Increasing consumer awareness regarding the health benefits of different nutrients in food has led to the requirement of assessing the effect of food processing approaches on the quality attributes. The present work focuses on understanding the effects of ultrasound (US) processing, mild heat pasteurization (65°C for 15 min), thermal pasteurization (80°C for 15 min) and their combination on physicochemical, microbiological properties and nutritional quality of pineapple juice through 60 days of storage at room temperature. Ultrasound treatment showed significantly lower browning degree. Ultrasound followed by ultrasound combined with mild heat pasteurization (UMP) treatments was effective in retaining the total phenolic content of pineapple juice as compared to the thermal treatment or the untreated juice sample at room temperature during 60 days of storage. Thermal pasteurization (TP) followed by ultrasound combined with mild heat pasteurization (UMP) and ultrasound (US) treatment, in increasing order, was found to be effective in delaying microbial growth in pineapple juice. This study demonstrates that ultrasound combined with mild heat pasteurization treatments could be able to effectively inactivate the microorganisms and pectin methylesterase in pineapple juice whilst preserving relatively high amount of phenols.
Considerable attention has been focused on vegetable and fruit juices recently due to the presence of health promoting compounds in the juices and ease of consumption especially for packaged juices. Juices play an important role in the daily nutrition and provide essential supplements. Pineapple (Ananas cosmosus) is one of the most important tropical fruits. Pineapple juice is famous for its sweet and sour taste as well as beneficial health compounds. The protective effects of this product have been associated with the presence of antioxidant compounds [
Spoilage in pineapple products is not only encountered by microbial contamination but also by enzymatic degradation which is generally not accepted by the consumer [
Thermal pasteurization is a common process used to inactivate pathogenic bacteria and some enzymes in juice. However, due to the high temperature used in the process, the nutritional and sensory properties of the pasteurized juice might be somewhat altered [
From scientific literature, it is apparent that some individual non-thermal methods are effective to inactivate microorganisms or reduce the log colony forming units (CFU) while not adversely affecting the sensory and nutritional quality.
Sonication (ultrasound) treatment, which is an emerging technology that is considered to be inexpensive, simple, reliable and environmentally friendly, has been studied for use in several applications including fruit juice processing [
Despite the numerous published reports on the effect of non-thermal treatments combined with mild heat pasteurization on fruit juices, there is a lack of information on pineapple juice. There is a need for alternative combined treatments that can preserve quality properties of pineapple juice. Therefore, the aim of this study was to evaluate the effect of mild heat pasteurization, thermal pasteurization, ultrasound, and their combination on the physico-chemical, microbiological properties and nutritional quality of pineapple juice during storage.
Freshly harvested pineapples (Ananas comosus) were purchased from a local market at Cotonou, Benin. The obtained fruits were washed, peeled and pressed mechanically in a household juice extractor. The extracted juice was further filtered using a vacuum filtration unit through Whatman filter paper No.1 and then stored in an aseptic manner and further subjected to different thermal and non-thermal treatments.
Pineapple juice obtained was divided into five groups, and each group was samples (250 ± 5 mL) at least three times using glass jars.
The different groups were subjected to the following treatments.
Untreated groups are the fresh pineapple juice without any treatment was used as control (C).
Water bath having a polycarbonate basin was used for pasteurizations treatments of juices. The Biobase water bath had special features such as thermostatic control, temperature resistance till 120˚C, operating temperature range of 25˚C - 100˚C with accuracy of ±0.3˚C and electric supply of 220 V/50 Hz. For mild heat pasteurization (MP), juice (250 ± 5 mL) was transferred into a clean sterile closed glass jar and was pasteurized at 65˚C temperature for 15 min. On the other hand, for thermal pasteurization (TP), juice (250 ± 5 mL) was transferred into a clean sterile closed glass jar and subjected to heat treatment at 80˚C for 15min. The jars were kept for constant shaking at 100 rpm to obtain homogenous conditions inside the samples throughout the treatment. Samples were cooled immediately after the heat treatment and stored at room temperature (26˚C ± 2˚C) till further analysis.
Ultrasound (US) treatments were carried out using Bioblock Scientific, Vibra-cell 75,115 (with probe diameter of 10 mm) at constant power of 500 W and frequency of 20 kHz for 15 min. The temperature was maintained below 65˚C using an ice bath around the reactor. The temperature of the juice was monitored using a thermometer and it was ensured that the temperature was below 65˚C.
Juices were also treated in a combined operation using ultrasound and mild heat pasteurization (UMP). Ultrasound parameters were set at the same optimized values as mentioned earlier. After ultrasound treatment, the juice was pasteurized at 65˚C for 15 min.
The processed juice was stored at room temperature (26˚C ± 2˚C) and used at regular intervals for different analysis. Measurements and analyses of the juices were performed on the following days of storage period; 0, 15th, 30th, 45th and 60th day. Three samples were prepared for each treatment. The experiment was done in triplicates.
The pH of the pineapple juice was measured using hand digital pH meter (Eutech Instruments, pH 2700, Singapore), precalibrated with buffers of pH 4.0 and 7.0.
Titratable acidity of the juice was measured by titrating with 0.1 N NaOH and the results were expressed as percentage of citric acid [
The browning degree (BD) of pineapple juice was analyzed using a spectrophotometric method described by Roig et al. [
Pectin methylesterase (PME) activity was measured using the method adapted by Aguiló-Aguayo, et al. [
where [NaOH] is NaOH concentration (0.05 N);
The total phenolic content of pineapple juice was determined according to a method adapted by Alothman, et al. [
All samples were analysed for mesophilic, thermophilic, psychrophilic bacteria (total bacterial counts), also for yeasts, moulds and coliforms during 8 weeks of storage at room temperature. Juice samples (1 mL) were decimal diluted serially with sterile 1 mg/mL peptone water and appropriate dilutions were poured on to the respective plates. Total bacterial counts were determined on plate count agar (PCA; Merck, Darmstadt, Germany) following incubation at 35˚C over 3 days for mesophilic bacteria, at 4˚C over 7 days for psychrophilic bacteria and 55˚C over 2 days for thermophilic bacteria. Yeasts and moulds were estimated on potato dextrose agar (Merck) and with incubation at (28˚C ± 1˚C) for 7 days. Coliforms were enumerated using Violet Red Bile Lactose Agar (Oxoid) at (36˚C ± 1˚C) for 48 hours. The number of individual colonies on plate was counted by visual observation in light. The actual CFU/100 mL of juice was then calculated using the following Equation (2):
The microbial counts were expressed as log colony forming units (CFU)/100 mL. All microbiological analyses for each batch were conducted at least in triplicate for each experiment.
All experiments were performed in triplicate. Data were expressed as mean ± standard deviation (SD). The Tukey’s test and one-way analysis of variance (ANOVA) used for multiple comparisons by the SPSS 17.0 (SPSS, Chicago, USA). Difference was considered to be statistically significant if P < 0.05.
Time (days) | ||||||
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Parameters | Treatments | 0 | 15 | 30 | 45 | 60 |
pH | Control | 4.10 ± 0.03a | 3.60 ± 0.09a | 3.39 ± 0.03a | 3.29 ± 0.01a | 3.21 ± 0.05a |
MP | 4.10 ± 0.03a | 4.06 ± 0.01b | 4.01 ± 0.07b | 4.09 ± 0.10b | 4.02 ± 0.02b | |
US | 4.10 ± 0.03a | 3.51 ± 0.06a | 3.39 ± 0.03a | 3.47 ± 0.02a | 3.44 ± 0.05a | |
UMP | 4.10 ± 0.03a | 4.02 ± 0.07b | 4.02 ± 0.03b | 4.06 ± 0.03b | 3.92 ± 0.06b | |
TP | 4.10 ± 0.03a | 4.01 ± 0.02b | 4.05 ± 0.05b | 4.02 ± 0.06b | 3.99 ± 0.09b | |
TA | Control | 8.83 ± 1.03a | 21.60 ± 1.07b | 23.27 ± 1.13b | 23.60 ± 1.02b | 23.90 ± 1.06b |
MP | 7.23 ± 1.10a | 7.33 ± 0.05a | 7.93 ± 1.02a | 9.00 ± 1.02a | 8.93 ± 1.04a | |
US | 6.60 ± 1.01a | 24.73 ± 1.04b | 26.27 ± 1.05b | 23.87 ± 0.96b | 26.80 ± 1.11b | |
UMP | 6.67 ± 1.03a | 7.80 ± 1.04a | 7.73 ± 1.09a | 8.40 ± 1.04a | 8.37 ± 1.01a | |
TP | 9.20 ± 1.06a | 7.47 ± 1.10a | 7.60 ± 1.05a | 7.40 ± 0.83a | 7.40 ± 1.05a | |
TSS | Control | 13.00 ± 1.03a | 13.00 ± 0.63a | 10.00 ± 1.05a | 7.00 ± 1.09a | 7.00 ± 0.89a |
MP | 15.00 ± 1.18a | 16.00 ± 0.78a | 16.00 ± 1.10b | 16.00 ± 1.02b | 15.00 ± 1.05b | |
US | 14.00 ± 1.06a | 14.50 ± 1.09a | 15.00 ± 0.67b | 15.50 ± 1.11b | 15.00 ± 1.03b | |
UMP | 15.00 ± 0.97a | 15.00 ± 1.21a | 16.00 ± 0.94b | 16.00 ± 1.07b | 15.50 ± 1.06b | |
TP | 15.50 ± 1.21a | 15.50 ± 1.04a | 16.00 ± 1.10b | 16.00 ± 1.04b | 15.00 ± 1.05b |
Values are mean ± standard deviation of triplicates. Data in same column with different letters are significantly different (P < 0.05). Control: fresh pineapple juice without any treatment; MP: juice subjected to mild heat treatment at 65˚C for 15 min; US: juice subjected to ultrasound treatment for 15 min; UMP: ultrasound combined with mild heat pasteurization; TP: juice subjected to heat treatment at 80˚C for 15 min.
treated samples (MP, UMP and TP) varied between 4.10 and 3.95 suggesting the least significant differences (P > 0.05) among these values. The pH of the control and ultrasound treatment was found to decrease significantly (P < 0.05). Titrable acidity values evolved inversely compared to pH values during two months of storage. No significant change of TSS was observed for all samples except the control. In general, the acidity and the hydrogen ion concentration (indirectly the pH) in any fruit product vary due to different types of biochemical reactions such as hydrolysis, oxidation, fermentation, and decomposition.
Increased acidity and lowest TSS in control sample might be due to spoilage and fermentation, resulted to the conversion of sugar to acids, carbon dioxide or alcohol [
Consumers consider product appearance to be the primary criterion for acceptance. Color has been considered to play a key role in fruit juice, preference and overall acceptance. It may even influence taste thresholds, sweetness perception and pleasantness. In this regard, it is important to bear in mind that for a given treatment, color maintenance should be given consideration to ensure that the outcome of the final product meets consumer choice and acceptability.
Browning degree of different pineapple juice samples are increased by increasing storage time (
During the first 15 days of storage, the thermal pasteurization and control following by ultrasound treatments, showed the lowest browning degree compared to other treatments. When the storage progressed to day 60, ultrasound sample showed significantly lower browning degree (P < 0.05). Spoilage of pineapple juice is not only encountered by microbial contamination but also by enzymatic degradation which is generally not accepted by the consumer [
Pectin methylesterase (PME), an ubiquitous enzyme found in plants, hydrolyses pectin resulting in decreased cloud stability and reduced viscosity due to pectin chain degradation. The effects of ultrasound and heat treatments on PME activity of pineapple juice during storage are shown in
The lower PME activity observed with US treatment, during storage could be due to the mechanical damage of the pectin methylesterase protein structure during sonication. On the other hand, the lower pectin methylesterase activity observed with thermal pasteurization treatment could be due to the heat pasteurisation. Pectin methylesterase is generally inactivated using conventional heat pasteurisation.
Studies show that the PME activity of pineapple was inactivated when heating temperature increased from 20˚C to 90˚C [
The effects of different treatments during the storage time on the total phenolic content of pineapple juice are shown in
(P < 0.05) decrease on the total phenolic content during storage. There was a significant difference between control and treated samples at end of storage. Ultrasound followed by ultrasound combined with mild heat pasteurization treatments were effective in retaining the total phenolic content of pineapple juice as compared to the thermal treatment or the untreated juice sample at room temperature during 60 days of storage. The results confirmed that the application of ultrasound treatment preserved the total phenolic level of fruit juices during the storage. Similar results have been reported for the studies of different fruit and vegetable juices [
Organisms which are usually responsible for spoilage of fruit juice include gram-negative, psychrotrophic bacteria, yeast and moulds. Figures 4(a)-(d) shows mesophilic, psychrophilic, yeasts and moulds counts of pineapple juice treated with US, MP, UMP, TP and control stored for a period of 60 days at room temperature.
Gradual growth of all microorganisms was seen during storage in all samples. However, some treatments retarded the microbial growth more than others. Generally, yeasts and moulds were present in relatively lower numbers during the storage. Coliforms and thermophilic were not founded in any samples. The highest amount of microorganisms was observed in control samples. Thermal pasteurization (TP) followed by ultrasound combined with mild heat pasteurization and ultrasound treatment, in increasing order, were found to be effective in delaying microbial growth in pineapple juice. Thermal pasteurization treatments were effective in retaining the microbial growth in pineapple juice during storage at room temperature. Unfortunately, due to the high temperature used in the process, the nutritional and sensory properties of the pasteurized juice were somewhat altered. Many researchers have also demonstrated that heating is the most widely accepted technique used for inactivating microorganisms in food, however there can be a significant change in the functional properties and contents of food, which tends to reduce the product quality and freshness [
Compared to the untreated (control) samples, treated samples had significantly (P < 0.05) longer shelf-life. The ultrasound and combination of non-thermal methods are a new approach for preservation of fruit juices that can enhance the microbiological safety while having lower impact on the organoleptic and nutrient properties of juices in comparison with the thermal processing.
The authors would like to thank Dr. Abdou Madjid O. Amoussa, for technical assistance.
Lagnika, C., Adjovi, Y.C.S., Lagnika, L., Gogohounga, F.O., Do-Sacramento, O., Koulony, R.K. and Sanni, A. (2017) Effect of Combining Ultrasound and Mild Heat Treatment on Physicochemical, Nutritional Quality and Microbiological Properties of Pineapple Juice. Food and Nutrition Sciences, 8, 227- 241. https://doi.org/10.4236/fns.2017.82015