African Crop Science Journal, Vol. 8. No. 1, pp. 93-98, 2000
PRODUCTION AND EVALUATION OF PRECOOKED DEHYDRATED UNRIPE BANANA SLICES
J. H. MUYONGA
Department of Food Science & Technology, Makerere University, P.O. Box 7062, Kampala, Uganda
(Received 16 June, 1999; accepted 19 November, 1999)
Code Number: CS00009
This study was aimed at developing a process for production of easy to prepare dehydrated banana slices. Steaming unripe bananas for 7 minutes followed by hand peel stripping, slicing and dehydration in air dryers produced slices with better rehydration properties than slices produced without steaming, even when the latter were dehydrated by freeze drying. No significant change in total aerobic counts or yeasts and moulds counts occurred in dehydrated banana slices packaged in 250 gauge polyethylene bags and stored at ambient temperature for 3 months. The slices were found to be high in starch (~68.5%) and minerals. When shallow fried, the dehydrated banana slices were found to have acceptable flavour, appearance, taste and texture. Overall, they were considered acceptable by sensory panelists. The findings of this study indicate that steaming, in addition to easing peeling and reducing discolouration, improves rehydration and reduces cook loss.
Key Words: Musa spp., dehydration, rehydration, pretreatments
Cette étude visait au developpement dun procès facile de production pour préparer des tranches de bananes déshydratées. La cuisson à la vapeur des bananes non mûres pendant 7 minutes suivie de lenlèvement des épluchures à la main, de la coupure en tranches et de lhydratation dans les séchoirs à lair a produit des tranches avec meilleures propriétes dhydratation que des tranches produites sans cuisson à la vapeur, même si ces dernières étaient déshyadratées par lyophilisation. Pas de changement significatif dans les comptages aériobiques ou levures et les nombres de moisissures ont apparu dans les tranches de banane déshydratées emballées dans les sachets de polyethylène de 250g et conservées à une temperature ambiante pendant 3 mois. Les tranches ont été trouvées plus riches en amidon (~68.5%) et en minéraux. Une fois séchées superficiellement, tranches de bananes déshydratées ont été trouvées avoir une saveur, apparence, un goût et texture acceptables. En général, elles étaient acceptées par les évaluateurs. Les résultats de cette études indiquent que la cuisson à la vapeur, en plus de faciliter lépluchage et la réduction de la décoloration, améliore la déshydratation et réduit les pertes de cuisson.
Mots Clés: Musa spp., déshydratation, reshydratation, prétraitements
Bananas and plantain (Musa spp.) are an important staple in Africa. Per capita consumption of bananas and plantain in E. Africa is estimated to exceed 400 kg. With an annual production of ~9.3 million tons, Uganda is the Worlds largest producer and consumer of bananas and plantain (FAO, 1997).
Traditionally, preparation of bananas in Uganda involves hand peeling of the mature unripe fruit, followed by boiling in water or steaming. The cooked bananas may be mashed or fried before consumption. Hand peeling is rather tedious and cumbersome and a substantial amount of pulp is lost with the peel.
While large volumes of bananas are produced in Africa and indeed in the World, industrial utilisation is limited. Infact, banana are still handled and prepared mostly by traditional methods such as the aforementioned. Major problems raised by this amidst increased urbanisation include; high transportation and handling costs associated with product bulk, short shelf life, high post harvest losses and limited variety for consumers.
Processing of unripe bananas into products which are shelf-stable, easy to handle and easy to prepare is a necessity in light of changing life styles. Dehydration is traditionally used in West Africa (Firmin, 1994; Darzie and Wainwright, 1995) and, to a limited extent, in East Africa (Aked and Kyamuhagire, 1996) to preserve unripe banana. The resulting products, however, have a dark colour due to browning which occurs during processing. Upon rehydration the product lacks the aesthetic properties associated with fresh cooked green bananas.
Steaming has been used by many workers to reduce enzymatic discolouration and to ease peeling of unripe banana prior to processing (Hanson, 1976; Suntharalingam and Ravindran, 1993). Wherever cooking is required, the cooking has been done after peeling (Hanson, 1976; Ukhun and Ukpebor, 1991).
In the current study, steaming was used to loosen the peel, control browning as well as precook unripe bananas prior to dehydration. The aims of this study were to develop a simple technique for processing precooked dehydrated banana slices and to determine the physico-chemical and sensory properties of dehydrated precooked banana.
MATERIALS AND METHODS
Production of dehydrated banana slices. Mature unripe cavendish bananas (Musa cavendishii), three quarter to full three quarter levels (Thompson and Burden, 1995) were obtained from a wholesale agent in Mysore, India, within 2 days after harvest. The fruits were processed into dry slices on the same day they were received.
The bananas were washed and steamed in a steam kettle (Krauss Maffei, Munchen, Germany) at 1 kg/cm2 (100-115°C) for 7 minutes. This time was adequate to eliminate discolouration as detected by colouri-metry and visual observation. It was also adequate to loosen the banana peel, allowing hand stripping of the peels. The fruits were removed from the steam kettle, left to cool and the peels were stripped-off by hand. The peels and pulp were weighed to determine the pulp to peel ratio. The peeled bananas were sliced using a slicer/shredder (Model UM 40, A Stephan U. Sohne, Weser, Germany) with a 4 mm slicing disk. The slices were dried in a cross flow hot air drier (Armstrong Smith, PVT Ltd., India), with air flow rate at 1.5 m/s for 4 hours. Each drier tray (80x40 cm) was loaded with 1.5 kg of banana slices and drying temperature was maintained between 60 65°C. The dry slices were left to cool to ambient temperature, weighed to determine the slices yield and then packaged in polyethylene bags (gauge 250). The bags were heat sealed and stored at ambient temperature (~25°C) for 92 days.
For production of slices by other dehydration methods, raw banana slices were dipped in a 1% solution of potassium metabisulphite for 5 minutes and dried either in a Virtis freeze drier (Virtis, NY.), vacuum drier (Pharmlab Industries, Bombay, India) at a vacuum of -0.7 to -0.9 kg/cm2 or in a cross flow air drier with drying temperature maintained between 60 65°C. The fresh banana pulp and the dry slices were analysed by methods described below. Prior to chemical analysis of slices, they were comminuted using a Mikro pulveriser (Ramlek and Co. Ltd., London) with a mesh of 0.25mm diameter. The experiment was duplicated.
Analytical methods. Moisture and crude protein were determined by AOAC (1997) methods (# 4.1.03 and 955.04, respectively). Ash, crude fiber, crude fat, starch, total sugars, tannins, vitamin C and titratable acidity were determined by methods described by Ranganna (1986). Ash determination was by dry ashing at 450°C, crude fat by soxhlet extraction, starch and sugars by the Lane and Eynon method after acid hydrolysis and citric acid inversion, respectively; tannins by titration with potassium permanganate, and vitamin C by the titrimetric method based on reduction of 2,6 dichlorophenolindophenol. Minerals were determined using a Perkin Elmer atomic absorption spectrometer (U.S. Instrument division, Norwalk, CT.) on digest of ash from ash determination. Copper, iron, magnesium, magnese and zinc were determined by absorption spectrometry (Ihnat, 1981) at wavelengths of 324.8, 248.3, 285.2, 279.5, and 213.9 nm, respectively. Potassium and sodium were determined by emission spectrometry at 766.5 and 589 nm, respectively.
To determine rehydration ratio and solubility in hot water , distilled water or a 1% salt solution was boiled and poured into a beaker containing the slices (200 ml per 20 g). The mixture was left to stand for 10 minutes, with intermittent stirring. The water was then drained and the weight of the slices determined. The weight of water absorbed was derived from the difference in the weight of the slices. An aliquot (25 ml) of the drained water or salt solution was transferred to a petri dish and evaporated to dryness using a water bath. The remaining moisture was evaporated using an oven air dryer at 80°C and -0.8 kg/cm2 until a constant weight was recorded. The weight of the recovered solids was used to calculate the solubility. The solubility represents the solids that leach into the water or brine solution during rehydration.
Hunter values, L, a, b and D E colour were determined for the slices using a Shimadzu colour measuring instrument (MPC-3100, Shimadzu Corporation, Japan).
Storage stability and sensory evaluation of slices from steamed banana. Moisture content and microbial load were determined on the day of drying and after 3 months storage under ambient conditions. Total aerobic counts, and yeast and mould counts were determined using plate count agar (Swanson et al., 1992) and potato dextrose agar (Mislivec et al., 1992) respectively.
Sensory analysis was conducted on the slices after 3 months storage. The slices were soaked in 1% salt solution for 10 minutes, drained and then shallow fried in sunflower oil for about 2 minutes. The fried slices were then cooled and presented to a 21 member panel consisting of scientific staff and trainees at the Central Food Technological Research Institute, Mysore, India. A 7 point hedonic scale (1-like extremely, 2-like moderately, 3-like slightly, 4-neither like nor dislike, 5- dislike slightly, 6 - dislike moderately and 7-dislike extremely) was used.
RESULTS AND DISCUSSIONS
Physico-chemical properties of dehydrated banana slices. The yield for slices was 20.8% and the pulp to peel ratio was 2.33. Suntharalingam and Ravindran (1993) reported pulp to peel safio of 1.8 and 1.9 for Alukehel and monthan bananas. The inconsistency may be due to differences in banana cultivars.The dry slices were high in starch and low in sugars (Table 1). The Vitamin C content of the slices was lower than that reported for fresh banana pulp (Izonfua et al., 1988; Barthkur and Arnold, 1990). This may be attributed to heat destruction of this constituent. In comparison to recommended dietary allowances for adults (National Academy of Sciences, 1989), the slices are good sources of the minerals determined. The fact that the iron in banana is 100% bio-available (Suntharalingam and Ravindran, 1993) makes it a very good source of this mineral. The minerals content are comparable to levels reported by Izonfuo and Omuaru (1988).
TABLE 1. Chemical composition of dehydrated unripe banana slices on dry basis
TABLE 2. Comparison of physico-chemical properties of banana slices dehydrated by different processes
Values in the same row with different superscripts differ significantly (P<0.05) as separated by LSD
Slices from steamed bananas imbibed more than 3 times their weight when soaked in either a 1% salt solution or water (Table 2). The rate of water imbibition was higher for the steamed than for freeze-dried slices (Fig. 1). Since good rehydration is a major challenge for dehydrated products, this is desirable. The freeze-dried unsteamed slices still remained quite hard to chew even after 20 minutes boiling. According to Sole (1996), even freeze-dried ripe banana slices take long to rehydrate and tend to stick to the teeth during chewing. The unsteamed vacuum oven and the cross flow air dried products had even poorer rehydration. In addition to good rehydration, steamed slices exhibited the least cook loss in hot water and 1% salt solution. This is desirable since most time the water used for cooking banana is not consumed. From the results, the cook loss would be higher in a salt solution than in water. Nutrients are therefore better retained when cooking or rehydration are done in water rather than in a salt solution. The increase in ionic strength of the solution apparently leads to increase in cook loss. The high rehydration can be attributed to starch gelatinisation (Doublier et al., 1986).
Figure 1. Rehydration pattern for freeze dried and steamed hot air dried unripe banana slices.
Sensory acceptability and storage stability of banana slices. All the attributes of the fried banana slices were found to be acceptable. The scores for appearance, taste, flavour, texture and overall acceptability were 2.23, 2.36, 2.64, 2.68 and 2.21, respectively. Although much of the latex is retained within the pulp during this process, astrigency, which is associated with tannins (Von Elbe and Schwartz, 1996) contained in the latex, was not identified as a problem by the panelists.
TABLE 3. Storage stability of dehydrated banana slices
Mean ±SD for triplicate determinations
Judging from the microbial load (Table 3), the product was stable beyond 3 months when stored at ambient temperature in 250 gauge polyethylene bags. The storage stability may be attributed to the low moisture content of the product. The product also showed no hygroscopic tendency.
Steaming is a useful and inexpensive pretreatment for production of dehydrated starchy foods. Other methods such as extrusion cooking, which give products with good rehydration, require high investment. Since much of the food dehydration in developing countries is done on a small scale, inexpensive and technically uncomplicated techniques are most appropriate. Steaming meets these criteria.
The author acknowledges The United Nations University for providing a research fellowship and thanks Dr. V. Prakash, Dr. W.E. Eipeson and Dr. R.S. Ramteke of The Central Food Technological Research Institute, Mysore, India for their advice.
©2000, African Crop Science Society
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