequent. Kiwifruit allergy is often cross-reactive with

other common allergies such as to pollens, particu-

larly birch pollen, latex, housedust mites, and some

other tropical fruit. Actinidin, the most abundant

protein in kiwifruit, has been postulated as a major

cause of kiwifruit allergy but it is not known whether

kiwifruit are the primary cause of the allergic reac-

tions, or are secondary to exposure to other allergens.

(See Food Intolerance: Food Allergies.)

Possible Medical Uses

0033Early Chinese pharmacopeia from the Tang Dynasty

onwards list a variety of medicinal uses for kiwifruit

or extracts from other parts of the plant. Various

Actinidia species have long been used in traditional

Chinese and Japanese medicine but their efficacy has

yet to be fully confirmed. The anticancer and anti-

mutagenic potential of the fruit are now being stud-

ied. The antioxidant capacity of kiwifruit likewise

requires more study.

0034Myoinositol is a major carbohydrate in some Acti-

nidia species and in ripe ‘Hayward’ fruit comprises

about 6–7% of the free sugars. Myoinositol supple-

mentation has been suggested for some medical con-

ditions, but the supplementary amounts proposed

would require consumption of 20 or more kiwifruit

every day.

Handling, Storage, and Marketing

0035Actinidia chinensis and A. deliciosa flower in mid to

late spring and the fruit are ready for harvest nearly

6 months later in late autumn. Harvest maturity is

KIWIFRUIT 3429

taken as the stage at which fruit can be harvested

from the vine and yet continue to develop to give an

acceptable final eating quality which meets con-

sumers’ expectations. Fruit that are picked too early

often have a poor color and flavor when ripened, a

shorter storage life, and a shorter shelf-life when

taken out of storage. However, fruit soften as they

remain on the vine and if they are picked too late they

may be not firm enough for handling, grading, and

subsequent storage. Harvest maturity cannot be

gaged by the external appearance of the fruit, and if

there are no heavy frosts or bird attack, the fruit can

hang on the vines until budbreak the following

season.

0036 Changes in the chemical composition of the fruit as

they mature on the vine are therefore used to deter-

mine when fruit may be safely picked. Maturity

values are taken on representative samples of fruit

from vines of an individual maturity area, an area in

which the vines are presumed to be similar through

uniformity of age, management, and growing envir-

onment. The harvest maturity of ‘Hayward’ kiwifruit

is usually assessed using the soluble solids content of

the fruit as determined using a refractometer. As the

fruit mature, the soluble solids content increases,

largely as a result of the conversion of starch to

sugars. In New Zealand, a minimum maturity index

of 6.2 Brix is normally used following a standardized

measurement procedure: in other countries, a min-

imum of 6.5 Brix has been set. Dry matter has also

been used as an additional harvest index attribute.

Other parameters may be more appropriate for dif-

ferent cultivars or different environments. The fruit of

‘Hort16A’, for example, are promoted in the market-

place for their yellow fruit flesh, but the chlorophyl in

the pericarp is lost relatively late in the season and the

harvest maturity indices currently recommended

therefore include attributes such as soluble solids

content (Brix), firmness, and hue angle.

0037 Although kiwifruit are relatively firm when picked,

they are still easily damaged if handled roughly. Fruit

of ‘Hort16A’ are particularly prone to damage be-

cause their sharp ‘beaks’ can cause small wounds

which allow the entry of fungal rots. Fruit are there-

fore picked by hand and grading equipment is

designed to minimize injury.

0038 Kiwifruit are seldom marketed immediately after

harvest and most will be sold only after a period in

storage. Botrytis stem-end rot was the cause of much

of the losses of stored kiwifruit, but curing of the fruit

can reduce the severity of rots. Fruit are not therefore

cooled immediately after harvest but are left at ambi-

ent temperature for several days. Curing presumably

allows the picking scar or wound to dry out, making

entry of Botrytis less likely. Fruit may then be stored

in bulk, either in conventional coolstores or in CA

stores, or packed immediately into the consumer

packs which are then stored until required. Tradition-

ally, fruit were packed into wooden trays containing

3.6 kg of fruit, but a variety of different-sized packs is

now common.

0039The aim in storage is to reduce fruit temperatures

and respiration to slow the rate of ripening. This is

achieved with kiwifruit by holding them at as low a

temperature as possible (0 + 0.5



C). Fruit will freeze

at about 1.5



C. The fruit may be stored for up to

6 or 7 months so to prevent the shriveling caused by

water loss, fruit are held in their packs within plastic

film liners and maintained at a relative humidity

greater than 95%.

0040Kiwifruit are very sensitive to ethylene and even

low levels of ethylene will promote ripening. It is

therefore critical that kiwifruit not be exposed to

ethylene from other fruits, e.g., apples, to ethylene

from ripening kiwifruit, or ethylene produced in the

exhausts of petrol-driven machinery. Cool stores are

routinely flushed with fresh air and there is increasing

use of ethylene scrubbers to remove any traces of

ethylene produced. Although ‘Hayward’ kiwifruit

survive lengthy periods in cool store, their storage

life and their quality can be enhanced even further

by CA storage and ethylene scrubbers are then essen-

tial. (See Controlled-atmosphere Storage: Effects on

Fruit and Vegetables; Ripening of Fruit.)

0041Fruit ripened after a few weeks in storage seem to

be of better quality than those ripened immediately

after harvest, but during prolonged storage, the pro-

cesses of ripening occur slowly over a long period:

fruit soften and there may be some loss of eating

quality. The intensity of the fruit flesh color may

diminish and there is a loss of volatile flavor com-

pounds.

0042Fruit that are newly harvested and stored for only a

brief period may reach consumers before they are

fully ripe, and an inexperienced customer trying to

cope with a rockhard kiwifruit is unlikely to repur-

chase. About one-fifth of the New Zealand crop is

therefore treated with ethylene either on board vessels

or at the destination before being distributed to the

markets. This insures that consumers are able to buy

‘ready-to-eat’ fruit. This does, of course, necessitate

much more careful stock management.

0043‘Hayward’ (green) kiwifruit are now available in

many markets throughout the year. For half the year,

supplies will come from the big southern hemisphere

producers, Chile or New Zealand, and for the other

half of the year from northern hemisphere countries

such as France, Greece, Italy, or the USA. New Zea-

land is so far the only large supplier of yellow kiwi-

fruit to international trade, but as cultivation of

3430 KIWIFRUIT

‘Hort16A’ or other A. chinensis cultivars starts in the

northern hemisphere, they too will be available for

much of the year.

004 4 ‘Hayward’ kiwifruit store remarkably well. Most

other cultivars of A. chinensis or A. deliciosa have

shorter storage lives. Some other Actinidia species are

being grown on a small scale, e.g., A. arguta, but their

fruit generally have a much more restricted storage

life.

Industrial Utilization

0045 Processing of kiwifruit into high-quality products is a

challenge, as much of the uniqueness of their subtle

flavors is lost. Furthermore, if green kiwifruit, such as

A. deliciosa ‘Hayward’, are processed, the bright-

green color of the chlorophyl is usually modified to

an unattractive, sludgy brown. Most juice or nectar

products or canned kiwifruit products do not retain

their green color unless frozen without heat treat-

ment. Drying likewise usually results in a loss of

flavor, green color, and vitamin C. It is therefore not

surprising that in most countries kiwifruit are grown

primarily for the fresh-fruit market and only fruit that

do not reach export quality standards are processed.

Extraction of chemical byproducts from waste fruit

remains a possibility.

004 6 In China, processing of kiwifruit is relatively more

important since storage and transport systems for

fresh fruit are still being developed. Between 20

and 30% of all Chinese kiwifruit production is cur-

rently processed into products such as fruit juices,

jams, wines, and spirits. Yellow-fleshed fruit of

A. chinensis are often preferred for processing be-

cause they are sweeter and the color withstands pro-

cessing better.

Kiwifruit in the Future

0047 Kiwifruit are amongst the most recent of all domesti-

cated fruits. Actinidia deliciosa has been cultivated

for just on a century, A. chinensis for little more

than 20 years. Although the kiwifruit currently avail-

able commercially throughout the world are pro-

moted for their contribution to health, they probably

do no more than set a minimum standard nutritionally

for kiwifruit, even if these standards are higher than

for most other fruits. The variation within the genus

Actinidia indicates that it is realistic to predict the

development of new kiwifruit cultivars with enhanced

nutritional qualities but containing smaller amounts

of the compounds having potentially adverse effects

on human health.

Seealso: Antioxidants:NaturalAntioxidants; Ascorbic

Acid:PropertiesandDetermination; Chlorophyl;

Controlled-atmosphere Storage:EffectsonFruitand

Vegetables; Dietary Fiber:PropertiesandSources;

Dietary Requirements of Adults; Elderly:Nutritional

ManagementofGeriatricPatients; Folic Acid:Properties

andDetermination; Food Intolerance:FoodAllergies;

Fruits of Temperate Climates:CommercialandDietary

Importance;FruitsoftheEricacae;FactorsAffecting

Quality; Ripening of Fruit; Starch:Structure,Properties,

andDetermination; Tocopherols:Propertiesand

Determination