Annals of Sri Lanka Department of Agriculture. 2007.9:19-26.
A STUDY OF PHYSICOCHEMICAL PROPERTIES AND QUALITY ASPECTS OF SLOW AND RAPID FROZEN CHOWCHOW (Sechium edule) S. EKANAYAKE1, H.A.C.G.K. JAYARATHNE2 and C.K. RANAWANA1 Food Research Unit, Horticulture Crops Research and Development Institute, Gannoruwa, Peradeniya 2 Post Graduate Institute of Science, University of Peradeniya, Peradeniya
1
ABSTRACT Chayote/chowchow (Sechium edule) is a perennial and a seasonal crop, which grows mostly in up country areas of Sri Lanka. The problem associated with chayote fruit is postharvest losses during storage due to shrivelling and sprouting at room temperature. Chayote fruit is susceptible to chilling injury under refrigerated storage conditions. Therefore alternative methods of freezing are needed to store chayote to retain its fresh quality. Chayote fruits were subject to slow and rapid freezing prior to storage at -20°C. The physicochemical properties, microbial growth and sensory properties were compared with fresh chayote after 1, 2 and 3 months of storage periods. The weight loss of rapid frozen chayote stored at -20 °C was less compared with slow frozen chayote stored at same temperature. Vitamin C content of fresh chayote was 10.8mg/100g, whereas this was further reduced to 3.1mg/100g in rapid frozen chayote and this was reduced by 2.6mg/100g after three months of storage. Undetectable vitamin C content was observed in slow frozen chayote. Average high values of firmness was observed in rapid frozen chayote (3.06kg/cm2) during storage whereas this was less in slow frozen chayote (1.37kg/cm2). Moisture content and total soluble solids were decreased in slow and rapid frozen chayote during storage and poor eating quality and external appearance were observed in slow frozen chayote. External appearance and physicochemical properties except vitamin C content of rapid frozen chayote did not deviate much with those of fresh chayote during storage and eating quality was highly acceptable. Very low microbial density was observed even after 3 months of storage of both slow (9 ± 2 colonies per 1g of sample) and rapid (5 ± 1 colonies per 1g of sample) frozen chayote. This study showed that rapid freezing can increase the shelf life of chayote while keeping its fresh qualities. KEYWORDS: Chayote, Frozen, Microbial, Physicochemical, Sensory, Storage.
INTRODUCTION The chayote/chowchow (Sechium edule) is a member of Family Cucurbitaceae and a native Mexican plant. In Sri Lanka, chowchow is grown in home gardens in upcountry areas including Kandy, Nuwara Eliya and Welimada districts. This is an underutilized vegetable and a neglected crop in Sri Lanka. Major production of chowchow in Sri Lanka can be observed
20 EKANAYAKE et al.
during February to August. Harvested fruits are used as a vegetable mostly in upcountry tea cultivated areas. Chayote is basically consumed by human and served in many ways: creamed buttered, fried, stuffed, baked, boiled, mashed, pickled, salads etc. Commercially, the largest market appears to be for pickling. The immature fruits can be eaten raw in salads and provide a good source of vitamin C (Lira, 1996). Chayote is also used in different food preparations in some countries. The softness of the flesh makes it particularly suitable for giving consistency to baby foods, juices, sources and pastes (Aung and Harris, 1996). In India and America, the fruit and roots are not only used as food for human consumption but also as fodder for cattle (Chakravarty, 1990). The edible parts of chayote are relatively low in nutritional value compared with other vegetables. Nevertheless, they are rich in several essential amino acids (Flores, 1989). Decoctions made from the fruits and leaves are used to relieve urine retention and burning sensation during urination or to dissolve kidney stones, and as a complementary treatment for arteriosclerosis and hypertension (Lira, 1996). Major postharvest problems associated with chayote are the losses due to shrivelling and sprouting during storage at room temperature (Vargas, 1991). Seed germination is a major storage problem if the fruit is held at room temperature above 10 째C (Alvarado et al., 1989). Chayote fruit is also susceptible to chilly injury and internal browning appears in fruits held at refrigeration conditions (Aung and Harris, 1996). Shrivelling can be minimized by packing fruits in polyethylene bags and storing at 5-7 째C but the fruits decay due to increase of moisture condensation resulting in the incidence of pathogens such as Mycovelosiella cucurbitiola and Fusarium oxysporum (Alvarado et al., 1989). Freezing of vegetables is well recognised all over the world as the best method of preservation of vegetables (Pruthi, 1999). Freezing maintains product quality (colour, flavour, texture and nutritional value) very close to the fresh ones. Although freezing is applied to preserve many widely used vegetables this has not been applied for this type of underutilised vegetables. Hence, present study was undertaken to identify suitable freezing techniques to prolong shelf life of fresh chayote to maintain good quality. MATERIALS AND METHODS Selection of variety for freezing California green and Monticello white varieties of fruits were harvested at 35-40 days maturity after flowering from farmer fields at Ohiya.
STORAGE STUDY OF CHOWCHOW (Sechium edule) 21
Assessment of quality of fresh chayote Physico-chemical properties such as average fruit weight, average fruit length, % moisture content (Ascar and Treptow, 1993), firmness of flesh (hand penitrometer) total soluble solid (TSS) content, Vitamin C content (Ascar and Treptow, 1993) pH (Kaushal and Bhat, 1999) were determined. The green variety, California green was selected for freezing due to the high availability and demand. Pre-freezing treatments Harvested chayote fruits were washed twice using chlorinated water (200ppm) while brushing to remove soil and dust particles and unwanted plant fragments (Ascar and Treptow, 1993). Washed fruits were cut into halves and seeds removed. These fruits were cut into thick slices (20mm) and placed in citric acid solution (0.5%) prior to blanching to avoid enzymatic browning. Pieces were blanched in boiling water for 2.5 minutes to inactive enzymes and cooled immediately with cold water (Madar, 1998). Slow freezing of chayote (Conventional deep freezing) Nylon mixed linear low density polythene (300 gauge), 14cm x 18cm size packets were used to package the blanched chayote. Blanched chayote slices (100g) in packages were frozen in chest freezer at -18 째C for 4 hours and thereafter stored in deep freezer at -20째C. Rapid freezing of chayote Rapid freezing of the chayote pieces was done by placing blanched chayote pieces (1kg) as a single layer in a clean Styrofoam box and pouring liquid nitrogen (1.5 l) directly on to all pieces. After freezing, the frozen pieces were packaged in Nylon mixed liner low density polyethylene packages (300 gauge) and stored in deep freezer at -20째C temperature. Assessment of quality of frozen products The moisture content, weight loss, pH, total soluble solids, vitamin C content, colour and firmness were tested initially, and after freezing at monthly intervals for three months, in slow and rapid frozen samples. Microbial examination of the samples Total colony count of the frozen products were examined after three month of storage using Colony Count Method (AOAC, 1984).
22 EKANAYAKE et al.
Sensory evaluation of slow and rapid frozen products Chayote curries were prepared from slow and rapid frozen chayote after three months of storage using a standardised procedure. Consumer preference was evaluated 30 untrained panellists. RESULTS AND DISCUSSION Physico-chemical properties of fresh chayote California green and Monticello white varieties are the two common varieties of chayote grown in Sri Lanka. These two varieties differ from each other only by its colour as presented in Table 1. Table 1. Physicochemical properties of fresh chayote varieties. Parameter Variety California green Monticello white Average fruit weight (g) Average fruit length (cm) Flesh colour(RHS colour charts) % Moisture pH TSS (0Brix) Firmness (kg/cm2) Vitamin C (mg/100g)
400 13.7 Yellow Green 145 D 94.3 6.86 4.7 3.5 10.8
405 13.8 Yellow 4 D 93.6 6.98 4.6 3.5 10.7
Physicochemical properties of frozen chayote Change in the moisture content was significantly different between slow and rapid frozen chayote tissues (Table 2). Slow frozen chayote showed gradual reduction of moisture content at 2nd month of storage period. Moisture losses of the packaged frozen foods mainly take place due to migration of moisture through the packaging material, weight loss and frost formation within the pack (Pruthi, 1999). However, packaging material used for freezing chayote in this experiment is highly moisture proof and flexible which is specially made for packaging frozen foods. Reduction of moisture content of slow frozen chayote may be due to frost formation as a result of temperature fluctuation during storage (Pruthi, 1999). High weight losses were observed in slow frozen chayote initially and during the freezer storage, whereas weight losses were low in rapid frozen chayote. Table 2 revealed that weight losses of slow and rapid frozen chayote were significantly different initially and at first, second and third months of storage at 5% significant level.
STORAGE STUDY OF CHOWCHOW (Sechium edule) 23 Table 2. Physicochemical properties of frozen chayote during freezer storage. Property Storage period (months) 0 1 2 3 Percent moisture content Treatment- Slow frozen 91.230b 89.333b 87.667b 84.167b Rapid frozen 93.960a 93.933a 93.773a 90.400a Percent weight loss Treatment - Slow frozen 34.560a 37.880a 35.747a 1.333b Rapid frozen 4.530b 4.553b 5.543b 7.220b Firmness (kg/cm2) Treatment - Slow frozen 1.500b 1.250b 1.417b 1.333b Rapid frozen 3.100a 3.167a 3.000a 3.000a Total soluble solids (TSS) Treatment - Slow frozen 2.800b 2.733b 2.933b 2.067b Rapid frozen 4.600a 4.533a 4.200a 4.000a pH Treatment - Slow frozen 6.930a 6.923a 6.956a 7.013a Rapid frozen 6.910 a 6.917a 6.947a 6.971a Vitamin C (mg/100g) Treatment - Slow frozen 0.000 0.000 0.000 0.000 Rapid frozen 3.110 2.907 2.730 2.624 Colour change Treatment - Slow frozen YG 145D YG 150D YG YG 150D Rapid frozen YG 145D YG 149D 150D YG 150D (YG- Yellow green group) YG 149D Means having a common letter/s are not significantly different by DMRT at 5%.
Firmness of slow and rapid frozen chayote showed a significant difference at the initial stage and throughout the storage period (Table 2). These values were not changed much during the storage period and showed an average of 3.06kg/cm2 in rapid frozen chayote and 1.37kg/cm2 in slow frozen chayote. A little reduction in firmness could be observed in rapid frozen chayote compared to fresh chayote (Table1). Total soluble solids (TSS) of both products differ significantly at initial and during storage at 5% level and low values were observed in slow frozen chayote (Table 2). TSS values of rapid frozen chayote slowly decreased with time and at 3rd month it showed only 14% reduction of TSS from its original TSS content whereas 40% reduction of TSS value was observed in slow frozen chayote. pH values of slow and rapid frozen chayote were not significantly different initially and during storage at 5% level (Table 2). Chayote is a low acid vegetable and its pH in fresh condition is also around 6.94. pH of the products can be changed during storage due to the preservatives added to the product (Hussein et al., 2003) and due to the microbial activity. But freezing is a food preservation method without adding
24 EKANAYAKE et al.
chemical preservatives and normal food spoilage organisms do not grow below -9.5°C (Pruthi, 1999). A loss in vitamin C was shown in slow frozen chayote, whereas lower values of vitamin C content were observed in rapid frozen chayote (Table 2). Absence of Vitamin C content in slow frozen chayote may be due to the destruction of the cells (Pruthi, 1999). Lower content of vitamin C in rapid frozen chayote may be due to water blanching and water cooling (Selman, 1978). Reduction of vitamin C content with time in rapid frozen chayote may be due to the temperature fluctuation and frost formation inside the pack during storage. Colour of frozen products during storage showed slight difference from its initial colour (Table 2). Blanching and water cooling affect the colour change of the frozen products. Loss of Vitamin C also affects the colour changes of the frozen fruits and vegetable products. Microbial examination of the frozen products Low bacterial colonies were observed in slow and rapid frozen samples after three months of storage (Table 3). Bacteria at this level are not harmful and can be destroyed through cooking. Table 3. Microbial population of frozen chayote after three months of storage. Treatment Average number of bacterial colonies per g of sample Slow frozen Rapid frozen
9±2 5±1
Sensory evaluation Summary of the sensory data showed that curries prepared using fresh and rapid frozen chayote had high values of average ranks for all the sensory properties when compared to curry prepared from slow frozen chayote (Table 4). Fresh and rapid frozen curries were highly accepted by consumers in respect of all sensory characteristics. Table 4. Summary of analysis of sensory data. Produc Sensory properties t Flavour Aroma Texture Chewnes s
Rapid Slow Fresh
M
AR
M
AR
M
AR
M
AR
External appearanc e M AR
4 1 4
37.1 13 41.5
3.5 2 4
38.1 12.9 40.5
4 1 4
41 13.5 45
3.5 1 4
3.5 13 42
3.5 1 4
36.6 13.3 41.6
Overall acceptabilit y M AR 4 1 4
37.4 10.9 43.2
STORAGE STUDY OF CHOWCHOW (Sechium edule) 25 p=0.0000 p=0.0000 p=0.000 p=0.0000 p=0.0000 M = Median AR = Average rank p = Probability.
p=0.0000
CONCLUSIONS Physico-chemical properties of rapid frozen chayote were comparable with fresh chayote. Consumer acceptability was high in the rapid frozen chayote. Retention of Vitamin C content of rapid frozen chayote may be improved using more uniform and faster steam blanching and cooling systems. Storage after slow freezing technique deteriorates the quality of fruits. Freezer storage below -20째C is successful for storing of rapid frozen chayote and final quality can be improved by maintaining a uniform temperature without fluctuation throughout the storage period. REFERENCES Alvardo, S., M.V. Saenz and E. Valverde. 1989. Evaluation of postharvest treatments for the preservation of the fruits of chayote (Scechium edule). Agron Costarricense 13:35-43. AOAC, 1984. Official Method of Analysis. Association of Official Analytical Chemists. USA. 579-580. Ascar, A. and H. Treptow. 1993. Quality assurance in tropical fruit processing. Springer Verlag, Berlin, Heidelberg, New York. 10-114. Aung, L.H. and C.M. Harris. 1996. Postharvest storage temperature and film wrap effects on quality of chayote. Journal of Horticultural Science 71(2): 297-304. Chakravarty, H.L. 1990. Cucurbits of India and their role in the development of the vegetable crops. In: Biology and utilisation of the cucurbitaceae. Eds. D.M. Bates., R.W. Robinson and C. Jeffrey, Pp 325-334. Cornell University press. Ithaca, New York. Flores, E. 1989. The chayote Scechium edule swartz (Cucurbitaceae) Revista de Biologis Tropical. 37:1-54. Hussein, S.S., M.A. Rehman and M. Iqbal. 2003. Studies on physicochemical, microbial and sensory evaluation of mango pulp storage with chemical preservatives. Pakistan Journal of Research Science 14: 1-9. Kaushal, B.B.I. and A. Bhat. 1999. Studies on physicochemical properties of fruit leather blended with sprouted soy slurry. Indian Food Packer 53(5): 18-21. Lira, S. 1996. Chayote. International plant genetic resource institute. http://www.ipgri. cigiar.org/publications/pdf/355.pdf. Madar, S.L. 1998. Freezing of vegetables. http://ohioline.osu.edu./hyg-fact/5000/5341.html. Pruthi, J.S. 1999. Quick freezing preservation of food principals, practices, R and D needs. Vol. 1, Foods of plant origin 2nd edn. Allied Publishers Ltd., New Delhi.
26 EKANAYAKE et al. Selman, J.D. 1978. Vitamin C losses from peas during blanching in hot water. Food Chemistry. 3(3), 19-97. Vargas, E. 1991.Chayote Schechium edule (Jacq.) swartz, Cucurbitaceae. Aspects technicos sorbe cuarenta y cino cultivos agricolas de Costa Rica Ministrio de Agriculture y Ganderia. Director General de Investigation Y extension. Agricola. San Jose De Costa Rica, 227-336.