The guava tree (Psidium guajava L.), which belongs to the Myrtaceae family, is considered one of the most important tropical fruit trees in the world with high palatability and pleasant flavour and odour1. The origin of the tree is tropical America and well grown in all warm areas of tropical and subtropical regions2. In Sudan, guava fruit is the most popular fruit after date, citrus, mango and banana. The most popular guava cultivars in Sudan are the pear- and apple-shaped fruit types with pink or white pulp. Both guava types are easily grown in the country with high productivity and usually harvested two to three times during the year. However, guava fruits are highly perishable; nearly 30% of their production are usually lost due to spoilage, mishandling, lack of transportation or cold storage facilities3.
Guava fruit is very rich in tannins, phenols, terpenes flavonoids, essential oils, saponins, carotenoids, fibres and fatty acids. The fruit is also an excellent source of pectin, minerals (potassium, copper and magnesium) and vitamins such as ascorbic acid (vitamin C), vitamin A, thiamine (B1), riboflavin (B2), pantothenic acid, folic acid and niacin4,5. The fruit is eaten fresh or processed into juice, jam, concentrate or other food products6,7. Therefore, the preservation of guava pulp concentrates not only supports guava-based industries but also the financial loss associated with its importing can be avoided. No previous studies have been carried out to investigate the effects of storage on the quality characteristics of Guava (Psidium guajava L.) fruit concentrates. Thus, the present study was conducted to evaluate the effects of storage on the quality characteristics of Guava pulp concentrates with respect to their physicochemical, chemical and microbiological properties.
MATERIALS AND METHODS
The guava fruits (Psidium guajava L.) with white and red pulp colour were obtained from Alkadaro and Aljeraf farms, Khartoum, Sudan. All the chemicals and reagents used in this study were of analytical grades. This study was conducted during 2008-2012.
Guava concentrate processing method: Freshly harvested fully ripened white and red guava fruits were quickly washed, sorted, weighed, graded, manually peeled, sliced and blanched. The fruit juice of the two samples was extracted by using a fruit pulpier (MDX-207823-HR, Reeves, USA), placed in a steam jacketed kettle (50 litre capacity, No. 1044-19-66, Gebrs-H.J Scheffers) and heated to 100°C until the total soluble solids (TSS) reached 19°Bx. Then the steam was turned off, and 0.03% sodium benzoate (CDH, England) was immediately added as a food preservative. Finally, the white and red guava concentrates were kept in cleaned tin containers (Cans: 3; Size: 700×404 inches; Capacity: 35.08 ounces), which were tightly closed, sterilized in boiled water (100°C) for 30 minutes, cooled to 30°C and stored at room temperature for further analysis.
Storage method and conditions: Both the white and red guava pulp concentrates produced in this study were stored at room temperature (25±5°C) for 13.5 months. During this period, representative samples from both guava pulp concentrates were taken every 45 days for analysis in order to determine the changes that occurred with respect to their physicochemical and chemical properties in comparison with their initial values before storage.
Physicochemical methods: TSS in the guava pulp concentrates was measured with a hand held refractometer (No. 002603, BS eclipse, UK) at 20°C as described by the AOAC8, while the pH of the samples was measured by using a pH-meter with two buffer solutions (pH 4.01 and 7.0) at 20°C to the nearest 0.01 pH units.
Chemical methods: The concentrations of total, reducing and non-reducing sugars and ascorbic acid of the different guava pulp concentrates were determined according to the standard method of the AOAC8. The non-enzymatic browning optical density (OD) and Titratable acidity percentages were determined according to Ranganna9.
Microbiological analysis methods: The microbiological quality and safety of the guava fruits concentrates before and during storage were assessed based on their total viable bacterial count (TVBC), total Coliform count (TCC), total Staphylococcus aureus count (TSAC), Lactic acid bacterial count (LABC), total Salmonella count (TSC), total yeast and mould count (TYMC), according to ICMSF10.
Statistical analysis: All statistical analyses were performed using the Statistical Package for Social Science (SPSS) version 16.0 for windows (SPSS Inc., Chicago, IL, USA). While, the mean values were tested and separated using Duncan’s Multiple Range Test (DMR) according to Steel et al.11.
RESULTS AND DISCUSSION
Physicochemical and chemical characteristics of guava pulp concentrates before storage: After the processing of the white and red guava pulp concentrates by evaporation under normal atmospheric pressure, the physicochemical and chemical characteristics of the two products were immediately investigated. The initial physicochemical and chemical characteristics of the white and red guava pulp concentrates was determined before storage as described by the AOAC8 and presented in Table 1.
Results showed the significant differences between the TSS of the white guava pulp concentrate (17.0%) and red guava pulp concentrate (19.0%). This variation in TSS may be due to the thermal processing conditions or the initial pectin contents in the fruits12,13. In contrast, the hydrogen ion concentrations (pH) (4.60, 4.53) and titratable acidity percentages (0.30%, 0.30%), in the white and red guava concentrates respectively, were found to be different non-significantly. In the white guava concentrate, the vitamin C concentration (ascorbic acid) was significantly (p≤0.05) higher (134.9 mg/100 g) than that of the red-guava concentrate (124.0 mg/100 g). According to the literature, the degradation of vitamin C in fruit juice or fruit concentrate is usually influenced by the processing method and temperature14. On the other hand, the red guava concentrate had higher concentrations of total sugars (16.14%) and reducing sugars (10.95%) with no marked variations in its OD levels (0.13 and 0.14). Sabato et al.15 reported that variations in sugar contents of different guava varieties is not only due to the physiological changes and polysaccharides metabolism that occur during the ripening process but also due to the effects of the thermal processing conditions during the production of guava concentrates.
Changes in the physicochemical and chemical characteristics of white and red pulp guava pulp concentrates during storage: One of the main objectives of this study was to determine the changes in the physicochemical and chemical properties of the white and red guava concentrates during storage. Therefore, representative samples from the two concentrates were taken every 45 days to evaluate the changes in their TSS (%), pH, acidity (%), ascorbic acid (mg/100 g), total sugars (%), reducing sugars (%), non-reducing sugars (%) and non-enzymatic browning optical density (OD).
Changes in physicochemical characteristics: Table 2 and 3 show changes in the TSS and pH of the white and red guava pulp concentrates, respectively, during storage. Table 2 indicates that the TSS increased gradually and significantly (p≤0.05) from 17.00-21.55% in white guava and from 19.00-22.96% in red guava by the end of the storage period. These results agree with the findings of Yang et al.16 and Jain et al.17. The increasing trends in the TSS percentage may be due to the conversion of polysaccharides into soluble sugars during storage.
In contrast, at the end of the storage period the pH values significantly (p≤0.05) decreased from 4.60-3.00 in the white guava concentrates and from 4.53-3.02 in the red guava concentrates. The decline in the pH values of the two guava concentrates during storage may be due to the conversion of soluble sugars into organic acids. It was observed that, the processing methods and increase in storage temperature and period decreased the pH levels and viscosity in fruit beverages and concentrates18-20.
Changes in chemical characteristics
Changes in acidity and ascorbic acid: Table 4 and 5 show changes in acidity and ascorbic acid concentration in the white and red guava concentrates during storage, respectively. The acidity of the two guava concentrates (as citric acid) increased with increasing storage time from 0.3% at the initial storage time (zero time) to 0.9% by the end of the storage time (Table 4).
In contrast, the levels of ascorbic acid (Table 5) in the white and red guava concentrates significantly (p≤0.05) decreased from 134.9 and 125.0 at the initial time of storage to 16.20 and 10.91 mg/100 g by the end of the storage period, respectively. These results agree with the previous studies17,21. The decrease in vitamin C was due to many factors, such as, oxygen, heat, light, storage conditions and type of containers22.
Changes in sugars: Changes in total, reducing and non-reducing sugars of the white and red concentrates during storage are shown in Table 6. The levels of total sugars in the white and red guava concentrates decreased significantly (p≤0.05) from 15.32 and 16.14% at the beginning of the storage period to 13.15 and 13.7% by the end of the storage period. In addition, the concentration of non-reducing sugars in the white and red guava concentrates decreased significantly (p≤0.05) from 6.20 and 5.19% at the initial time to 0.04 and 0.06% by the end of the storage period. Similar results were reported in previous studies20,23.