Abstract: Restructured mango gel was prepared by alginate texturization of mango pulp using sodium alginate, glucono-δ-lactone and calcium hydrogen orthophosphate. The requisite quantities of the chemical additives were mixed, added together to the neutralized mango pulp in a mixer and allowed to set at 4°C for 16 h. The mango gel was cut into 1.5 cm cubes and stabilized by Hurdle Technology (HT) through adjustment of water activity to 0.89, pH 4.4 and pasteurization in polypropylene pouches. Storage studies were carried out in foil laminate packs at three different temperatures i.e. 4°C, RT (20-33°C) and 37°C and evaluated for chemical parameters, Hunter colour values as well as microbiological and organoleptic characteristics. Studies showed that HT preserved texturized mango gel could be kept in acceptable condition up to 12 months at RT and 6 months at 37°C. During storage a considerable reduction in sulphur dioxide, carotenoids and an increase in acidity were observed. Decrease in L, a, b values was also observed in all the samples stored under different temperatures. The product was also found to be microbiologically stable and safe up to 12 m.
INTRODUCTION
Mango is one of the most popular and highly prized desert fruit of the tropics. It is liked by the people for its rich luscious aromatic flavour and a delicious taste with evenly blended sweetness and acidity. Nutritionally, it is a rich source of carotenoids, organic acids, polyphenols, minerals etc. India produces 10.8 million metric ton of mango annually (FAO, 2004). Most of the mangoes are consumed as fresh. In India both raw and ripe mangoes are used for making various product like pickle, mango bar, canned mango slices, canned mango pulp, nectar squashes, ready-to-serve beverage, juice, jam, osmotically dehydrated mango etc. (Ramteke et al., 1999). However the major product processed for export is canned mango pulp followed by mango chutney and canned slices in brine. Although the export trade on mango products worldwide is meager compared to orange, pineapple etc, the research efforts on extension of shelf life and value addition to mango as well as improvement in quality are continued. Effect of various coating materials and formulations on the shelf life of mangoes during storage has been evaluated (Thittoa et al., 2002; Nuzab et al., 2005). Martinez-Ferrer et al. (2002) conducted studies on modified atmosphere packaging of minimally processed mango and reported that the shelf life could be extended upto 25 days at 5°C with good colour, flavour, aroma and texture. Effect of different hydrocolloids on the quality of mango product has been investigated by Gujral and Brar (2003). Jaya and Das (2004) reported the effect of maltodextrin, glycerol mono stearate and tricalcium phosphate on vacuum dried mango powder. Quality attributes of mango soy fortified yoghurt with added stabilizer has been discussed by Pradyuman and Mishra (2004). In spite of so many products from mango available commercially the scope for development of value added products is quite open considering the enormous production of different varieties of mango in India. Development of value added product by alginate texturization of fruit pulp has been known for many years (Luh et al., 1976; Hannigan, 1983; Glicksmann, 1983). Sodium alginate is chosen as a suitable hydrocolloid because of its ability to form thermostable gel which facilitates further processing by thermal treatment. Alginate texturization of fruit pulp and juices has already been reported (Nussinovitch and Peleg, 1990; Kaletune et al., 1990; Moquet et al., 1992). Fruit-alginate interactions and their effect on gel formation are reviewed with regards to the use of these gels in novel, restructured fruit product (Mancini and McHugh, 2000).
However most of these studies involved textural characteristics of the gel rather than their physico chemical, sensory and shelf stability characteristics.
The objective of the present investigation was to develop a shelf stable product from mango by alginate texturization and subsequent processing to evaluate the same during storage under different temperature conditions.
MATERIALS AND METHODS
Processing
Canned mango pulp (Badami variety) in tin cans (A1) was purchased from the
local manufacturer (M/s Globe Foods Limited, Mysore) and used for texturization
at our laboratory Mysore, INDIA. Total soluble solids, pH and acidity of the
pulp were 18° Brix, 3.9 and 0.28%, respectively. The process flow diagram
for restructuring of mango pulp as well as its stabilization is given in Fig.
1. In brief the process consist of neutralization, addition of chemical
additives i.e. sodium alginate (Across) glucono-δ-lactone (Sigma) and calcium
hydrogen phosphate (Across) and blending. The mixture was allowed to set for
16 h at 5°C, cut into 1.5 cm cubes and stabilized by hurdle technology as
described by Jayaraman et al. (1997) by soak infusion for 4 h in a sugar
solution (70° B) containing citric acid and potassium metabisulphite. The
osmosed cubes after draining were packed in polypropylene pouches (300 G; 50
g) and pasteurized for 20 min in boiling water.
| Fig. 1: | Flow diagram for the preparation of resturctured mango gel |
Analytical Methods
Moisture was determined by drying in a vacuum oven (Pathak electrical, Mumbai)
at 60°C. Total soluble solids were measured using hand refractometers (Erma,
Japan) of 0-28° B and 28-54° B ranges and expressed as °Brix. pH
of the macerated sample was measured with a digital pH meter (Century, India).
Titratable acidity was determined by titrating against 0.1N NaOH upto a pH of
8.4 using digital pH meter (AOAC, 1980). Reducing and total sugar contents were
determined by Lane and Eynon method as reported by Ranganna (1986). Total carotenoids
in the sample were determined as described by Ranganna (1986). Sulphur Dioxide
was estimated by AOAC (1980) method using rosaniline hydrochloride.
Colour parameters of the product were measured by a colour difference meter, Chromoflash (Model 2810, Ashco, India) using 10° standard observer and illuminant D65 and expressed as L, a, b, c and h where L represents lightness index; a and -a, redness and greenness, respectively; b and-b, yellowness and blueness, respectively; c and h, chroma and hue according to Hunter L, a, b system.
Storage Studies
The inpack pasteurized samples were further packed in paper (60 gsm)-aluminium
foil (0.02 mm)-polyethylene (75 μ) (PFP) laminated pouches and kept for
storage under three different temperature conditions of low temperature (0-5°C),
ambient temperature (RT, 20-33°C) and 37°C. Samples were taken out every
two months and subjected to sensory evaluation and physico-chemical analyses.
Microbiological analysis initially and during storage for Total Plate Count
(TPC), coliform, spores, yeast and mold was carried out, using the methods recommended
by APHA (1976).
Sensory Evaluation
Sensory evaluation of each sample at each time interval, from either of
the storage temperature conditions was carried out by ten-semi trained panelists
drawn from the scientific staff of the laboratory for colour, aroma, taste,
texture and overall acceptability using a nine point hedonic scale as recommended
by IS 6273 (1971) where 9 was equivalent to like extremely and 1 dislike extremely.
Statistical Analysis
The data obtained from both the physico-chemical analysis and the sensory
evaluations were analysed statistically for analysis of variance (ANOVA) using
a Completely Randomized Design (CRD) and Least Significant Difference (LSD)
at p≤0.05 using Software (Stat Soft, USA).
RESULTS AND DISCUSSION
Preliminary experiments on the basis of trial and error involving sensory evaluation showed that 90 % pulp was suitable to obtain a gel of acceptable quality from alginate texturized mango pulp. Though it was observed that the gel formation took place at all level of mango pulp, we have attempted a higher pulp level to obtain a gel product with good flavour and taste characteristic product. So that maximum pulp level was used for the gel formation. In our experiment we found that using high level of pulp (90%) did not affect the gel strength as was observed by Mouquet et al. (1992). However, the concentration of alginate was kept at 1% considering the texture of the gel so that it can withstand thermal stabilization during further processing. The process for gel formation depended on a controlled reaction where soluble sodium alginate reacted with sparingly soluble calcium salt under conditions of suitable pH and and concentration. Rate of reaction is very critical for smooth gel formation. Rapid lowering of pH as well as release of calcium generally produces the grainy texture. The choice of calcium salt is also very important (Glicksman, 1969). Hence in our process neutralization, addition of glucano-δ-lactone and calcium hydrogen orthophosphate facilitated in obtaining a smooth as well as firm gel.
The gel was subsequently stabilized by hurdle technology using these hurdles viz., pH, aw and in pack pasteurization. A pH just below 4.37 was achieved using citric acid in the soaked solution for 2½ h. During the same period the aw 0.84 was achieved. The soaked material was pasteurized in polypropylene pouches for 20 min, so that the integrity of the restructured cubes was not affected. Restructured mango gel when processed by hurdle technology was found to have water activity of 0.89 and pH 4.37. It was reported earlier by Kaletune et al. (1990) as well as Nussinovitch and Paleg (1990) that large quantities of fruit pulp produced weak gel and were not texturally very effective. Mouquet et al. (1992) adjusted the brix level of the pulp used for gelation at 50°C. So that the high soluble dry matter content was achieved which could reduce the cost of drying stage if required and process was carried at higher temperature. However in our investigation we have achieved the gel formation at 20° brix at low temperature and utilize the same gel for further stabilization by soaking in a 70° brix sugar solution. The °brix of the final stabilized gel was found to be 45°, so that the flavour of mango is not affected much.
It was observed that during 12 months storage at 5°C, RT (20-33°C) and 37°C, moisture, pH, sulphur dioxide and carotenoids decreased whereas °brix and acidity increased steadily (Table 1). Changes in these parameters were highest at 37°C followed by RT and 5°C. Increase in acidity and lowering of pH might be due to liberation of acid from glucono-δ-lactone in water. Reduction in moisture content in the sample may be due to syneresis which in turn increased the °brix. Loss of sulphur dioxide was found to be drastic particularly at RT and 37°C. Loss of SO2 was found to be significant (p≤0.05) at both RT and 37°C while loss of carotenoids was significant at 37°C. In general it was observed that changes in all the physico chemical parameters were more significant at 37°C as compared to those at 5°C and RT.
Colour Parameters
‘L’ value which indicates lightness decreased with storage time
as well as temperature. Changes in ‘L’ value of the samples kept
at 5°C were not substantial while it was quite considerable at both RT and
37°C. Decrease in ‘L’ value indicated darkening of the sample
colour as the storage progressed.
| Table 1: | Changes in the physicochemical properties of restructured
mango stored under different temperature conditions (n = 3) |
| Means with same superscript within same coloumn are not significantly different at p≤0.05 | |
The same was reflected in case of ‘a’ value (redness) which showed increasing trend as the storage time advanced as well as under higher storage temperatures. Decrease in ‘L’ values and increase in ‘a’ values indicated browning of the product during storage (Table 2). Generally high water activity in the range of 0.7-0.9 favours the development non-enzymatic browning a higher rate (Williams, 1976). Increase in browning could also be due to substantial loss of sulphur dioxide during storage (Table 1). The decrease in yellowness or ‘b’ value during storage may be attributed to loss of carotenoids. Chroma (C) or saturation index equivalent to purity in the CIE system, steadily decreased during the entire storage period from its initial value of 32.02 to 26.12 (at RT) and 22.51 (at 37°C) at the end of 12 m storage. Magnitude of decrease was found to be higher at 37°C. Hue angle (h), measures the shift in colour from the a axis, the decrease in ‘h’ signifies the increase in red colour. In this case ‘h’ was found to decrease steadily as the storage progressed.
Sensory Evaluation
Sensory characteristics of shelf stable restructured mango in terms of colour,
aroma, taste, texture and overall acceptability have been presented in Table
3. The scores in hedonic scale obtained periodically at two months interval
upto 12 m under three different temperature conditions were statistically evaluated.
Gradual decrease in the scores of all the attributes was observed as the storage
time progressed. Significant differences (p≤0.05) were noticed in case of
colour, aroma and OAA at 37°C as compared to those in case of 5°C and
RT stored samples from 4 m storage onwards. However in case of taste and texture
significant differences were observed from 6 and 8 m storage onwards, respectively.
It may be inferred that colour and aroma influenced the OAA more than taste
and texture. Texture was found to be less influenced by the temperature. It
may be concluded from the overall acceptability score that the product was acceptable
organoleptically upto 12 m at room temperature and 6 m at 37°C.
Microbiological Analysis
The data in Table 4 showed that TPC, coliform, spores,
yeast and mold were negligible initially as well as during storage up to 12
m under all the temperature conditions. It indicate that hurdles applied i.e.
aw, pH and pasteurization were quite effective to stabilize the product
microbiologically.
| Table 2: | Changes in the colour values of restructured mango stored
under different temperature conditions (n = 3) |
| Table 3: | Changes in the over all acceptability scores of restructured
mango stored under different temperature condition (n = 10) |
| Means with same superscript within same coloumn are not significantly different at p≤0.05 | |
| Table 5: | Changes in the microbial content of restructured mango stored
under different temperature conditions (n = 3) |
CONCLUSION
A shelf stable restructured mango product can be developed by alginate texturization and stabilized using aw, pH and pasteurization as well as the product was acceptable up to 12 m under ambient (20-33°C) and 6 months at 37°C temperature conditions when packed in polypropylene followed by foil laminate pouches. The product was also microbiologically stable during the storage period. The most significant conclusion may be drawn from our investigation was the stability of the hurdle process restructured mango gel i.e., 12 m at ambient temperature and 6 m at 37°C.