PROCESSING AND QUALITY EVALUATION OF SOURSOP (ANNONA MURICATA L) NECTAR

PROCESSING AND QUALITY EVALUATION OF SOURSOP ( A N " A MURZCATA L) NECTAR

MARY PETERS', NEELA BADRE* and EDWARD COMISSIONG'

'Food Technology Unit Faculty of Engineering

'Department of Food Production School of Agriculture

Faculty of Agriculture and Natural Sciences Universiq of the West Indies St. Augustine, Trinidad, W.I.

Accepted for Publication July 26, 2000

ABSTRACT

Soursop (Annona muricata L) nectar was processed from pasteurized unstored or pasteurized frozen pulp. Nectars of pH 3.6-3.7 with 0.1 % xunthan gum were producedfrom either 6" or 8" Brix pulp and increased to 13" or 15 " Brix by addition of sucrose. The &feet of storage (4C or 30C for 12 weeks) was investigated on nectar pH, titratable acidity FA), browning, consistency, microbes and sensory attribute. At 8 weeks of storage, consistency was similar in all nectars, except for thinning of nectar from pasteurized, unstored pulp (8"- 15 "Brix) stored at 4C for 12 weeks. Nectars produced from frozen pulp had significantly @ C 0.01) lowerpH and higher TA than nectars from unstoredpulp. Browning increased in all products except nectar produced from unstored 8" Brix pulp adjusted to 15" Brix and then stored at 4C. This nectar was most highly ranked and had an overall rating of being liked moderately to liked very much afrer 12 weeks of storage. Microbial growth occurred in all nectars between 8-12 weeks of storage.

Corresponding author. TEL: 868-662-2002 ext. 3211 or 2090; FAX: 868-663-9686; E-mail: [email protected] or badrimCarib-1ink.net

Journal of Food Quality 24 (2001) 361-374. All Rights Reserved. "Copyright 2001 by Food & Nutrition Press, Inc., Trumbull, Connecticur. 361

362 M. PETERS. N. BADRIE and E. COMISSIONG

INTRODUCTION

Agricultural diversification is important for the Caribbean in the 21st century. Some under-exploited tropical fruits such as soursop (Annonu muricutu L) can be utilized in the development of exotic processed products, thus adding value to tropical fruits, reducing import substitution and increasing foreign exchange. A 1994 survey conducted on fruit processing enterprises in the Caribbean revealed that the large processing enterprises use imported juices, pulps and concentrates primarily from extra-regional sources to produce a range of carbonated and noncarbonated beverages in cans, bottles or aseptic packaging (Francis 1995). Soursop is known in most Spanish-speaking countries as guanabana (Morton 1987) and is found throughout the Caribbean, primarily in humid lowlands usually as a backyard garden or small farm crop (Sanchez-Nieva et ul. 1953; Rajack 1990). The major fruiting season is from January to April, but the trees bear fruits throughout the year (McComie 1987).

Soursop fruit are highly perishable, being very susceptible to bruising and decay. Thus, they must be handled with extreme care. The soursop juice can be exported in its fresh form because it retains its flavor even after deep freezing (Satney 1994). Sanchez-Nieva et ul. (1953) reported that the pulp darkens on storage at 4-16C which limits its distribution and utilization. The soursop pulp is white, juicy and highly aromatic and thus ideal for the preparation of high-quality nectars (Sanchez-Nieva et al. 1953; Morton 1987). Soursop fruit are eaten fresh or made into pulp, drinks, jelly, custards, syrup and ice cream (Barbeau 1998; Rajack 1990).

The commercial utilization of this fruit is hindered by the lack of adequate processing techniques. The objectives of our research were to utilize both pasteurized, unstored or pasteurized frozen soursop pulp in the production of nectars and to investigate the effects of storage on the nectar quality attributes.

MATERIALS AND METHODS

soursop Processing Fully mature, firm and “Bums” soursop fruits (Annonu muricutu L) were

purchased at a Central Market, Trinidad, W.I. The fruits were ripened at room temperature (30C) for 2 days to allow full flavor and aroma development (Rajack 1990). The fruit were washed and sanitized with 20 ppm NaOCl at pH 6.5, air-dried and allowed to ripen. Fruit were stored at 5C until required for pulping. As outlined by Camejo (1994) the fruit were hand peeled, cored and pulped in a Dixie paddle pulperlfinisher (Model 18, Dixie Canning Company, Athens, Georgia) utilizing screens of 3.18 mm and 0.69 mm. Seeds were loosened from the adhering pulp by agitation, as the seeds contained a

QUALITY OF SOURSOP NECTAR 363

yellow-nondrying oil which is an irritant poison (Morton 1987). Losses (%) of peel with core and pulp yield were measured on three batches of 25 soursop fruit. The pulp was either heat-treated at 90C for 1 min in 1L tubs with 100 ppm of sodium metabisulphite, cooled with water (30C), then frozen at -18C or the fresh pulp was given no prior heat treatment (pasteurized, unstored) and utilized immediately in nectar production. Figure 1 outlines the general steps in the processing of the experimental nectar. All the nectars were processed to meet minimum specifications in terms of fruit content and "Brix (Codex Alimentarius 1992). The pH of the nectar was adjusted with 0.4% citric acid to 3.6-3.7, heated to 9OC for 1 min, sodiummetabisulphite (100 ppm) added, filled in green bottles (300 mL) and pasteurized in boiling water for 15 min followed by cooling in water (30C). Aswan et al. (1980) recommended filling soursop drink in green bottles to minimize quality changes.

Trials

In the preliminary work, both unpasteurized, unstored soursop pulp and pasteurized frozen soursop pulp (-182) were used. The frozen pulp was thawed in cool running water (7-IOC) prior to nectar production. Soursop pulp was diluted with distilled water to 6" or 8" Brix and the total soluble solids (SS) increased to 13" or 15" Brix with granulated sucrose. After two days of storage at 30C, nectars produced from unpasteurized, unstored pulp, developed a distinct and fermented (alcoholic) odor, changed color from yellowish-white to brown and gas bubbles were observed in the bottles. Nectar treatments produced from pasteurized, frozen pulp were 6"-13" Brix (T,), 6"-15" Brix (Td, 8"-13" Brix (TJ and 8"-15" Brix (T4) were subjected to sensory evaluation.

Based on the preliminary trial, the two most highly ranked nectars from sensory evaluation were T2 and T4 modified by the addition of xanthan gum (0.2%) to prevent separation into layers. These nectars were either produced from pasteurized unstored pulp by modification of T, (T,) and T4 (T6) or from pasteurized, frozen pulp by modification of T, (T,) and T4 (T,). Based on the sensory means from T-test, nectar T, (8"-15" Brix) was selected for further experimentation.

Experimental

In the final experiment, nectar T8 was modified by reducing xanthan gum from 0.2% to 0.175, as panelists had indicated that nectar with 0.2% xanthan gum was too thick. Nectars were produced either from unstored pasteurized pulp of 8"-15" Brix, (T,F) or pasteurized frozen pulp of 8"-15" Brix (T,S) as shown in Fig. 1. The bottles were stored either at 30C or 4C for 12 weeks. Each treatment was replicated. Two cases each consisting of 24 bottles (300 mL) were produced from each treatment.

364 M. PETERS, N. BADRIE and E. COMISSIONG

ripe fruits -1

wash in chlorinated water -1

hand-peelhemove core .l

weigh peel.core.pulp -1

sieve pulp through pulper/finisher -1

store at 4 c' -1

pasleurise pulp at 90 C for I niiii -1

cool pulp (30 c \WtCI) -1

-1 thaw (4 C )

-1 adjust pH with citric acid

-1 adjust "Briu

-1 add xanthan gum

-1 heat to 90 C for 1 niin

-1 add sodiuni nietabisulphite

-1 f i l l hot in bottles

-1 hot -water pasteurise

-1 invert bottles

-1 cool

freeze pulp ( - ISC ) \

FIG. 1 . PROCESSING STEPS IN THE PRODUCTION OF SOURSOP NECTAR

Analyses

Analyses for pH, SS, "Brix, consistency, browning and residual sulfur dioxide were performed (triplicate) for final treatments according to the method of Ranganna (1977). The pH was measured using a laboratory micro-processor pH meter (H1 8521, Hanna Instruments, Inc., Milwaukee, WI) and total SS


"Brix using a Reichert-Jung hand-held refractometer (0-50 "Brix, Cambridge Instruments, Inc., Lexington, MA). The method of Lamb (1961) was used to measure titratable acidity (TA), which was determined as % citric acid.

Consistency of nectar (equilibrated at 30C) was a measure of flow of the nectar in distance (cm) traveled in 5 s down an inclined plane using a Bostwick consistometer (Central Scientific Co., Fairfax, VA).

The degree of browning was determined by measuring absorbance at 400 nm using a UVNIS spectrophotometer (Lambda 3B UV/VIS spectrophotometer , Perkin Elmer, Inc., Norwalk, Conn) of 4 g samples diluted to 100 mL with 60% ethanol and left for 12 h then filtered (0.22 pm).

Microbiological evaluation was performed on pulp and nectars T,F and T,S that were stored at 30C and 4C at week 8 and week 12 of storage. The bottles were swabbed with 95% ethanol prior to removal of samples for pour plates of Plate Count Agar (PCA, Oxoid Hampshire, UK) and Potato Dextrose Agar (PDA) and incubated at 32C for 48 h (Andrews 1992).

Sensory Evaluation

Sensory evaluation was conducted in individual booths at the Food Technology Laboratory, Department of Chemical Engineering, University of the West Indies. The panelists were students and staff members who had been involved in sensory evaluation and were familiar with the sensory attributes of other types of nectar. Sensory evaluation was conducted at week 8 and 12 of storage.

In the preliminary work, 24 semitrained panelists ranked nectar products from frozen pulp T,, T,, T, and T4 (#I most liked sample, #4 least liked sample) for taste, odor, mouthfeel and color. The two most liked samples were selected for further experimentation. For product research and development purposes, where many samples must be ranked, 8-25 semitrained members can provide a preliminary estimate of consumer response (Kramer and Twigg 1979). Ranking has the advantage of simplicity in instructions to subjects (Lawless and Heymann 1998) and was used as a form of preference testing with multiple samples.

In the second trial, 16 semi-trained panelists rated products T,, T,, T, and T, on a 9-pt hedonic scale (9-liked extremely, 5-neither liked nor disliked, 1- disliked extremely) as described with comments by Lawless and Heymann (1998).

The experimental nectars, T,F and T,S, stored at 30C and 4C were ranked at week 8 of storage (#I most liked, #4 least liked) by a panel of 17 semitrained individuals. At week 12 of storage, hedonic sensory evaluation was conducted on all experimental nectars based on the 9-pt scale (as described before).


Statistical Analysis

T-test, Friedman test was performed on ranks. Analysis of variance (MINITAB 11.21, 1996) was carried out to determine differences in sensory scores given to the nectars. Data means were separated by least significant difference (LSD p c0.05).

RESULTS AND DISCUSSION

Pulp Analysis

The average yield of pulp from peeled fruit was 43.8 2 1.9% [MSOfficel]. It was lower than the 69.5% average obtained by Sanchez-Nieva er al. (1970) and the average of 53.68% obtained by Camejo (1994). The peel with core loss ranged from 25.7 to 48.7% which was higher than the range of 22.6-37.1% obtained by Sanchez-Nieva er al. (1970).

Unstored pulp (no initial heat treatment) had a lower pH, higher microbial counts and more browning than the pasteurized frozen pulp (Table 1). The unpasteurized, unstored pulp had total count of microorganisms ranging from 7.0 x -5.1 x lo6 CFU mL-’ and yeasts and molds were 4.4 x 1051.0 x 10s CFU mL-l. The total microbial counts of unstored pulp were higher than those obtained by Camejo (1994) of 2.0 x 10s - 2.9 x lo5 CFU mL-’ and Umme ef al. (1997) of 1 .O x lo2 CFU d-’. Also, the yeast and mold counts were higher than those obtained by Umme er al. (1997) of 8.5 x 102 CFU mL-’. There was a decrease in the microbial load upon pasteurization of the pulp.

TABLE 1 . CHARACTERISTICS OF SOURSOP PULP

Treatment Abs PH ”Brrx Total count, Ycast and CFU mL ’ molds. CFU

mL ’ 5 1x106 1 0 x 1 0 ~

FP 0 022 4 06-4 1s 1.1 5-17 j 2 oUio4- 2 4x10’- 4 0vlOJ 2 8 X 1 O 2

SP 0 019 445-449 145-174 3 4 ~ 1 0 ’ - I 0x10’-

F 0 024 3 88-4 19 19 5-17 5 7 Iu10‘- 4 4uIO4-

3 7x10’ I9uiO’

F - unpasteurized, unstored pulp FP - pasteurized, unstored pulp, SP - pulp stored at -18C for 4 weeks


Sensory Evaluation

Table 2 indicated that the first and second ranked overall treatments were T4 (8"-15" Brix) and T, (6"- 5" Brix), respectively. Treatment T4 was most liked for its odor, and mouthfeel, while it was second most liked for taste and color. Nectar T, was included for further work as it was the second ranked treatment overall and most liked for its taste. Morton (1987), describes the aroma of the pulp as somewhat 'pineapple-like', with a unique musky, and subacid to acid flavor. Dilution of the nectar pulp in water was required as a lower content of fruit ingredient is desirable in nectar production due to the high pulp content and the strong flavor of soursop. Also, it resulted in a greater volume (production) of nectar.

TABLE 2. SENSORY EVALUATION OF SOURSOP NECTAR FROM PASTEURIZED

FROZEN NECTAR (PRELIMINARY TREATMENT)

Treatment Taste Odor Color hloutlifecl Overall

T I 2 75a 3 50a 3 69a 3 50a 3 36a

T2 1 50c 2 50b 2 8 l b 2 15b 2 39b

T3 2 25b 2 50b 1 56d 2 25c 2 45b

T4 2 25b 150c 2 19c I 50d I 8 6 c

LSD 0 48 0 56 0 49 0 46 0 54

Illearl

Means with the same letters in columns are not significantly different (p>O.O5) Rank: #1 most L i e d to #4 least liked

Number of panelists: 24 X 2 replications T1 6"-13"B; T2 6"-15"B; T3 S0-13"B; T4 8"-15"B

There were no significant differences (p>O.O5) in mouthfeel and taste between treatments T,, T,, T, and T,. Based on the hedonic scores for mouthfeel, T8 (8"-15"Brix) had the highest score 7.69 compared with T, (7.60), T, (7.13) and T, (7.05), with all treatments being liked moderately to liked very much. Hence, treatment T, was chosen for further work based on its sensory score for mouthfeel. The addition of 0.10% xanthan gum resulted in a desirable mouthfeel and also prevented separation of nectar pulp during storage. Xanthan has high thermal stability, is stable in acidic systems, is ideal for stabilizing suspensions and is useful as a thickening agent (BeMiller and Whistler 1996; Belitz and Grosch 1999).

368 M. PETERS. N. BADRIE and E. COMISSIONG

Analyses

A product's shelf-life can be determined based on the exhibition of unacceptable physical, chemical, microbiological or sensory characteristics for consumption (Vargas ef al. 1989). Thus the determination of a product's shelf- life is important in that product's development. There were no differences (p>O.O5) in pH (Table 3) or TA (data not shown) of nectar due to storage temperature or length of storage. Nectars produced from frozen (T8S) pulp had significantly (pCO.01) lower pH and higher TA (0.649-0.747% citric acid) compared with the pH or TA (0.213-0.286% citric acid) of nectars from unstored pulp (T8F). More citric acid was required to adjust the pH of nectar from frozen pulp to 3.6-3.7 compared with nectars from unstored soursop pulp. Table 1 indicates that the frozen soursop pulp had a higher pH, thus requiring a greater addition of citric acid for pH adjustment compared with the unstored soursop pulp. Aswan er al. (1980) found that there was no change in pH of soursop drink stored in refrigerated condition but a slight increase in samples that were not refrigerated. The action of pectinesterase is thought to have contributed to a decrease in the pH of papaya puree by deesterification of the carboxyl groups of pectin (Jagtiani er al. 1988). On storage, there may have been incomplete inactivation of pectinesterase in soursop nectars, with more pectinesterase activity in nectars from frozen pulp, thus resulting in lower pH of nectars compared with nectars from unstored pulp. The acid component of soursop is composed of 2 parts malic acid and 1 part citric acid (Brekke 1992). Aswan et al. (1980), found that there was no change in TA in soursop drink under refrigerated storage but there was a slight decrease in samples that were not refrigerated.

TABLE 3. EFFECT OF TREATMENT AND STORAGE ON pH OF NECTAR

Storage ( s ~ k s ) Treatment 0 3 6 9 12 TsF, 4 C 3 64a 4 14a 4 18a 4 29a 4 Ola TRF, 30C 3 64a 4 30a 4 I la 4 3 l a 3 97a TsS, 4 c 3 60a 3 83h 3 55h 3 83b 3 50h TnS. 30 C 3 60a 3 78b 3 74b 3 70b 3 47b LSD 0 15 0 21 0 22 0 29 0 29

Means with the same letters in columns are not significantly different (p < 0.05) T8F nectar (S0-15'B) made from pasteurized, unstored pulp, and nectar stored at 4C T,F nectar (8a-150B) made from pasteurized, unstored pulp, and nectar stored at 30C T,S nectar (8"-15"B) made from pasteurized, frozen pulp, stored at 4C T,S nectar (8"-15"B) made from p a s t e u d , frozen pulp, stored at 30C Average of 6 readings


All treatments had a constant 15"Brix during the 3 months of storage. No variation was also observed by Aswan et al. (1980) during storage of soursop drink. Soursop pulp contains 18% carbohydrate, of which sucrose is the predominant sugar, 61 %, glucose, 22%, and fructose, 17% (Brekke 1992). The general standard for fruit nectars states that in no case shall the composition of the fruit ingredients be less than 25% m/m (by mass) and SS shall not exceed 20% m/m as determined by a refractometer at 20C and measured in "Brix (Codex Alimentarius 1992).

At 8 weeks of storage, the consistency (apparent viscosity) of all nectars was constant at 22 c d 5 s. There was a slight decrease in consistency (22.5 cm/5 s) only in nectar T,F at 4C on week 12 of storage. The enzymes pectinesterase has been found in soursop pulp (Hui 1992). Pectinesterase is one of the most heat-resistant enzymes found in soursop fruit, which could cause gelation and precipitation of pectin in puree and juice with resulting loss of cloud. The slight thinning of this nectar by week 12 of storage could have been due to the presence of some pectinesterase activity.

By week 12 of storage, only nectar T,F produced from pasteurized unstored pulp and stored at 4C remained unchanged in browning during storage (Table 4). Initially, though not significantly different (p>0.05), nectars from unstored pulp were more brown than nectars produced from frozen pulp. However, on storage, nectars from the stored pulp became more brown than those from unstored pulp. Enzymatic browning is the discoloration that results when

TABLE 4. CHANGES" IN BROWNING OF NECTAR

Storage (wks) Treatment WkO Wk3 Wk12 TEF, 4C 0 026a 0 026a 0 026a TEF, 30C 0 026a 0 035b 0 045c TES, 4C 0 Ol5a 0 017a 0 066b T8S. 30C 0 014a 0 012a 0 067b LSD 0014 0 010 0 01 1

"Means (56 absorbance) followed by the same letters in columns are not significantly different (p CO.05). T,F nectar (8"-15"B) made from pasteurized, unstored pulp and nectar stored at 4C T,F nectar (8"-15'B) made from pasteurized, unstored and nectar stored at 30C T,S nectar (8'-15'B) made from pasteurized, frozen pulp and nectar stored at 4C T,S nectar (8"-15"B) made from pasteurized, frozen pulp and nectar stored at 30C Average of 6 readings


monophenolic compounds of plants, in the presence of atmospheric oxygen and polyphenol oxidase (PPO) are hydroxylated to Odiphenols and the latter oxidized to 0-quinones (Mayer and Hare1 1979; Vamos-Vigyazo 1981; McEvily et al. 1992). It is expected that there would have been some inhibition of PPO by the pH of the soursop nectar since optimum activity for PPO activity is between PH 5 and 7. Also, the enzyme is heat labile (Institute of Food Technologist's Expert Panel on Food Safety and Nutrition 1993). There was no residual sulfur dioxide in any of the nectars at week 12 of storage (data not shown). Sulfiting agents are added to foods to perform various functions (Taylor et al. 1986; Lindsay 1996). Ascorbic acid, sodium bisulfite and thio compounds prevent browning due to the reduction of the initial product U-benzoquinone, back to the substrate, thereby preventing melanin formation (Whitaker 1996). It is expected that as sodium metabisulphite was depleted, browning occurred, since some enzymes may have still been active. To minimize browning in soursop nectars, we recommend filling of bottles under vacuum and the addition of an antioxidant, such as ascorbic acid.

Microbial Analysis

Nil growth (< 10 CFU mL") in total microbial counts or yeasts and molds was recorded at week 8 of storage for all treatments. Between weeks 8 and 12 of storage, nectars from frozen pulp stored at 4C or 30C had higher total microbial counts, ranging from 2.0 x lo3 - 1.5 x lo4 CFU mL-' compared with nectars produced from unstored pulp of 1.0 x lo2 - 2.0 x 10' CFU mL-'. Growth of yeasts and molds was observed only at week 12 of storage, with more growth in nectars stored at 30C (1 x lo3 - 1 x lo', CFU mL-') than in nectars stored at 4C (2 x l@ - 1 x lo3 CFU mL-I). Bacteria capable of growing in food at pH 4.6 or below can be regarded as aciduric or acidophilic (Ray 1996).

Sensory Evaluation on Storage

Based on ranks for overall liking of nectars, T,F which was stored at 4C was significantly (p<0.05) most liked, followed by T,F stored at 30C, T,S stored at 4C and T,S at 30C, at week 8 of storage (Table 5 ) . Nectars produced from unstored pulp were liked more than those produced from frozen pulp for color, taste and mouthfeel. Panelists commented that nectars made from frozen pulp were very acidic, which were reflected in the pH values (Table 3) and TA. By week 12 of storage only nectar T8F which was stored at 4C had an overall score of 7.5 (liked moderately to liked very much). The sensory attribute scores given to this product were 8.00 (liked very much) each for odor, mouthfeel, and taste but color had an average score of 6.00 (liked slightly). All the other nectars (T8F stored at 30C or T,S stored at 30C and 4C) had scores for odor,

QUALITY OF SOURSOP NECTAR 37 1

Nectars TpF,4C rlt., 3UC T p S . 4C TxS ,30C LSD

taste, mouthfeel, color and overall quality of less than 5.00 (dislike category). Panelists indicated that the soursop nectars were very aromatic and distinctive. Iwaoka ef al. (1993), identified a number of volatiles in a methylene extract of ripe soursop (day 5 or 6) with methyl butanoate, methyl (E) - 2 - butenoate, methyl hexanoate, methyl (E) - 2 - hexenoate being present in the highest amounts.

Odor I Color Taste Mouthfeel Overall rank 225b 165, I25a l2Sa I 59a

175a 2 88c 3 ooc 3 ooc 2 66b 3 25bc 3 88d 3 75d 3 75d 3 67c 0 52 0 52 0 57 0 54 0 45

2 75bc 2 13b 2ob--- 2UUb - 2 2 % -

TABLE 5. RANKS FOR NECTAR TREATMENTS ON WEEK 8 OF STORAGE

"Means with the same letters in columns are not significantly different (p < 0.05) Rank: #1 most liked to #4 least liked T8F nectar (8"-15"B) made from pasteurized, unstored pulp and nectar stored at 4C T,F nectar (8"-15"B) made from pasteurized, unstored pulp and nectar stored at 30C T,S nectar (8"-15"B) made from pasteurized, frozen pulp and nectar stored at 4 C T,S nectar (8"-15"B) made from pasteurized, frozen pulp and nectar stored 30C Number of panelists-17

CONCLUSIONS

At week 8 of storage, soursop nectar T,F (8" Brix increased to 15" Brix) produced from pasteurized unstored soursop pulp with inclusion of 0.1% xanthan gum and stored at 4C was ranked as being liked the most. It had an overall score (7.5) of being liked slightly to liked moderately at week 12 of storage. Panelists rated taste, mouthfeel and odor as being liked very much (8.00 each) while color (6.00) was neither liked nor disliked. At 8 weeks of storage consistency was similar in all nectars. There was a slight thinning and no change in browning of nectar T,F stored at 4C of week 12. Growth of microorganisms was observed after week 8 of storage for all nectars with lower microbial counts recorded in nectars stored at 4C than those stored at 30C.

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PROCESSING AND QUALITY EVALUATION OF SOURSOP (ANNONA MURICATA L) NECTAR

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