Abstract

Changes in sensory attributes of vegetables over time under different conditions have been reported, however, little has been done regarding profiling and assessing changes in sensory attributes of raw leafy vegetables particularly Solanum aethiopicum (S.) and Amaranthus lividus (L.). This study therefore fills an important knowledge gap of profiling sensory attributes and assessing changes in color, texture and appearance of S.aethiopicum and A.lividus leafy vegetables over time after harvest. A complete randomized design in a 3 ×3 factorial arrangement (each vegetable sample was subjected to three treatments (Time of the day) and three replicates) and data was collected by use of quantitative descriptive sensory analysis. Descriptive data was entered into Microsoft excel spread sheets, averages computed and graphs generated. The data was further subjected to ANOVA and a least significant difference test was used to compare means of samples for all attributes at 95% confidence interval. Correlation analysis using Statistical Package for Social Scientients’ (SPSS version 16.0) was also performed to assess relationship between sensory attributes. Descriptive sensory analysis results showed that all 9:00hrs samples were rated highly for each attribute compared to the 12:00hrs and 15:00hrs samples. ANOVA results for S. aethiopicum showed statistical significant (p<0.05) difference for all the attributes except for light green color of leaf stalk (p<0.05) whereas that for A. lividus showed significant differences for moist appearance, well spread appearance, smoothness and overall quality. Correlation results showed significant positive relationship (p<0.05) among attributes. This study observed that sensory attributes of leafy vegetables change with time after harvest andtraders are therefore encouraged to adopt local cooling systems to help preserve the sensory attributes of vegetables.

1. Introduction

Sensory quality of fresh produce is a very important attribute that is always required by consumers ¹. Attributes such as freshness, brightness, colour, absence of defect and cleanliness are essential for product acceptance by the consumer ^{1, 2} and may influence sensory shelf life of vegetables.

Mechanical damage, temperature, enzymatic senescence processes and browning of fresh vegetables negatively affect their sensory properties and finally result in the rejection of affected products by consumers ². Cutting method of leaves during food preparation has also been reported to cause changes in the sensory quality of vegetables ³. However, various mechanisms have been explored in maintaining sensory attributes. The applications of natural preservatives, plus optimal storage conditions, the use of packaging material have been reported to maintain sensory attributes in vegetables for some time. ^{2, 4, 5}. Refrigeration at household and supermarkets is practiced; moreover, local cooling techniques such as charcoal cooler have also been invented. Despite the available mechanisms of maintaining sensory attributes, farmers and traders in Uganda have not widely explored them especially for leafy vegetables. This is attributed to the expensive equipment and high maintenance costs, unavailability of the required equipment and tools in the local markets, lack of sanitary and packaging facilities and poor marketing systems. Moreover, they lack knowledge and appreciation for the need to maintain quality of vegetables ⁶.

From literature, little has been done regarding profiling and assessing sensory attributes of raw leafy vegetables particularly S. aethiopicum and A. lividus. However, changes in sensory attributes of vegetables over time under different conditions have been reported ^{2, 7, 8}. This study fills an important knowledge gap of profiling sensory attributes and assessing changes in color, texture and appearance of S. aethiopicum and A. lividus leafy vegetables over time after harvest.

2. Materials and Methods

Two leafy vegetable types; S.aethiopicum (Shum) and A.lividus (Linn) were purposively selected because they are the most commonly cultivated and marketed vegetables in Central Uganda. They are grown for their ready market, palatability and high nutritive value ⁹ and yet little about their sensory properties is known.

Vegetable samples were picked from the markets at 9:00hr,12:00hr and 15:00hr after ascertaining that they were from the same farm; this was done through farm visits during harvesting and following up the truck during transportation up to the market. The samples that were picked were then wrapped in a black polythene bag and placed in a cool ice box and transported to the laboratory at the Department of Food Technology and Nutrition for sensory evaluation. Sensory evaluation was done as the vegetable samples came in at the respective hours.

Quantitative descriptive sensory analysis was used to profile and investigate changes in sensory attributes of vegetables with time after harvest (9:00hrs/morning, 12:00hrs/noon and 15:00hrs/evening). S. aethiopicum and A. lividus vegetable samples were picked from the markets at these respective times of the day after ascertaining that they were from the same source (farm).The samples that were picked were wrapped in a black polythene bag and then placed in a cool box and transported for further laboratory analysis at the Department of Food Technology and Nutrition.

Seven (3 females and 4 males) trained and experienced panelists were selected to participate in the development of sensory profiles for assessment of changes for the two types of leafy vegetables. They were chosen based on their ability to provide similar responses on similar vegetable samples on repeated occasions, interest in the study and availability for the duration of the study. Panelists were exposed to a two- day training session of one hour so as to develop a clear definition for each attribute of importance in the respective vegetables. Each panelist received a representative specimen and with training, they were able to increase their sensitivity and ability to discriminate between the sensory attributes of the leafy vegetables. Descriptors and definitions of the characteristics of the vegetables were collectively agreed on and developed by the panelists.

A 5 point category scale ranging from 1 to 5 was used to measure the intensity of each sensory attribute (appearance, color and texture) for the two vegetables; where 1 denoted the least intense and unacceptable condition (Highly unacceptable) and 5 denoted the most intense and acceptable condition (Highly acceptable). The overall quality was also rated using the same scale. A 5 point scale was chosen and modified from the ‘just about right scale’ as suggested that this kind of scale generates both intensity and acceptability at the same time ¹⁰. ⁷ also used the same denotation on the category scale except that they used a 9 point scale.

Attributes described were appearance, colour and texture using descriptors generated during the training; appearance had three descriptors (moist, well spread leaves and whole leaves) for both S. aethiopicum and A. lividus leafy vegetables, colour also had three descriptors (Dark green top face, pale green bottom face and light green/purplish stalks) for S. aethiopicum. Colour was described differently for A. lividus (green with purple patches on top face of the leaf, purple with green patches bottom face of the leafy and purple leaf stalk). Texture had two descriptors for S. aethiopicum (rough leaf surface and firm/stiff leaves) and three for A. lividus (smooth top face of the leaf, rough bottom face of the leaf and firm/stiff leaves).

On arrival from the market, the roots and soil were removed and a portion of approximately 300g of raw vegetable sample was served on plastic trays and coded with a three digit figure. One type of vegetable was served at a time. A score sheet for evaluation accompanied the sample. All samples were evaluated by a trained panel according to methods by ¹⁰. Samples were evaluated under a well-balanced light and each panelist sat in an individual booth.

A complete randomized design in which each vegetable sample was subjected to three treatments (hour of the day) and three replicates was used resulting in a total of 18 vegetable samples for both S. aethiopicum and A. lividus.

The results were subjected to descriptive analysis, and when it was confirmed that the data followed a normal distribution, one-way analysis of variance was performed and mean comparison between the treatments was by least significant difference test (LSD) with confidence level of 95% using Statistical Package for Social Scientists software (SPSS version 16.0) . Correlation matrices of the sensory attributes and overall quality data were also performed using Pearson’s correlation coefficients.

3. Results and Discussion

Sensory profiles of eight (8) and nine (9) descriptors were developed for S. aethiopicum and A. lividus; respectively using quantitative descriptive sensory analysis (Table 1 & Table 2). The final list of descriptors developed by the panelists encompassed; appearance, color and texture. Attributes and definitions for S. aethiopicum indicated three attributes for appearance, three for color and two for texture (Table 1). Attributes and definitions for A. lividus indicated three attributes for appearance, three for color and three for texture (Table 2), with definitions for each attribute and overall quality for both types of vegetables.

Descriptive Sensory Analysis was carried at 9:00hrs, 12:00hrs and 15:00hrs. Results show all 9:00hrs samples being rated highly for each attribute compared to the 12:00hrs and15:00hrs samples for both types of vegetables (Figure 1 & Figure 2).

Changes in sensory attributes over time after harvest were observed and all 9:00hrs samples were rated highly for each attribute compared to the 12:00hrs and 15:00hrs samples .This implies that during morning hours (up to 9:00hrs), changes due to environmental and postharvest practices are minimal due to the little heat from the sun and therefore this can be a recommended time for purchase of leafy vegetables. Literature on this kind of study is limited, however, some studies have looked at changes in sensory attributes of non-leafy vegetables such as carrots and other products such as wine, cheese, honey over a storage period ^{11, 12, 13, 14}. Those that have looked at leafy vegetables like lettuce, Chicory have subjected them to conditions like cooking methods and storage period ^{2, 3, 4, 5, 6, 7}. This particular study has looked sensory changes in one day of sale. This is because vegetables are expected to be consumed when still fresh and most of the traders sell their vegetables in one day.

Appearance characteristics for S. aethiopicum were rated highest at 9:00hrs. However as time increased, more negative change was observed. From Figure1a, an observable decrease in appearance between 9:00hrs, 12:00hrs and 15:00hrs were observed. Despite the higher rating of other attributes for A. lividus at 9:00hrs, appearance characteristics were rated highest at 12:00hrs. As expected, change in appearance characteristics were observed at 15:00hrs. The observed changes in appearance attributes can be attributed to the high temperatures that cause loss of moisture making the leaves appear wilted and folded. Wilted appearance is not a desired sensory attribute and this can cause rejection of the vegetable by the consumer since appearance is one of the important quality attributes ¹. The loss of moistness is associated with loss in moisture content which later leads to loss of vigor ².

Color for both types of vegetables rated highly for all the times (9:00hrs, 12:00hrs and 15:00hrs) as compared to other sensory attributes. This implies that color deteriorates but at a slow rate over time. It can be assumed that processes such as oxidation and enzymatic browning that are responsible for color change have no significant effect on vegetables within a day of harvest. Also cool temperatures during rainy days are assumed to maintain an attractive color on the vegetables. This could be another reason for the high color intensity rating by the panelists.

There was an observed deterioration in texture characteristics for both vegetables. However, deterioration was greater in S. aethiopicum as compared to A. lividus. The low intensity ratings of texture could be attributed to the improper handling of the vegetables from farm and in the market. Roughness and firm leaves described the texture of the vegetables and the later scored the least intensity. This implies that the rough handling, improper transport means and high temperatures of the day caused the leaves to lose their firmness and hence the poor texture observed. The loss in firmness can also be attributed to dehydration ³. A decrease in firmness during day 1 of cold storage of fresh Broccoli was also reported ¹⁵.

Overall samples were evaluated as of acceptable quality though A. lividus samples were more acceptable than those of S. aethiopicum. Generally, 9:00hrs samples were rated highest while evening samples rated lowest in overall quality. This shows evidence that the sensory quality of vegetables without refrigeration deteriorates in one day. Leafy vegetables should therefore be consumed on the day of harvest if no other means of preservation are available.

ANOVA results showed significant (p<0.05) differences in all attributes of S. aethiopicum over time after harvest except for light green color of the leaf stalks. For A. lividus, significant (p<0.05) differences were observed for moist appearance, well spread appearance, smoothness and overall quality. These results are related to the descriptive analysis results above where there is an observed variation the sensory attributes particularly for S. aethiopicum over time after harvest. These findings are related to those by ^{2, 12} who reported significant (p<0.05) differences in the intensity ratings of all attributes such as color and freshness (appearance) during storage. Significant (p<0.05) mean differences were also observed in S. aethiopicum for moist appearance and firmness and none for A. lividus between 9:00hrs and 12:00hrs. Between 12:00hrs and 15:00hrs, significant (p<0.05) mean differences were observed in well spread appearance and texture characteristics for S. aethiopicum. For A. lividus, significant (p<0.05) mean differences were observed in moist appearance, well spread appearance, smooth leaf texture and overall quality. From this observation, appearance and texture characteristics for both types of vegetables deteriorated more than other attributes and these highly impact on acceptability of the respective vegetables in the market. From the statistical analysis, it appears that S. aethiopicum is more prone to degradation compared to A. lividus. This observation can be attributed to characteristics of leafy structure specific to each type of vegetable ².

Sensory attributes for both types of vegetables had a positive significant (p<0.05) relationship(r=0.5 & above) among each other. These findings relate to that of ¹⁶ where attributes like texture, aroma, and flavor and after taste had significant correlation coefficients. Stronger positive correlations of 0.8 were observed for more attributes of S. aethiopicum compared to that A. lividus. More related to these findings are those reported by ¹⁷ where positive correlations between aftertaste and texture descriptors in fresh orange juice were observed. Furthermore the strong positive correlations were observed between color characteristic for both vegetables. Color is an important attribute in the choice of any product in the market. The loss or change in color for example yellowing of vegetables may mean spoilage. There was a strong relationship between firm texture and well spread appearance in S. aethiopicum vegetable (p=0.00, r=0.812) and not in A.lividus. The loss of moisture causes loss in vigor which then affects the firmness and the well spread appearance of the leaves. Strong positive correlations among sensory attributes were also observed in bread ¹⁸. For vegetables, overall quality and well spread appearance (p=0.00, r=0.843; p=0.000, r=0.832) respectively had a strong relationship. This implies that appearance has a great impact on the acceptable quality of these vegetables. Loss in appearance characteristics means loss in quality and acceptability as well. It is therefore important to preserve these appearance characteristics. Traders in the market sprinkle water on the vegetables from time to time in an attempt to preserve these characteristics. However, the best option would be keeping temperatures low for example by refrigeration/cooling system which is sustainable.

4. Conclusion

From the findings, it can be concluded that sensory attributes of leafy vegetables deteriorate with time after harvest especially when there is no cooling system in place. Due to the lack of cooling systems, consumers are encouraged to purchase leafy vegetables in the morning hours since morning (9:00hrs) samples showed highly acceptable sensory attributes. Traders are also encouraged to adopt local cooling systems like charcoal coolers to help preserve the sensory attributes of vegetables.

References