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Introduction
Pineapple is one of the most attractive agricultural products due to its nutritional and sensorial characteristics, which have positioned it as one of the most consumed fruits around the world 1), (2. Among the many presentations available, minimally processed pineapple is one of the most popular in many supermarkets and food service chains 3), (4). According to 5), the texture, color, and aroma of pineapple are the main criteria for determining its quality. In fact, perception and all these properties are crucial in food process engineering 6), (7. Minimal processing of fruits and vegetables can trigger chemical reactions that affect their acceptability 8. Indeed, an increase in metabolic processes can cause significant changes in the physicochemical properties of texture and color and the microbiological load, thus limiting the shelf life of the food 3), (9. Therefore, sustainable forms of conservation have been developed to extend the shelf life of these products. An example of these methods is edible coating, which is defined as a thin layer of material deposited on food as a cover 10, 11. Edible coatings not only act as packaging systems during storage, but they also delay deterioration, enhance quality attributes, and can act against microorganisms 12), (13. Aloe vera gel is used to produce edible coatings due to its ability to form colloidal solutions and its antimicrobial properties 14), (15. In addition, this material has been demonstrated to be adequate for preserving the physicochemical properties of texture, the microbiological load, and colorimetric parameters of pineapple 16. For instance, a previous study reported the effectiveness of aloe vera combined with cassava starch to preserve the physicochemical properties and concentration of vitamin m C in IV-range pineapple 17. The main objective of this research is to evaluate the effect of the edible coatings based on aloe vera and cassava starch for preserving texture, color, and microbiological parameters in minimally processed pineapple.
Materials and methods
Raw material preparation
Pineapples of the Honey Gold variety were selected, washed, and disinfected in 50 ppm of chlorinated water. Afterwards, they were peeled and cut into 3 cm2 x 3 cm rectangular prisms. Aloe vera leaves were washed, disinfected with sodium hypochlorite at 50 ppm, and peeled to obtain the gel. The gel was washed with distilled water, triturated, and homogenized.
Preparation of the solutions and coating of the fruit
The edible coatings for three treatments (T1, T2, and T3) were prepared following the formulations in Table I. The starch was heated and agitated in a water bath until it gelatinized at a temperature of 67 °C at 1.200 rpm. After that, aloe vera was added to the starch under agitation at 85 °C until a homogenized solution was obtained. The samples were covered by immersion for 1 min in the prepared solutions. The coated samples were air-dried at room temperature and packed in polystyrene trays. Finally, the samples were sealed with a commercial film (polyvinyl chloride) and stored at 4 °C for 16 days.
Table I Formulation of the evaluated treatments’ solutions
| Treatments | Aloe vera | Starch | Glycerol (% w/w) | Tween 80 (%w/w) | Olive oil (%w/w) |
|---|---|---|---|---|---|
| T1 | 75 | 25 | 1% | 0,05% | 0,7% |
| T2 | 50 | 50 | 1% | 0,05% | 0,7% |
| T3 | 25 | 75 | 1% | 0,05% | 0,7% |
| T4 | 0 | 0 | 0 | 0 | 0 |
A texture analyzer (TA-XT model plus, Stable MicroSystems, Vienna Court, England) with a uniaxial compression strength of 75 % and a speed of 1 mm/s was used for the measurements. The Texture Expert Software analyzed the data for hardness, cohesiveness, springiness, adhesiveness, and chewiness. The samples were analyzed on the 16th day of storage and performed in triplicate.
Colorimetric parameters
The color was measured directly with a Minolta CR-400 chroma meter (Konica Minolta Sensing, INC., Osaka, Japan), using the CIE color space L*, a*, and b*. The equipment was set up for illuminant D65 at a 2° observer angle and calibrated using a standard white reflector plate 18.
Microbiological test
Microbiological parameters were analyzed by counting according to the corresponding Colombian Technical Standards (NTC): aerobic mesophilic bacteria (NTC 4519) 19, total coliforms and fecal coliforms (NTC 4516) 20, and yeasts (NTC 4132) 21.
Statistical análisis
In order to process the analysis results, a Completely Random Design (CRD) with three treatments and a control sample was used. All analyses were carried out in triplicate. The behavior of each variable was measured via an analysis of variance (ANOVA) and a Tukey test at a significance level of 5 %. The STAT_GRAPHICS Centurion XVI.I statistical software was used.
Results and discusión
Texture profile análisis
The texture profile analysis is depicted in Table II. The T2 treatment obtained values close to the control (T4). Likewise, the statistical analysis of this treatment (T2) showed no significant differences in parameters such as adhesion and elasticity when compared to the control. T2 reported a higher hardness (103,35 N) during the storage time, which indicates that a greater force is required to compress it. This value is closely related to cohesiveness, as the particles in the food require greater force to be compressed (i.e., the cohesion showed a behavior that is proportional to hardness).
Table II Minimally processed pineapple texture profile coated with aloe vera/starch
| Treatments | Hardness (N) | Cohesiveness | Springiness | Adhesiveness (N) | Chewiness (N) |
|---|---|---|---|---|---|
| T1 | 83,89±0,9a | 0,10±0,01a | 0,51±1,05a | -0.60±0,02 a | 4,28±1,32a |
| T2 | 103,34±0,8b | 0,18±0,05c | 0,43±0,58a | -1,30±0,12 b | 8,00±1,05c |
| T3 | 84,75±1,1a | 0,11±0,80a | 0,47±0,79a | -0,66±0,34 a | 4,38±0,02a |
| T4 | 102,9±0,4a | 0,15±0,12b | 0,44±0,31a | -1,22±0,08 b | 6,81±0,16b |
Note: Lowercase letters correspond to the Tukey test, and values that share an equal letter indicate that there is no statistically significant difference.
Similarly, T1 and T3 were found to have the highest elasticity values. Regarding the adhesión values, a negative force was observed, which demonstrates the sticky behavior of the sample on the palate. It was also found that T2 and T4 were the least adhesive, which means that a lower force is required to remove the food from the palate. Finally, the chewiness values were higher in T2 (8,0N) and T4 (6,81N). The texture profile analysis showed that the 50/50 ratio was the most efficient in comparison with the control. The studies reported by 22 established that the changes in the hardness of pineapple are due to the increase of polygalacturonase and the activity of β-galactosidase and pectin methylesterase. Similarly, factors such as the loss of pressure from the cellular turgor, the loss of extracellular air, and the denaturation or degradation of the cell wall constituents cause the fruit to soften 23. Likewise, the type of packaging used in fourth range products can contribute to the loss of texture 22, and the los of water during storage leads to further deterioration 16. However, it has been demonstrated that the use of edible coatings such as those applied by 16 and 24 can maintain the texture of fruits for a longer storage time in comparison with non-coated ones. In previous studies, we demonstrated the effects of edible coatings on water loss control while maintaining the texture of minimally processed pineapple 17.
Colorimetric parameters
Table III shows the results obtained for the coordinates L*, a*, and b*, where significant differences in T2 regarding L* are evidenced with respect to the other treatments. For the a* coordinate, significant differences between the different treatments can be observed in comparison with the control (T4). All the treatments except the control sample had negative values, indicating a light green coloration. The b* coordinate showed no significant differences between treatments. The T4 treatment showed the best luminosity (L*) value, with an average value of 64,65. Similar results were reported in 16. This study found that edible-coated samples had lower L* values, attributing this effect to the opacity of the pineapple’s surface, which depends on the concentration of aloe vera gel in the coating. The chromatic coordinates a* and b* for the treatments T3 and T1 showed optimal mean values (a*=-0,898 for T3 and b*=-37,58 for T1). It has been demonstrated that luminosity is associated with the action of the enzyme polyphenol oxidase, the gradual loss of water, or the surface dehydration of the fruit 25. Similarly, the changes in coordinates a* and b* are likely related to the acceleration of metabolic processes in the fruit, causing undesirable reactions such as enzymatic browning 26), (27. Finally, according to the results shown in Table III, the colorimetric parameters of the samples are not significantly affected by the concentration of cassava starch.
Table III Colorimetric analysis of IV-range pineapple coated with the different treatments
| Treatments | L* | a* | b* |
|---|---|---|---|
| T1 | 60,61±0,21 a | -0,283±0,03 a | 37,58±0,58 a |
| T2 | 62,74±0,13 b | -0,724±0,12 a | 35,93±0,76 a |
| T3 | 62,58±0,08 a | -0,898±0,17 a | 33,41±0,45 a |
| T4 | 64,65±0,15 a | 0,048±0,12 b | 37,48±1,02 a |
Note: Lowercase letters correspond to the Tukey test, and values that share an equal letter indicate that there is no statistically significant difference.
Microbiological test
The microbiological results regarding aerobic mesophilic bacteria and coliforms were within those established by the Colombian Technical Standard (NTC 6005) for minimally processed fruit 29. For coliforms, the values obtained for all the samples during the experiment were at a good quality level (<10) (according to the NTC, the values for m and M are 101 to 102 , respectively). Similarly, no significant growth of aerobic mesophilic bacteria was evidenced in the samples, since, in this study, the maximum value obtained was 4,7x105 (according to the NTC, the values for m and M are 105 to 106 , respectively). However, in treatments with a higher concentration of aloe vera (T1 and T2), the presence of microorganisms was lower in comparison with T3, which had a higher amount of starch. This behavior is most likely due to the polymer matrix, which increases the bioavailability of the substrate while also stimulating the growth of microorganisms 27. In addition, it has been reported that acemannan is the main active compound with antimicrobial activity present in aloe vera 30. Therefore, the lower growth of aerobic mesophilic bacteria and coliforms can be attributed to the active components of aloe vera gel 31. These findings are consistent with previous studies 16), (32), (33. The values obtained for yeast were above those established in Colombian regulations 34. In particular, for the fruit pulp category, the maximum value at an acceptable level is 3.000. In this study, the results showed higher values in all the treatments. However, while comparing T1 to T2 and T3 on day 6, the results showed that a higher concentration of aloe vera inhibited the growth of this type of microorganism. Likewise, it has been demonstrated that the presence of yeasts in minimally processed pineapple is not only the result of microbiological contamination during processing, but that these microorganisms are also part of the endophytic microbiota of pineapple 28. Moreover, despite being washed with a chlorinated solution, certain microorganisms can survive within cells or in areas where the solution cannot penetrate 35), (36.
Table IV Microbiological analysis of IV-range pineapple coated with the different treatments
| Day 0 | ||||
|---|---|---|---|---|
| Analysis/Treatments | T1 | T2 | T3 | T4 |
| Aerobic mesophilic bacteria CFU/g | <10 | <10 | <10 | <10 |
| Yeasts CFU/g | <10 | <10 | <10 | <10 |
| MNP Fecal coliforms CFU/g | <3 | <3 | <3 | <3 |
| MNP Total coliforms CFU/g | <3 | <3 | <3 | <3 |
| Day 6 | ||||
| Analysis/Treatments | T1 | T2 | T3 | T4 |
| Aerobic mesophilic bacteria CFU/g | 12.600 | 19.000 | 29.700 | 35.000 |
| Yeasts CFU/g | 5.000 | 9.300 | 11.000 | 10.000 |
| MNP Fecal coliforms CFU/g | <3 | <3 | <3 | <3 |
| MNP Total coliforms CFU/g | <3 | <3 | <3 | <3 |
| Day 12 | ||||
| Analysis/Treatments | T1 | T2 | T3 | T4 |
| Aerobic mesophilic bacteria CFU/g | 14.000 | 15.000 | 18.000 | 47.000 |
| yeasts CFU/g | 8.600 | 10.000 | 12.400 | 25.000 |
| MNP Fecal coliforms CFU/g | <3 | <3 | <3 | <3 |
| MNP Total coliforms CFU/g | <3 | <3 | <3 | <3 |
Conclusions
Edible coatings are an option for preserving minimally processed pineapple. This type of packaging allows maintaining certain properties of the fruit for long periods of time. In this study, it was demonstrated that a coating with 50 % starch and 50 % aloe vera can ensure a good texture profile in Honey Gold pineapple for 12 days, achieving a firmer product with higher values of hardness and cohesion when compared to uncoated pineapple. However, the coatings can decrease the luminosity of samples, as seen in comparison with the control. In addition, good activity against microorganisms such as mesophiles and fecal and total coliforms was evidenced, but the activity against yeast was weak.
