.

uce

f.

:du

Maturation and,

Maturity Indices0-

ux

Michael S. Reid

g-

d:

3

MATURATION AND MATURITY INDICES 55

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.

DEFINITION 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. L maturus ripe):l: Based on slow, careful consideration; 2a (1):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 mature and ripe to be distinct terms for differentstages of fruit development (fig. 6.1). Mature is best defined by 2a (1)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 mor,e 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.

INDICES 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 commodity's development, and it also implies the

56 CHAPTER 6

~

Figure 6.1

Horticultural maturity in relation to developmentalstagesof the plant.(Watadaet al. 1984)

<::0

'j;;";:'E

Development

.s:'"OJCI

I

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. vVith

many crops the need for labor and equip-ment for harvesting and handling is seasonal.In order to plan operations efficiently, grow-ers need to predict the likely s~arting and fin-ishing dates for the harvest of each commod-ity. Objective maturity indices me vital foraccurate prediction of harvest dates.

Growth

Maturation

Physiological---maturity ---

n~ipening ---

Senescence

Horticultural maturity

Sprouts n__- Stemsand leaves nn-asparagus,celery,lettuce, cabbage

Inflorescences

artichoke,broccoli,cauliflower

Partially developedfruitcucumber,-greenbean,okra, sweet corn

Fully developedfruitapple,-pear,citrus, tomato

Rootsand tubers Seeds

carrot, onion, potato dribean

?~~_dlings

Cut and potted Pottedfoliage flowering Cut

nurserystock plants flowersOrnamentalcrops

Seeds

CHARACTERISTICS OF A t1ATURITYINDEX

Maturity measures made by producers, han-dlers, and quality control personnel must besimple, readily performed in the field ororchard, and require relativelyjinexpensiveequipment. The index should preferably beobjective (a measurement) ratl;J.erthan sub-jective (an evaluation), The inClex 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 ':'lith 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-

factory maturity index continues,Two rather different problems will be

addressed here, The first probh~m 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. For

both problems, similar techniq:ues may beappropriate, but the ways in which they areapplied differ.

DEVelOPING A MATURITY, INDEX

Many features of fruits and vegetables havebeen used in attempting to provide adequateestimates of maturity. Examples of those thathave been proposed, or that are presently in

need for techniques to measure maturity.The maturity index for a commodity is ameasurement or measurements that can be

used 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 the

latest) shipments of particular commodities.

~---

.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 maturity.

Table 6.1. Maturity indices for selected fruits and vegetables

Index Examples,- --- --' ' "' ----------..-

Elapseddaysfrom full bloom to harvest Apples,pears--- .~--_._----------

Mean heat units during development Peas,apples, sweet corn- - -- -- .---

Developmentof abscissionlayer Somemelons,apples,feijoas" " "'--- ~-_.-

Surfacemorphologyandstructure Cuticleformationon grapes,tomatoes

Nettingof somemelonsGlossof somefruits (develop-mentof wax)

----.------------.------

Size All fruits andmanyvegetables

Cherries,watermelons,potatoesSpecificgravity

Shape Angularityof bananafingersFullcheeksof mangoesCompactnessof broccoliandcauliflower

lettuce, cabbage,Brusselssprouts~ .---..

Solidity

Texturalproperties:- - - -- - n

...----..---------------

Firmness Apples,pears,stonefruits--~-~- ~ .-------

Tenderness Peas

Externalcolor All fruits andmostvegetables--- ---...----------......-----------

Internalcolorandstructure Formationof jellylikematerialin tomato fruits

Flesh color of some fruits--------------....--------

Compositionalfactors:.-- . __n--

Starchcontent

Sugarcontent

Apples,pears--- - -

Apples, pears, stone fruits, grapes- ~--

Acidcontent,sugar/acidratio Pomegranates, citrus, papaya,melons, kiwifruit--Citrus fruits

-----.-

Juice content--.-Oil content

,

Avocados

Astringency(tannincontent) Persimmons,dates- -- - ~--

Internal ethyleneconcentration Apples,pears--------------_.-------

MATURATION AND MATURITY INDICES 57

. 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 INDICES

Chronological 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 before

they 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

58 CHAPTER 6

Accurate devices employin~ state-of-the-artelectronics and optics nowipermit objectivecolor measurements. As th~ price of suchdevices has fallen, they ha~e replaced com-

parison techniques in mant cases. For exam-

ple, digital color examinatitn is now used inthe sorting of mechanically! harvested pro-cessing tomatoes.

Texture. Maturation of ftuits is often

accompanied by softening; evermature veg-etables frequently become ffbrous or tough.

These textural properties car be used todetermine maturity. They a~e measured with

I

Table6.2. Methodsof maturitydetermination I

[ -.r --, ' r---+ - I N~~~--~

i

l

i~ d;;~r;;;; .W:~:~:~""m;",,;o"... !.. S"bi"';.' I_ObJ';';"'-t"~~"~

I

!~estr:~~=

full bloom i i I I II. ,.. I I

I-~~~~~;;~~~-~~~~;~~~r~~~~~;-~: ~~~-t.l ---~ -- -I

.

i- ---~---!---~~- -1.

1---~-

ll

! b..

I I ! !.

~clsslon ayer~~ !! I: i 1

! :~~,ce"",ct",'. -- i~,:;:;";;;,"ri;gd~;I~ W~lght! . -'-_oj=-, -J - t 1 : - ~

ISpecificgravity I?ensitygradientsolutions, i i x I I x IiSh~P; - - - --I~~~~e~:~o~~~~;i~~~-~~~~~~~ r ~---1 : ~ +---!- -- x I

I ---+- r j t- I ~ISolidity IFeel,bulk density,gammarays, ! x I xi: x I

1 p:~~y~-~ ~--- ~- l ~- t--~ ~! Textural properties: I ; I ! I i I

!Firmness IFirmnesstesters,d~formation i --r ~ I ~-+---i I

p,-,d.m,,;:-"":: JTeod"".m",,= == - i := T-:::: t- ,-\-r=:~I Toughness

I

Texturometer,fibrometer(also:: I x I x I I.

!

j chemicalmethodsfor deter-. I ! I i Ii I mination of polysaccharides) _L---~ I l--~ IIColor,external Lightreflectance ' I x II I x I

ICoI~~.i~t~;~;I '-~j~~;lt~~;~~~;:~~:~aY~d :-- ._-~ l -- --~---~-- -\--l---~--I

I

.

I I. h .. I I I

I

I Ig t emission i ! I

I--~- ! Visualexamination i x +-+~ I

I-".:~':~;'?"'""'~~;;;;;'d"i"~- --I . ~ ,1 .1. ~-d If-- I i

I Starch content IKItest,otherchemicaltests I I x i x t-- f[ sugar..c:~~~~__J_~nd refractometer,chemicaltests ! ~ x ! x Ii .

~:,~~~~:~;;;---I~:~'m":"""'_--1- -- -I-:=~- :-tj -.=-1

U~l:~]~~~:~:~~~~l... ~J ~_: .f-= ~+r= ~--JI

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 amaturity 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 aretherefore commonly used to assess fruitmaturity (fig 6.3). Color swatches may beused to determine external or internal color.

I..

instruments that measure the force required

to push a probe of known diameter throughthe flesh of the fruit or vegetable (fig 6.4).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 accompaniedby profound changes in their chemical com-position. Many of these changes have beenused in studies of maturation, but relativelyfew have provided satisfactory maturityindices because they usually require destruc-tive sampling and complex chemical analy-sis. Chemical changes that are used for

Figure 6.2

Colorimeterusedto measuresurfacecolorof apples.

Figure 6.3---~---,---'-- '--' '--'----

Color matching usedfor maturity grading.

III

"'00""°' '"0 ""omi 'N",WFigure 6.4

59

--' -- -- -----

Usingthe UCfirmnesstester to measurethnessof apples.

Figure 6.6~ ---

Treatingcutappleswith an iodinesolutionrevealsthedisappearanceof starch(whichstainsdark)a~applesmature.

60 CHAPTER 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 oldoil content measurement for avocados,which has been replaced by the determina-tion of percent dry weight because of thetime-consuming and complex nature of oildetermination.

French scientists have developed an inter-esting approach to objective maturity (andquality) determination of harvested melons;they remove a slender cylinder of flesh fromeach melon and rapidly determine its sugarcontent by measuring the refractive index ofthe juice. The outer portion of the cylinder isreplaced, and the melon is accepted or reject-ed based on the sugar reading.

New opportunities in chemical analysisare exemplified by the development of near-infrared technologies for examining the com-position of fruits and vegetables, and rapidsensor technology for determining volatileprofiles in harvested products. The former isable to measure sugars in fruits nondestruc-tively, and the latter is sufficiently rapid toenable determination of melon maturity inthe field. Researchers have found, for exam-ple, that the sugar content of peaches can beaccurately determined using near-infraredabsorption profiles. As melons mature, theirproduction of aroma volatiles increases dra-

matically. An instrument has been developedthat enables this increased production to bean indicator of harvest readiness.

Physiological changes. The maturation ofcommodities is associated with changes intheir physiology, as measured by changingpatterns 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 productionand respiration among similar individuals ofthe same commodity. The techniques are alsocomplex and expensive to implement on acommercial scale. Nevertheless, the rate of

ethylene production of a sample of apples isused by some producers to establish the

maturity of the apples, and it is particularly

-- --~ --

used to identify those that W.

~ll be.

suited to

long-term controlled atmosp~ere storage.

PREDICTING MAT

Predicting when a commodi will mature ismore complex than assessin its maturity ator after harvest. The basic re uirement for

prediction is a measurement hose changeduring the commodity's deve opment can bemodeled mathematically to r veal a patternor patterns of change. Once he pattern ofchange is established for the easurement,measurements made early in the season canbe compared with the patter in order topredict the date at which the commodityshould reach minimum acce table maturity.The way in which this strate y has beenapplied can best be illustrate by the follow-ing examples.

APPLES

Although the literature on th prediction ofmaturity in apples is volumi ous, no trulysatisfactory method has yet b en proposed,The use of climatic data to p edict the dateof harvest by a modification f the "daysfrom full bloom" index note in table 6.1,even when adapted by using 'days from theT stage" has provided only eneral predic-tions of the harvest date.

In an attempt to provide a ore satisfactoryprediction of the maturation ate, researchers

have examined a number of hanges thatoccur during fruit developme t. Measure-ment of respiration, ethylene roduction,sugar content, starch content and the chang-ing firmness of fruit each fail d to meet someof the criteria for a satisfacto maturityindex, and they proved to be 00 variable topermit prediction of maturati n date. The"starch pattern," an old meth d of determin-ing apple maturity, refined by assigningscores to a range of patterns, as proved tobe a good index. Changes in he mean starchindex score during the period prior to har-vest are readily analyzed as a inear regres-sion, and the date of minimal acceptablematurity and can be predicte several weeksin advance (fig. 6.7).

AVOCADOSThe State of California for ma

1Y years pro-

mulgated a minimum oil cont nt as thematurity standard for avocad s. This index

.

s

;:

Figure 6.7---

has been unsatisfactory, since it is difficult to

apply and because some avocados that have

more than the minimum oil requirement

may be lacking in flavor quality. However,raising the minimum oil content might elim-

inate from the market particular avocadovarieties whose flavor quality is adequate at

a low oil content. Using taste panel evalua-

Figure 6.9

MATURATION AND MATURITY INDICES 61

tions to determine quality, rese rchers have

shown that the patterns of dry eight accu-mulation, or the growth of avo ado fruit, canbe used not only to determine hen mini-

mum acceptable maturity has b en achievedbut also to predict the date whe it will

be achieved (fig 6,8). Taste pan 1scoresincreased as oil content increas d, and oil

Relationshipbetweendryweightandoil contento~avocado,(Leeet al. 1983)

-----

Changein starchindex valuesfor maturing GrannySmith apples.

.---.

6

~ 5"C,:E 4~[ 3

.=.v

~ 2VI

020

Mar

Figure 6.8--- ---

°. ..°..

Q y = 0.085x + 0.300

(:t 0.009) (:t 0.003)

23 8

May Jun

1:en'0;~>--SC 15OJvQjc..

25

20

Y = 10.67 + 1.01

r =0,96

10

Percent oil15 20

Changingacceptabilityandoil content of maturing avocados,(LeeandYoung1983)

Figure 6.10

Changesin selectedchemicaland physicalparametelrsof kiwifruitduring growth and development.

10

9

8

6OJ

i5

; 5~

4

015

Sep

015Oct

15Nov

15Dee

15Jan

Growthperiod1- -11--111--IV-V- Fuliymature

J.

J.

J. Weight

...Totalsolids

J.J.J.J.J. .. ...Ash..

00 10 20 30

Timeafter anthesis (wks)

J.J. A

100

75

E~...]

50 QjCo. 1:en

~25

0

.

!

1.0

.

0,8 ~0.6 ~.=.0.4 .'(

/0

..

40 50

°4..

I5 21 7

AprHarvest date

l300

025

2020

'0 15

-y.

rJ'i.=.

-i= 10. .10. '"

'0VI

55

---~

62 CHAPTER 6

content was found to be closely correlatedwith the percent dry weight (fig 6.9).Consequently, the California minimummaturity index was changed from oil contentto percent dry weight.

KIWIFRUIT

As a prelude to developing a maturity indexfor kiwifruit, researchers measured changesin a wide range of chemical and physicalcharacteristics during growth and develop-ment (fig. 6.10). This information was com-pared to storage life and taste panel resultsto decide on possible methods for determin-

Figure 6.11

Changesin fruit firmnessduring maturation of kiwifruit.

g~ 20OJ<::

§

Figure 6.12

Kiwifruit maturation

25

1977

1974

100 120

Districtaverages

15

60 80

Day

,-~ '-~--~-~'---~" '~-'--~-' -~~--~~-,. "~~ -,~,--,~-,~~,-~--,, '-"~

Changesin solublesolids content during the 6 weeks before harvestofkiwifruit canbe usedto predict harvestdate.

~~ 6"0VI

Kiwifruit maturation

districtaverages

Lif80

.

100

Day

120

~-'-~-

ing, and if possible predictin , the time ofminimal acceptable maturity for this crop. Itseemed possible that soluble solids contentand fruit firmness might pro ide a suitablematurity index, but in repea ing the experi-ments over several seasons, hanges in thefirmness of the fruit were fo nd to be highlyvariable and unrelated to fm t quality (fig.6.11). In New Zealand, a mi imum maturityindex of 6.25% soluble solid has now been

used for many years. The ch nge in solublesolids in the 6 weeks prior t the normalharvest date can be used, wi h regressionanalysis, to predict the date f harvest fordifferent orchards, seasons, nd growing dis-tricts (fig. 6.12).

REFERENCES

Arthey, V D. 1975. Quality of hort}cultural products.

New York: Wiley 228 pp.

Eskin, N. A. M., ed. 1989. Quality nd preservation of

vegetables. Boca Raton, FL: C C Press. 313 pp.

-. 1991. Quality and preser ation of fruits.

Boca Raton, FL: CRC Press. 12 pp.

Hulme, A. C, ed. 1971. The bioch mistry of fruits

and their products. Vol. 2. N w York: Academic

Press. 788 pp.

Kader, A. A. 1999. Fruit maturitY'

t,pening, and qual,

ity relationships. Acta Hort. 85:203-208.

Lee, S. K., and R. E. Young. 1983. rowth measure.ment as an indication of avo

Am. Soc. Hort. Sci. 108:395-

Lee, S. K., R E. Young, P M. Schif man, and C W

Coggins Jr. 1983. Maturity st dies of avocado

fruit based on picking dates nd dry weight.].Am. Sac. Hort. Sci. 108:390- 94.

Pattee, H~E., ed. 1985. Evaluation of quality of fruits

and vegetables. Westport, CT AVI. 410 pp.

Ryall, A. L., and W]. Lipton. 197 . Handling, trans-

portation and storage of fruit and vegetables.

Vol.1,Vegetablesand melor . 2nd ed. Westport,CT AVL 588 pp.

Ryall, A. L., and W T. Pentzer. 19

~:2.Handling, trans-portation and storage of fruit and vegetables.

Vol. 2, Fruits and tree nuts. 2 d ed. Westport,

CT AVL 610 pp.

Seymour, G. G.,]. E. Taylor, and

1993. Biochemistry of fruit ri

Chapman and Hall. 454 pp.

Watada, A. E., R. C Herner, A. A.

Romani, and G. L. Staby. 198.

the description of developme

cultural crops. HortScience l'

. A. Tucker, eds.

ening. London:

.ader, R.J.

. Terminology for

tal stages of horti-:20-21.

--'