ration andty Indices

Michael S. Reid

\ N D M A T U R I T Y I N D I C E S

The first step in the postharvest life of the product is the moment ofharvest. For most fresh produce, harvest is manual, so the picker isresponsible for deciding whether the produce has reached the correctmaturity for harvest. The maturity of harvested perishable commodi-ties has an important bearing on their storage life and quality andmay affect the way they are handled, transported, and marketed. Anunderstanding of the meaning and measurement of maturity is there-fore central to postharvest technology. The meaning of the termmature, the importance of maturity determination, and some exam-ples of approaches to determining and applying a satisfactory index ofmaturity, are discussed in this chapter.

DEFIN IT ION OF MATURITY

To most people mature and ripe mean the same thing when describingfruit. For example, mature is defined in Webster's dictionary as: "mature(fr.Lmaturus ripe):l: Based on slow, careful consideration;2a (I):having completed natural growth and development: RIPE (2): havingundergone maturation, b: having attained a final or desired state; 3a: ofor relating to a condition of full development." In postharvest physiol-ogy we consider mqture and ripe to be distinct terms for differentstages of fruit development (fig. 6.1). Mature is best defined by 2a (l)above as "having completed natural growth and development"; forfruits, it is defined in the U.S. Grade standards as "that stage whichwill ensure proper completion of the ripening process." This latter def-inition lacks precision in that it fails to define "proper completion ofthe ripening process." Most postharvest technologists consider that thedefinition should be "that stage at which a commodity has reached asufficient stage of development that after harvesting and postharvesthandling (including ripening, where required), its quality will be atleast the minimum acceptable to the ultimate consumer."

Horticultural maturity is the stage of development at which a plantor plant part possesses the prerequisites for use by consumers for aparticular purpose. A given commodity may be horticulturally matureat any stage of development (see fig. 6.1). For example, sprouts orseedlings are horticulturally mature in the early stage of development,whereas most vegetative tissues, flowers, fruits, and underground stor-age organs become horticulturally mature in the midstage, and seedsand nuts in the late stage, of development. For some commodities,horticultural maturity is reached at more than one stage of develop-ment, depending on the desired use of the product. In zucchinisquash, for example, the mature product can be the fully open flower,the young fruit, or the fully developed fruit.

A qualitative difference in the relationship between maturity andedibility distinguishes many fruits from vegetables. In many fruits,such as mature (but green) bananas, the eating quality at maturitywill be far less than optimal. The fruit becomes edible only after prop-er ripening has taken place. In contrast, in most vegetables, optimalmaturity coincides with optimal eating quality

IND ICES OF MATURITY

The definition of maturity as the stage of development giving mini-mum acceptable quality to the ultimate consumer implies a measur-able point in the commoditys development, and it also implies the

5 5

\odl

t[

oftha

dE

F

g n :

!B:

r r :

Ine':

b.--:

*:;i.:-"

t - _ _ :

L

SL

b:-.

5 5 C H A P T E R 6

Figure 5.1

Horticultural maturity in relation to developmental stages of the plant.(watada et al. 1984)

Physiological_ _r!3!UtiIL---

j ^ ,

__f i rpenrng ___

Senescence I

Horticultural maturity

Sprouts ____- Stems and leaves ____.asparagus, celery lettuce, cabbage

Inflorescencesartictrokl, nroccoti, cautii'l6wer

lgjtEllyjlyelgps{ f ryjlcucumber, green bean, okra, sweet corn

Fully developed fruit _apple, pear, citrus, tomato

Roots and tubers Seeds----ca-no, onnn, pouto dry be-an

Cut and potted Pottedfoliage flowering Cut

!g_._dlingr nurr.ry rto,k S..dt. - - - . - - - .

need for techniques to measure maturity.The maturity index for a commodity is ameasurement or measurements that can beused to determine whether a particularexample of the commodity is mature. Theseindices are important to the trade in freshfruits and vegetables for several reasons.

Trade regulations. Regulations publishedby grower groups, marketing orders, or legal-ly appointed authorities (such as the statedepartments of agriculture and the USDA)frequently include a statement of the mini-mum (and sometimes maximum) maturityacceptable for a given commodity. Objectivematurity standards are available for relativelyfew commodities, and most regulations relyon subjective judgments related to the broaddefinitions quoted above.

Marketing strategy. In most markets thelaws of supply and demand create priceincentives for the earliest (or sometimes thelatest) shipments of particular commodities.

This encourages growers and shippers toexpedite or delay harvesting their crop totake advantage of premium prices. The mini-mum maturity statements in the grade stan-dards exist to prevent the sale of immatureor overmature product and the consequentloss of consumer confidence. Objectivematurity indices enable growers to knowwhether their commodity can be harvestedwhen the market is buoyant.

Efficient use of labor resources. Withmany crops the need for labor and equip-ment for harvesting and handling is seasonal.ln order to plan operations efficiently, grow-ers need to predict the likely starting and fin-ishing dates for the harvest of each commod-ity. Objective maturity indices are vital foraccurate prediction of harvest dates.

C H A R A C T E R I S T I C S O F A M A T U R I T YINDEXMaturity measures made by producers, han-dlers, and quality control personnel must besimple, readily performed in the field ororchard, and require relatively inexpensiveequipment. The index should preferably beobjective (a measurement) rather than sub-jective (an evaluation). The index must con-sistently relate to the quality and postharvestlife of the commodity for all growers, dis-tricts, and years. If possible, the indexshould be nondestructive.

The search for an objective determinationof maturity has occupied the attention ofmany horticulturists working with a widerange of commodities for many years. Thenumber of satisfactory indices that have beensuggested is nevertheless rather small, andfor most commodities the search for a satis-factorv maturitv index continues.

Two rather different problems will beaddressed here. The first problem is how tomeasure maturity at harvest or at a subse-quent inspection point. The second andmore complex problem is how to predict thetime at which a commodity will mature. Forboth problems, similar techniques may beappropriate, but the ways in which they areapplied differ.

DEYELOPING A MATURITY INDEXMany features of fruits and vegetables havebeen used in attempting to provide adequateestimates of maturiqr Examples of those thathave been proposed, or that are presently in

o

q o6

use, are shown in table 6.1. The wide range ofmethods that have been devised to measurethese features are summarized in table 6.2.

The strategy for developing a maturityindex is

. To determine changes in the commoditythroughout its development.

. To look for a feature (size, color, solidity,etc.) whose changes correlate well with thestages of the commodity's development.

. To use storage trials and organolepticassays (taste panels) to determine thevalue (or level),of the maturity index thatdefines minimum acceptable maturitv

6.1. Maturity indices for selected fruits and vegetables

H.t Examples

&ped days from full bloom to harvestApples, pears

han heat units durinq development Peas, apples, sweet corn

helopment of abscission layer Some melons, apples, feijoas

9rface morphology and structure Cuticle formation on grapes,tomat0esNetting of some melonsGloss of some fruits (develop'ment of wax)

gze All fruits and many vegetables

. When the relationship between changes inthe maturity index quantity and the qualityand storage life of the commodity has beendetermined, an index value can be assignedfor the minimal acceptable maturity

. To test the index over several years and inseveral growing locations to ensure that itconsistently reflects the quality of the har-vested product.

FEATURES USED AS MATURITY INDICESChronological featuresFor certain crops (fast-rotation vegetables,such as radish, and perennial tree cropsgrowing in short summer environments),maturity can be defined chronologically, forexample, as days from planting or as daysfrom flowering. Chronological indices are sel-dom perfect, but they do permit a degree ofplanning, and they are widely used. For somecrops, the chronological method is refined bycalculating heat units accumulated duringthe growing period, which modulates thechronological index according to the weatherpattern during the growing season.

Physical featuresA wide range of physical features are used toassess the maturity of various commodities.

Size, shape, and surface characteristics.Changes in the size, shape, or surface charac-teristics of fruits and vegetables are common-ly used as maturity indices. For example, veg-etables in particular are harvested when theyhave reached a marketable size and beforethey become too large. Maturity in bananas isdetermined by measuring the diameter of thefingers; changes in the surface gloss or feel(waxiness) are used as a practical tool in har-vesting of some melons such as honeydew(see chapter 33, table 33.4).

Abscission. In many fruits, during thelater stages of maturation and the start ofripening, a special band of cells, the abscis-sion zone, develops on the stalk (pedicel)that attaches the fruit to the plant. Theabscission zone permits the fruit to separatefrom the plant. Measuring the developmentof this zone (degree of separation) is possiblythe oldest of all maturity indices. Abscissionforce (the force required to pull the fruitfrom the tree) is not generally used as a for-mal maturity index, but the development ofthe abscission zone, or "slip," in the netted

5 7

rd

f

F

I'E

E

='

ficFrE

IG

-ilt

B

nnl

rGtcL

lpecific gravity

Irape

Cherries, yvat-e_rmelons, potatoes

Angularity of banana fingersFull cheeks of mangoesCompactness of broccoli andcauliflower

Lettuce, cabbagg Brussels sprouts

Frmness Apples, pears, stone fruitsdli$- Tenderness

fxtemal color

Peas

All fruits and most veoetables

htemal color and structure Formation of jellylike materialin tomato fruits

Flesh color of some fruits(ornpositional factors:

Starch content Apples, pears

Sugar content Apples, pears, stone fruits, grapes

Acid content, sugar/acid ratio Pomegranates, citrus, papaya,melons. kiwifruit

Juice content Citrus fruits

oil content Avocados

futringency (tannin content) Persimmons, dates

Internal ethylene concentration Apples, pears

5 8 1 H A P T E R 6

muskmelons (see chapter 33, fig. 33.9) isused to determine their maturity.

Color. The color change that accompaniesmaturation in many fruits is widely used as a

maturity index. Objective measurement ofcolor requires expensive equipment (fig 6.2),and although the human eye is unable togive a good evaluation of a single color, it isextremely sensitive to differences betweencolors. Color comparison techniques are

therefore commonly used to assess fruit

maturity (fig 6.3). Color swatches may beused to determine external or internal color.

Table 5.2. Methods of maturiw determination

Accurate devices employing state-of-the-anelectronics and optics now permit objectir-ecolor measurements. As the price of suchdevices has fallen, they have replaced com-parison techniques in many cases. Forple, digital color examination is now usedthe sorting of mechanically harvested pro-cessing tomatoes.

Texture. Maturation of fruits is oftenaccompanied by softening; overmature l

etables frequently become fibrous or tougL

These textural properties can be used to

determine maturity. They are measured

Norl

Objective Destructive

x

lndex Method of determination

Elapsed days fromfull bloom

Computation

Mean heat units Comoutation from weather data

Visual or force of separation t -xDevelopment ol

abscission layer

VisualSurface structure. ,5tze

Textural properties:

Firmness

Colo[ external

Color, internal

Compositional factors:

Various measuring devices, weight

Densig gradient solutions,flotation techniquet vol/wt

Dimensions. ratio charts

Feel, bulk density, gamma rays,X-rays

mination of polysaccharides)

Light reflectanceVisual color charts

Light transmittance, delayedlight emissionVisual examination

t.'.pli.;, dry'r; IDry matter

Starch content Kl test. other chemical tests

Sugar content Hand refractometer chemical tests

Acid content

tuia. ,ont.r't

0il contenl

Fenic chloride test

Titration, chemical tests

Tannin content

Gas chromatography

instruments that measure the force requiredto push a probe of known diameter throughthe flesh of the fruit or vegetable (fig 6.a).The solidity of lettuce, cabbage, and Brusselssprouts is an important quality and maturitycharacteristic. In the case of lettuce, gamma-ray equipment has been devised to measurehead firmness, but the technique has notbeen adopted commercially

Chemical changes. The maturation offruits and vegetables is often accompaniedbv profound changes in their chemical com-position. Many of these changes have beenused in studies of maturation, but relatively[e*' have provided satisfactory maturityindices because they usually require destruc-tir-e sampling and complex chemical analy-sis. Chemical changes that are used for

ligure 5.2

M A T U R A T T o N A N D M A T U R T T y r N D t c E s 5 9

Figure 6.4

Using the UC firmness tester to measure the flesh firm-ness of apples.

Figure 6.5

Golorimeter used to measure surface color of apples.

: |Y ' ' , i , ' i i i i i i i l l i i i i i i i : r i i !

used for maturity grading.

Measuring soluble solids content with a refractometer,

Figure 6.5

Treating cut apples with an iodine solution reveals thedisappearance of starch (which stains dark) as applesmature.

6 0 1 H A P T E R 6

maturity estimation include the change intotal soluble solids, measured using a refrac-

tometer (fig. 6.5); changes in the distribu-tion of starch in the flesh of the commodity,measured using a starch-iodine reaction (fig.

6.6); acidity, determined by titration; andthe sugar to acid ratio, which is used as thelegal maturity index for citrus.

The unsatisfactory nature of chemicaltests for maturity is exemplified by the old

oil content measurement for avocados,which has been replaced by the determina-tion of pergent dry weight because of the

time-consuming and complex nature of oil

determination.French scientists have developed an inter-

esting approach to objective maturity (and

quality) determination of harvested melons;

they remove a slender cylinder of flesh from

each melon and rapidly determine its sugar

content by measuring the refractive index of

the juice. The outer portion of the cylinder is

replaced, and the melon is accepted or reject-

ed based on the sugar reading.New opportunities in chemical analysis

are exemplified by the development of near-

infrared technologies for examining the com-position of fruits and vegetables, and rapid

sensor technology for determining volatileprofiles in harvested products. The former is

able to measure sugars in fruits nondestruc-

tively, and the latter is sufficiently rapid to

enable determination of melon maturity in

the field. Researchers have found, for exam-ple, that the sugar content of peaches can be

accurately determined using near-infrared

absorption profiles. As melons mature, their

production of aroma volatiles increases dra-

matically. An instrument has been developed

that enables this increased production to be

an indicator of harvest readiness.Physiological changes. The maturation of

commodities is associated with changes in

their physiology, as measured by changing

patterns of respiration and ethylene produc-

tion. The problem with using these charac-

teristics in assessing maturity is the variabili-

ty in absolute rates of ethylene production

and respiration among similar individuals of

the same commodity. The techniques are also

complex and expensive to implement on a

commercial scale. Nevertheless, the rate of

ethylene production of a sample of apples is

used by some producers to establish the

maturity of the apples, and it is particularly

used to identify those that will be suited tolong-term controlled atmosphere storage.

PREDICT ING MATURITY

Predicting when a commodity will mature ismore complex than assessing its maturity ator after harvest. The basic requirement forprediction is a measurement whose changeduring the commodity's development can bemodeled mathematically to reveal a patternor patterns of change. Once the pattern ofchange is established for the measurement,measurements made early in the season canbe compared with the pattern in order topredict the date at which the commodity

should reach minimum acceptable maturiq-.The way in which this strategy has beenapplied can best be illustrated by the follou-

ing examples.

A P P L E SAlthough the literature on the prediction of

maturity in apples is voluminous, no truly

satisfactory method has yet been proposed.The use of climatic data to predict the dateof harvest by a modification of the "days

from full bloom" index noted in table 6.1,even when adapted by using "days from the'T'stage" has provided only general predic-

tions of the harvest date.In an attempt to provide a more satisfac

prediction of the maturation date, resea

have examined a number of changes that

occur during fruit development. Measure-

ment of respiration, ethylene production,

sugar content, starch content, and the

ing firmness of fruit each failed to meet

of the criteria for a satisfactory maturity

index, and they proved to be too variable

permit prediction of maturation date. The"starch pattern," an old method of dete

ing apple maturity, refined by assigning

scores to arange ofpatterns, has proved

be a good index. Changes in the mean

index score during the period prior to har-

vest are readily analyzed as a linear regres-

sion, and the date of minimal acceptable

maturity and can be predicted several

in advance (fig. 6.7).

AVoCADOSThe State of California for many years

mulgated a minimum oil content as the

maturity standard for avocados. This

has been unsatisfactorv. since it is difficult toapply and because some avocados that havemore than the minimum oil requirementmay be lacking in flavor quality. However,raising the minimum oil content might elim-inate from the market particular avocado

, varieties whose flavor quality is adequate ata low oil content. Using taste panel evalua-

Granny Smith apples.

y=0.085x+0.300(t 0.00e) (t 0.003)

Harvest date

oil content of maturing avocados. (Lee and

l 5Jan

tions to determine quality, researchers haveshown that the patterns of dry weight accu-mulation, or the growth of avocado fruit, canbe used not only to determine when mini-mum acceptable maturity has been achievedbut also to predict the date when it willbe achieved (fig 6.8). Taste panel scoresincreased as oil content increased, and oil

Figure 6.9

Relationship between dry weight and oil content of avocado. (Leeet al. 1 983)

1 0Percent oil

Figure 6.10

Changes in selected chemical and physical parameters of kiwifruitduring growth and development.

Growth periodl+ - l l - +l l l* - lv+ v - Ful ly mature

^ ^ ^ ^ ^

#Ash

20 30Time after anthesis (wks)

5 l

.9o3

E

o

t

{f l L- 0

o

t l o

od

EN

( n L

EoE

25

1 .0

0.8 Fr

0.6 €

0.4 <

0

B

b t u

s

Y = 1 0 . 6 7 + 1 . 0 1 Xr = 0.96

*r"-*al

6 2 c H A P T E R 6

content was found to be closely correlated

with the percent dry weight (fig 6.9).Consequently, the California minimummaturity index was changed from oil contentto percent drY weight.

KIWIFRUITAs a prelude to developing a maturity index

for kiwifruit, researchers measured changesin a wide range of chemical and physical

characteristics during growth and develop-ment (fig.,6 t0) This information was com-

pared to storage life and taste panel results

to decide on possible methods for determin-

Figure 5.11

Changes in {ruit firmness during maturation ol kiwifruit.

Figure 5.12

Changes in soluble solids content during the 6 weeks before harvest of

kiwifruit can be used to predict harvest date'

Kiwifruit maturationdistrict averages

1 0 0Day

ing, and if possible predicting, the time of Iminimal acceptable maturity for this crop. It Iseemed possible that soluble solids content Iand fruit firmness might provide a suitable Imaturity index, but in repeating the experi- Iments over several seasons, changes in the Ifirmness of the fruit were found to be highly Ivariable and unrelated to fruit quality (fig.

I6.11). In New Zealand, a minimum matudty Iindex of 6.25olo soluble solids has now been Iused for many years. The change in soluble Isolids in the 6 weeks prior to the normal Iharvest date can be used, with regression Ianalysis, to predict the date of harvest for Idifferent orchards, seasons, and growing dis-

|t r ic ts ( f is .6 . l2) .

I

^,,-r""t-l,,u,Ji*il^L X]?;ffirityand p,.,.'.uutio,-, or I-1"ti:L::ffil:";::..:L:":Tlill *

|,",J:T::""? t,:.f,.T,:jii',::,, of frui,s I

XT':T'T*cts Vol 2 NewYork:Academic

I

""o"l;i;i;f;:il::ffi :iil'-:',T:i# o"" I

"''';l;1lil,j;Jllli ;.ll:T:,ffi:- I

3 z vE

s

=

80