UNIVERSITI PUTRA MALAYSIA

THERMAL PROCESS REQUIREMENT AND STORAGE STABILITY OF CANNED GUAVA DRINK

ISHAK BIN SULAIMAN

FSMB 1993 3

THERMAL PROCESS �1Rl'.}U'lff AND S'roRAGE STABILITY OF CANNED GUAVA DRINK

BY

ISHAK BIN SULAIMAN

Thesis Sul:mi.tted in Fulfilment of the Requirements for the Degree of Master of Science in the Faculty

of Food Science and Biotechnology, Universiti PertanianMalaysia

February, 1993

I l ike to take the opportunity to acknowl edge my

i ndebtedness to the chai rman of my supervisory carmi ttee, Dr.

Asbi Ali and the ccmn:i. ttee members, Dr . Sal mah Yusof and Dr.

Jinap Sel amat for their guidance, supervision and suggestions

throughout the runni ng of the project and the preparation of the

thesis.

My sincere gratitude is also extended to the Mini stry of

Educati on, Malaysia for providi ng me the fi nancial support under

Training Scheme Fel lowship (Worl d Bank Loan VII) .

I would l ike tQ thank my col l eagues and fel l ow friends in

the Facul ty of Food Science and Biotechnology, Uni versi ti

Pertani an Mal aysia for their val uabl e assistance .

Final l y, I l ike to affectionately acknowl edge my wife,

Shamsiah, for her constant encouragement and support .

11

t

TABLE OF <XIfl'9fl'S

Page

ACKNCML�S • • • • • • • • • • • • • • • • • • • • • • • • • I • • • • • • • • • • • • • • • ii

LIST OF TABLES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vi

LIST OF FIGURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . viii

ABSTRACT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . , . . . . . . . . . . . . . . . x

ABSTRAK . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xiii

I INTRODUCTION 1

I I LlTERA'1t1RE REVIEW 5

Guava . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Bioch�cal Characteristics of Guava 6

Guava Processing . . . . . . . . . . . . . . . .. . . . . . . . . . . . 11

Thermal Processing of Fruit Juices and Beverages 15

Enzyme Inactivation in Fruit Juices 23

Pectinesterase Inactivation as Basis of Themel Process Evaluation . . . . . . , . . . . . . . . . . . . . 31

storage Stability of Fruit Juices and Beverages 38

III THERMOSTABILITY OF PECTINESTERASE IN GJAVA DRINK 43

Introduction . . . . , . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . 43

Materials and Methods . . . . . . . . . . . . . " " . . . . . . . . . . " . . . 45

Materials . . . . . . . . . . . . . . , . . . . , . . . . . . . . . . . . . . . . . 45

iii

IV

v

Preparation of Guava Drink . . , . . . . . . . . . . . .. . , . . . 45

Detennination of Therrral Inactivation Time of Pectinesterase ... . . . . . . . . . . . . . . . . . . . . . . . . . . 46

Quantitative Estimation of Pectinesterase Activity . . tI • • • • • • • • • • • • • • • • • • • • • • • • , • • • • • • • • • • 47

Calculation of Lethality of the 'come-up Time' . . 47

Determination of Decimal Reduction Time

Results and Discussion . . . . . . . . .. . . . . . . . . . . .. . . . . . . .

50

50

Thermal Inactivation Time of Pectinesterase 50

Decimal Reduction Time of Pectinesterase

Conclusion . . . . . . . . , . . . , . . . . . . . . . . . . . . . . . . . . , . . , , . .

THERMAL PROCESS REQUIREMENT OF CANNED GUAVA DRINK • •

Introduction .. . . . .. . . . . . . . . . . . .. . . . , . . . . . . . . . , . . . .

Materials and Methods

Materials . . . . . . . . . . . . . . , . . . . . . . . . . . . . . . . . . .. , . .

Heat Penetration Studies . . . . . , . . . . . . . . . .. . . . . .

calculations of Process Time

58

63

65

65

67

67

67

68

Microbiological Examdnation of Processed cans .. 69

Results and Discussion . . ... . .. . . , . . . ... . . . . . ... . . .

Thermal Process calculation • • • • , • • • • • • • • II • • • • •

Microbiological Safety of Process Evolved

Conclusion

STORAGE STABILITY OF CANNED GUAVA DRINK

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Materials and Methods .. . . . .. .. . . . . , . .. . . . . . . . . .. . . .. .. . .

iv

71

71

78

82

84

84

85

Processing and Sample Preparation

Analyses of Guava Drink SaJTI)le . . . . . . . . . . . . . . ..

Ascorbic Acid Measurement .................... .

Browning Measurement ......................... .

Total Soluble Solids and pH Measurements . . . • . . . .

Titratable Acidity Measurement ............... .

Cloud stability Measurement .................. .

Viscosity Measurement

Orgarl.ol eptic study . . . . . . . . . . . . . . . . . . . . . . . . . . . .

statistical Treatment

Results and Discussion

Ascorbic Acid Degradation . . . • • . . . . . . • • • . . . . . . .

The Development of Browning

Cloud stability ...................... " . . . . .. . . .

Changes in Viscosity

Changes in Acidity and pH . • . . . • . . . . . . • • • • . . . • .

Changes in Total Soluble Solid content ....... .

Sensory Evaluation . I I • • • • • • I I I I • • • • • • • • • • • • • I I

COncl'llSion ., . . . . . I • • • • • • • • • • • • • • • • • • • • • , • • • • • • • • • •

VI SUMMARY AND �ATIONS ...................... .

BIBLlOORAPtIY .............. , . . . . . . . , .. . . . . , . . . . . . . . . . .... .

APPmDlCES • • • • • • • • • • • • • • . • • • • • • • • • • . • • • • • • • . , . . • • • • . . • . . .

BIOGRAPHICAL SKETCH . .. . . . . . . . . . . . . . . .. . . . . . . . . . . . � . . . . . . .

v

85

86

86

87

87

87

88

89

89

89

90

90

97

101

105

108

112

112

118

119

125

139

152

LIST OF TABLES

Table Page

1 Nutrient and Proximate Composition of Guava Fruit 7

2 Classification of Canned Foods on Basis of Processing Requirements . . ...................... . . .. 18

3 Range of Processing Temperatures Used for Fruit Products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

4 Flash Pasteurisation Conditions for Fruit Juices 24

5 Batch Process Requirements for Canned Fruit Juices 26

6 Thermal Inactivation Data of Pectinesterase in Various Types of Fruits . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

7 F, D and Z-Values of Pectinesterase in Guava Drink 57

8 Process Time Calculated by Fo�la Method to Achieve 1.5 D Process for Guava Drink canned in 300 x 305 Carls . . , . ,' • • • . , . • • • . , • • • • • • t, ' • • • • • • • • • , I , , 76

9 The Process Times for Different F Values 10 Means of Sensory Panel Score for COlour, Flavour,

80

Taste, Cloudiness and Overall Acceptability . . . . ... . 114

11 ANOVA Table for Vitamin C in canned Guava Drink D'uI"ing storage .. ... . . . . .. . ..... .. .... . . , ........ ,.. 142

12 ANOVA Table for Browning in Canned Guava .Drink During storage ........ " . . . . . . .. .. . . ....... . . . . . . , . . , 1 43

13 ANOVA Table for Cloud Loss in canned Guava Drink During storage ... " . . . . . . . . II • • • • • • • • • • • • • • • • I • • • • • • • 144

14 ANOVA Table for Viscosity in Canned Guava Drink During storage ... . ... .... . . . ..... III • • • • • • • • • • • • • • , • • 145

15 ANOVA Table for Total Acidity in Canned Guava Drink D'uI"ing storage . . • . . . . . . . . • . . . • . . . . . . . . . . . . . . . 146

vi

16 "PJ.-{OVA Table for pH of Canned Guava Drink During storage . " " .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. 147

17 "PJ.-{OVA Table for Total Soluble Solid in Canned Guava Drink During Storage . . . . . . . . . . . . . . . . . . . . . . . . . 148

18 Analysis of Variance of Sensory Scores for Canned Guava Drinlt .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. , 149

vii

LIST OF FIClJRES

Figure Page

1 Pectinesterase Inactivation at 900C for Various Heating Times and Soluble Solid Concentrations (100 - 3SOsrix) .......... , ........... , . . . . . . . . . . . .. . 34

2 A Typical Heat Penetration Curve and Lethality CUrve of Came-up Time .............................. 49

3 Percent Residual Activity of Pectinesterase as a Function of Heating Temperature and Time . . . . • . . . . . . 52

4 Uncorrected Thermal Inactivation Time CUrve of Pectinesterase . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . , 54

5 Thermal Inactivation Time and Thermal Resistance CUrves of Pectinesterase in Guava Drink ............ 55

6 Thermal Inactivation Rates CUrves of Pectinesterase . 60

7 Heat Penetration Curves of Canned Guava Drink

8 Heating CUrves of canned Guava Drink

9 Cooling CUrves' of canned Guava Drink

10 Process Time as a Function of Heat Treatment

72

73

74

Level and Fill-in Temperature ...................... 79

11 Percent Retention of Ascorbic Acid in canned Guava Drink as a Function of Storage Temperature and Heat Treattnent Level ........ , . . . . . . . .. . . . . . . . 11 • • • • • 91

:2 Log Percent Retention of Ascorbic Acid in Canned Guava Drink as a Function of storage Temperature and Heat Treatment Level ........................... 95

13

14

Browning in Canned Guava Drink as Affected by storage Temperature and Heat Treatment Level

Cloud Loss in canned Guava Drink as Affected by Storage Temperature- and Heat Treatment Level

viii

99

102

15 Viscosity in canned Guava Drink as Affected by storage Temperature and Heat Treatment Level . . . . . . .

16 Total Titratable Acidity in canned Guava Drink as Affected by storage Temperature and Heat Treatment

106

Level . . , . . , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109

17 PH of canned Guava Drink as Affected by storage Temperature and Heat Treatment Level . • . . . . . . . • . . . . . 111

18 Total Sol uble Solid in canned Guava Drink as Affected by Storage Temperature and Heat Treatment Level 113

ix

Abstract of the Thesis Subndtted to the senate of the Universiti Pertanian Malaysia in Fulfilment of the

Requirements for the Degree of Master of Science

'J.'H!»W, PROCESS �IREJotm'l' AND STCJWlE STABILITY OF CANNED GJAVA mINK

By

ISfAK BIN SULAIMAN

February , 1993

Chairman Asbi bin Ali, Ph.D.

Faculty Food Science and Biotechnology

A thermal process for canned guava drink was evolved on the

basis of inactivation of pectinesterase (PE) , the most heat

resistant enzyme naturally present in guava. The

thermostability of PE in guava drink was deterrrdned according

to the thermal inactivation time tube method. The kinetics of

themal inactivation of PE in guava drink showed it obeyed

first-order reactions. '!'he therrral inactivation time curve of

the enzyme was analogous to the themal death time curve of

bacteria. As the heat resistance of spoilage rrUcroorganisms is

lower than PE in acidic conditions, the enzyme was used in the

thermal process evaluation of canned guava drink.

x

The F and D values of PE in the guava drink (pH 3.7) at

100.4oC were 1.0 minute and 0.67 minute, respecti vel y . The F

value was equivalent to 1.5 D. This data was used as the

basis for the calculation of process time by the formula method.

The heat penetration study was carried out in 300 x 305 cans.

The �rocess requirements under different initial temperatures

were determined. It was found that when the filling temperature

was 750C, to achieve F i�O.4 = 1 . 0 , no processing was required if

1.5 D process was considered as the minimum lethal i ty . For 2 D

process, the filling temperature required was 80oC.

The microbial and storage stability s�udy of the canned

drink showed that the process rendered the product

microbiologically safe and organoleptically acceptable. The 1.5

D was the minimum safe process for the canned guava drink .

However, a 2 D process is recommended for commercial processing,

when variation of PE concentration in guava is taken into

consideration.

Significant changes during storage of canned guava drink

processed with 2 D and 3 D indicated the effect of heat

treatment levels on the quality of the product. storage

temperature had appreciable effect on the ascorbic acid content,

colour, cloudiness, acidity, pH, viscosity and total soluble

solid. Sensory quality scores were most affected by storage

xi

temperatures rather than the heat processing levels . Thus, a 2

D process and storage at refrigerated terrperatures (SoC) is

recarmended for canned guava drink as it was the most stabl e

during storage .

xii

Abstrak Tesis Yang Dikemukakan Kepada Senat Universiti Pertanian Malaysia Sebagai Memenuhi Syarat

Keperluan untuk Ijazah Master Sains

� PDtPROSESM 'lDHA OM RES'l'ABILAN PEl'fYIMPMAN MI� JltMBJ BA'l'U l:\ALMl TIN

Oleh

IsmK BIN SULAIMAN

Februari, 1993

Pengerusi: Asbi bin Ali, Ph.D.

Fakul ti Sains Makanan dan Bioteknologi

Suatu ketetapan pemprosesan terma untuk rrQnuman jambu batu

dalam tin telah dibuat berdasarkan penyahaktifan enzim

pektinesterase (PE). FE merupakan enzim semulajadi yang

terdapat dalam jambu batu. Kestabilan terma PE di dalam minuman

jambu batu telah ditentukan mengikut kaedah tiub masa

penyahaktifan terma. Kinetik bagi penyahaktifan terma FE

menunjukkan ianya mematuhi tindakbalas tertib-pertama.

Keluk masa penyahaktifan terma (TIT) bagi enzim tersebut

menyerupai keluk masa kenusnahan terma (TOT) bakteria. Dalam

keadaan berasid, kerintangan haba bagi mikroorganisma perosak

1 ebih rendah daripada FE. Dengan demikian, enzim tersebut

xiii

digunakan sebagai asas penilaian perrprosesan terma pengetinan

minunan jambu batu.

Nilai-nilai F dan D bagi PE di dalam �numan jambu batu ( pH

3 . 7) pada 100. 4°C adal ah masing-masing 1.0 min dan O. 67 min.

Nilai F adalah setara dengan 1. 5 D. Data ini digunakan sebagai

asas pengiraan mas a perrprosesan dengan menggunakan kaedah

formula . Ujian penusukan haba telah dijalankan dalam tin

berukuran 300 x 305. Keperl uan perrprosesan bagi suhu awal yang

berl ainan telah ditentukan. Didapati jika pemprosesan dilakukan

berdasarkan kemusnahan �nimum 1.5 D , tiada perrprosesan

diperlukan bila suhu pengisian 7SoC . Bagi pElTQ?rosesan 2 Df suhu

pengisian yang diperlukan adalah SOoC .

Kajian kestabilan mikrob dan penyimpanan minuman jambu batu

menunjukkan pemprosesan tersebut selamat daripada segi

mikrobiologi dan baik daripada segi organoleptik . Pemprosesan

1.5 D adalah tahap minimum yang selamat bagi pengetinan minuman

jambu batu. Walau bagairranapun , pefIi>rosesan 2 D disyorkan untuk

pemprosesan komersial apabila variasi kepekatan PE di dalam

jambu batu diambil kira.

Perubahan bererti serrasa penyimpanan minuman jambu batu

yang diproses pada 2 D dan 3 D menunjukkan kesan paras perl akuan

haba terhadap kuali ti produk . Suhu penyimpanan memberi kesan

xiv

�ang ketara ke atas kandungan asid askorbik, warna, kekel adakan,

keasidan, pH, kelikatan dan jumlah pepejal l arut. Suhu

penyilll?anan rnemberi kesan yang 1 ebih ketara pada skor atdbut

deria berbanding dengan paras pemprosesan haba . Ol eh itu,

PEI'I'Prosesan 2 D dan penyinpanan dingin (SoC) disyorkan bagi

min\lTBIl jambu batu yang ditin . Keadaan ini menghasilkan minuman

yang paling stabil semasa penyinpanan.

xv

0IAPl'ER I

INTROIXJCTION

Guava (Psidium guajava L.) is one of the fruits that has

been cultivated on a commercial scale. There are approximately

1,322 hectares of guava farms in Peninsular Malaysia. Its total

production for the year 1990 was about 25,200 metric tonnes.

About 70% of this was of the Vietnamese variety, and 26% of the

Taiwan variety. The seedless, Glohmsahlee and Kampuchea

varieties constitute around 1% to 2%, respectively (FAMA, 1988;

F;MA, 1989; Malaysia 1989).

Salmah (1985) found that the Vietnamese variety contained

the highest amount of vitarrdn C compared to several other local

guava varieties. Aside fram its high ascorbic acid content, the

flavour of guava is its most distinguishing characteristic.

Guava of the Vietnamese variety was found to be suitable for

processing (Salmah and Suhaila, 1987).

Guava can be processed into various products such as puree,

juice, nectar, jellies, jams, cheese, ketchup, juice powder and

canned product. Guava drink is one of the tropical fruit

beverages that has a good demand in the world market.

1

2

Thermal processing is an important step in guava processing ,

Fruit products such as puree and juices should be processed at

such temperatures and for such time periods as would render them

sterile without impairing their flavour and the nutritional

values. Therefore, it is important to find the optimum time and

temperature relationship required for the pasteurisation of

fruit juices.

Different fruit juices require different amount of heating.

The severity of the heat treatment and the resul ting extension

of shelf life are determined IOOstly by the pH of the product.

Since fruit juices are considered as acid food, severe heat

treatment is not necessary. It is sufficient to process them at

1000C or below (Woodroof, 1986).

Guava contains enzymes such as ascorbic acid oxidase,

polyphenoloxidase, peroxidase and pectinesterase.

Pectinesterase is the most heat stable enzyme in guava pulps

(Garces, 1963; Nath and Ranganna, 1983). In guava pulp (Garces,

1963), the inactivation time of peroxidase was found to be

minimum (7.5 minutes at 650C) and that of pectinesterase l'1as

found to be rraximum (5 minutes at 98oC). The presence of

pectinesterase will hydrolyse the pectin molecules resulting in

sedimentation of the colloidal particles in puree and juices.

3

The enzymes as well as the organisms present in the juice

have to be inactivated in order to avoid undesirable changes

during storage. The�l inactivation is extensively used as it

simultaneously destroys ndcroorganisms (Whitaker, 1972) .

TEfIl'eratures of 700 to 75°C are sufficient to destroy nomal

ndcrobial population. However, to inactivate pectinesterase,

heating at temperatures of 900 to 1000C is required (Desrosier,

1977) .

Since pectinesterase is the most heat stable enzyme, it has

to be considered in guava juice processing. Lund (1975b)

suggested considering enzymes instead of microorganisms in

thermal process evaluation of acid foods. In thermal process

calculation based on enzyme inactivation, the most heat­

resistant enzyme is selected as the basis of the process, just

as the lOCIst heat-resistant pathogenic or spoilage organism is

used for microbial inactivation.

The�l processing of guava pulp has been evaluated based on

destruction of pectinesterase and microorganisms. It was found

that the thermal resistance of pectinesterase was higher than

that of microorganisms (Nath and Ranganna, 1983).

According to Chan (1983), pasteurisation of guava nectar is

best done in a plate heat exchanger and a few seconds at a

temperature of 73.90C or more are usually sufficient. However,

4

Luh (1980a) recommended that flash pasteurisation of guava drink

(nectar) should be done at 82. 2° - 87 . 80C for 60 seconds.

The objecti ve of the study was to establ i sh the thetl'T'al

process requirement for processing of canned guava drink. Since

pectinesterase is the most heat stabl e, the therroostabil ity of

the enzyme was dete�ned and used as the basis for the thetl'T'al

process eval uation . storage stabi lity of the canned guava drink

processed at different heat treatment levels and stored at

different temperatures was also evaluated .

<lIAPl'm II

LITmATORE RlWIBf

Guava

Traditionally, production of fruits in Malaysia has been

aimed mainly to meet the demands of the fresh market. However I

wi th the country's interest in general fruit processing and

development of the food products, the trend has been to upgrade

fruit growing frem the present subsistence level to a more

profit oriented plantation scale. Guava is one of the fruit

items that has been cultivated on a ccmnercial scale (FAMA,

1989). The potential of the guava industry in Malaysia is

excellent (Chan and Tee, 1976).

The guava (Psidi urn guajava L.) varieties found in Malaysia

are au 3, GU 4, GU 5, GU 6, GU 7, GU 8 (Kampuchea), GO 9

(Gl ohmsahlee or Hamsad), GU 10 (Kloontau), Maha 1, Pahang 4,

Serdang Bharu, Lebai Mel aka, Beaumont, Laknow, Indian guava,

Allahabad sateda, Taiwan rormd, Tai wan long, Vietnamese, Hong

Kong Pink, Mal acca seedless, Bentong seedless, Thai seedless and

Indonesian seedless (Chan and Tee, 1976; FAMA, 1989; Malaysia,

1989). However, only a few varieties are grown on large scale.

Based on the guava production in 1987, about 70% was of the

5

6

vietnamese variety, followed by Taiwan variety, 26%. The

seedless, Hamsad and Kampuchea varieties constitute around 1%

to 2%, respectively (FAMA, 1988) .

Biochemdcal Characteristics of Guava

Nutritionally guava is an excellent source of ascorbic acid.

Wilson ( 1980) reported that the average vitarrdn C content was as

high as 336.8 ITg/ 100 9 fruit. By far, the guava is more

nutritious than most of the irrq;>orted fruits (such as apple,

grape, pear, mandarin orange, avocado) in tet'TTG of vitamin C

content (Chan and Tee, 1976) . Ascorbic acid is not distributed

uniformly in the fruit tissues. The skin and outer flesh of

guava fruit contain most of the ascorbic acid (Agnihotri et al.,

1962; El-Zorkani, 1998; Salmah, 1985; Braverman, 1963) .

High vitarrdn C content is an important factor in selecting

fruit types for processing. Salmah ( 1985) found that the

Vietnamese variety contained the highest amount of vitamin C

(180 rng/100 9 fruit) compared to several other local guava

varieties. Ripening the fruits to a yellow-green stage would

further increase the vitamin C content (Salmah et ai., 1988).

The total sugar val ues in guava range from 4.3% to 9.0%

(Wilson, 1980). Glucose is the predarrnnant sugar, followed by

Table 1

Nutrient and Proximate Composition of Guava Fruit ---------------------------------------- ----------------------------------------------------------

Composition

Proximate Composition

Energy Moisture Protein Fat Carbohydrate Fibre Ash

Mineral s

Calcium Phosphorus Iron Sodium Potassi':lffi

Vitamins

Ascorbic acid Carotene RE B1 B2 Retinol Niacin

* Average

46 . 0 kcal 81 .2 9

1 . 1 g 0 . 2 9

10 . 0 9 6 . 8 9 0 . 7 9

33 . 0 rrg 15 .0 rrg

1 . 2rrg 23 .0 rrg 29 . 0 rrg

152 . 0 rrg 60 .0 J.lg 10 . 0 .llg

0 . 1 rrg 50 . 0 .llg

o .llg 1. 1rrg

================================================ *per 100 g edible portion

Adapted from Siang et al. (1988)

7

8

fructose and sucrose ( Salmah, 1989) . The total soluble solids

of the mature and ripened Vietnamese variety range fran 6% to 7%

( Salmah et al., 1988) . The predaninant organic acids in a

white-variety guava was citric acid . Mal ic and glycolic acids

present in much l esser amounts (Fang, 1965) . The acidity of

guavas range fran 0 . 33% to 0 .99% and the pH fran 3 .0 to 5 .4 .

The JOOre acid fruits are better for processing than sweeter

fruits (Wi lson, 1980; Luh, 1980a; Salmah, 1989) .

The acidity plays an important part in the fl avour

acceptance of the fruit and its processed products . Because of

the l ow acid, high sugar and total soluble sol ids contents, the

sugar-acid ratio and brix-acid ratio of guava are general ly high

but quite variabl e (Wilson , 1980) . For the Vietnamese variety,

the highest val ue of the brix-acid ratio was 16 . 3 , obtained

after ripening the fruits to a yel l ow-green stage ( Salmah et

al. , 1988) .

Guava is a potential source of pectin ( Pruthi et al., 1960;

Verma and Srivastava, 1966; Shastri and Shastri, 1975; Wilson,

1980). Values for total pectin range fran 0 . 5% to 1 . 8% . The

methoxyl content of purified guava pectin is rel atively l ow

( about 55%) , but pectin with characteristics sirrdlar to those of

commercial pectins has been prepared. The yield of pectin

t t

9

(about 11\ on dry weight basis) is larger than yields fran

citrus or pasion fruit peel (Muroki and Saint-Hilaire, 1977).

Good gel can be obtained at 65\ sol ubI e sol ids at pH 2 .1 to 2 .4

(Luh, 1980a).

The function of pectin in fruit juices is to hold the

colloidal material together and maintain them dispersed in

solution, thus contributing to the haze (commonly referred to as

, cloud') or body of the juice. Other product characteristics

affected by pectin include viscosity, colour stability and

flavour.

The enzyme, pectinesterase is also present in guava

(Rieckehoff and Rios, 1956; Garces, 1963; Shastri and Shastri ,

1975). They found most of the activity of the enzyme

associated with the peel and considerably less activity

associated with the flesh. The enzyme activity was fairly high

at immature stage, dec�eased considerably at mature stage, again

increased as the fruits became ripe. The acti vi ty was maxinrum

at overipe stage (Pal and Selvaraj, 1979). With the presence of

pectinesterase, pectin molecules can easily be hydrolysed

resul ting in sedimentation of the colloidal particles (cloud

loss) in frui t puree and juices. In guava processing,

pectinesterase has to be iHactivated.