Abstract

Preservation of highly perishable farm produce at household level is very essential in promoting food security. In this paper, we examine the effects of freezing and drying on the sensory attributes of okra. Fresh-frozen, blanched-and-frozen, and sun-dried okra samples were preserved for eight weeks. Portions of the preserved okra samples (treatments) and fresh okra (control) were cooked in equal quantities of lightly salted water. A preference test was then conducted using a panel of 20 judges. Paired sample t-tests between the treatments and the control showed that the blanched and frozen okra was the preferred preserved sample type, with sensory attributes closest to those of fresh okra. Given that consumers are more likely to accept preserved vegetables that retain most of their sensory qualities, it is recommended that blanched and freezing are employed to preserve fresh okra at the household level.

1. Introduction

According to the Swiss Agency for Development and Cooperation (SDC) ¹, food security and nutrition are major global challenges. Although the proportion of undernourished people in developing nations has decreased by almost half since 1990 (from 23.3 percent in 1990-92 to 12.9 percent in 2014-16), almost 800 million people worldwide still suffer from hunger and are unable to consume enough food for a healthy life ^{1, 2}. In addition, 2 billion people suffer from micronutrient deficiencies. Currently, only 3 million people escape chronic undernourishment annually, whereas 60 million people would need to do so annually to meet the target of 800 million people by 2030 to achieve a world without hunger ¹. According to the 2002 World Food Summit, food security exists when all people have access at all times to safe, sufficient, and nutritious food to meet their dietary needs and food preferences for an active and healthy life ³. Considering the global food supply and the population growth rate, it has been estimated that food production will need to increase by 70% to feed the world’s population in 2050 ^{4, 5}. Considering the many challenges facing agricultural food production in developing countries, it will be difficult to meet this target by 2050 ^{4, 6, 7}.

Food loss and waste have been reported to account for a third of food produced for human utilization worldwide ^{8, 9}. According to Ridolfi et al. [9], research funded by the Food and Agriculture Organization of the United Nations (FAO) on postharvest loss has shown that approximately a third (1.3 billion metric tons) of food produced for human consumption is lost worldwide annually. The study indicated that sub-Saharan African countries recorded relatively high proportions of food loss and waste compared to other low-income countries. Disproportionally greater losses during production, postharvest handling, and storage occur for food crops, vegetables, roots, and tubers. Food is lost when edible food mass decreases at any stage of the food supply chain. Food loss and waste must be reduced to improve food security ^{5, 10}.

The Economist Intelligence Unit ¹¹ rates countries’ food security using a food security index (on a 0–100% scale) based on three dimensions— food availability, affordability, and safety. In 2014, Ghana’s food security score was 43%, which corresponded to a moderate degree of food security. In many developing countries such as Ghana, fresh vegetables are grown mostly by smallholder farmers. Seasonal production of fresh vegetables makes them abundant during the rainy season and scarce in other seasons ^{12, 13, 14}. Ridolfi et al. ¹⁴ and the Affognon et al. ⁷ have estimated that Ghana loses approximately 20–50% of its fruits, vegetables, roots, and tubers annually. In Ghana, vegetables, fruits, and nuts account for approximately 10% of total food output and 23% of household consumption expenditures. This disparity in the percentages of food production output and household expenditures on fruit and vegetable in Ghana suggests that these food categories are important in the Ghanaian diet and that their adequate provision is a concern to both producers and consumers in Ghana ^{15, 16}. Data on food preservation at the household level in Ghana show that households process large proportions of cereal and tuber into flour and dough and process nuts into paste and oil for storage. However, fruits and vegetables typically receive less processing attention from households ^{16, 17}. A study conducted by the International Centre of Insect Physiology and Ecology (ICIPE) on major crops loss in Ghana during postharvest handling and storage indicated that okra suffers the largest loss percentage (30.0%), followed by tomatoes (28.8.%), yams (12.8%), mangos (8.7%), and maize (6.2%) ¹⁰. Okra is sold in almost every market in Africa and is one of the most commonly consumed vegetables in Ghana ¹⁸. Immature green okra and leaves are mainly cooked as vegetables. Dried okra is ground into powder, preserved, and used in stews and soups. Fresh okra consists primarily of water (85%), fat (0.5%), protein (4%), and carbohydrate (5.4%). Okra has a high quantity of folate, which is a vital nutrient for healthy prenatal brain growth and development ¹⁹.

Okra is one of the major vegetable crops cultivated by smallholder farmers in Ghana, with an estimated annual production output of approximately 1,548–4,507 metric tons ^{16, 20}. Okra has a short shelf life, as the fresh pods deteriorate rapidly under ambient conditions. Okra is harvested and sold fresh in almost all markets in Ghana. Nevertheless, okra is prone to aging and fibrosis after harvest because of its high initial moisture content (88–90% wet matter) ^{18, 20}. During the main production season, large amounts of fresh okra deteriorate. This situation often compels producers to sell their harvested okra pods at less than 50 percent of the expected price, and retailers sometimes throw away unsold okra pods at the end of the market day to avoid incurring extra costs for transporting unsellable produce back home ¹⁰. Okra is usually found in dehydrated form during the dry season, when it is scarce and expensive ¹⁸.

Postharvest preservation makes local foods available and less costly even during off-seasons. Small-scale food preservation practices can ensure the presence of various vitamins and minerals in daily diets. Therefore, research on small-scale preservation of highly perishable foods at the household level to prevent loss is timely. Although okra may be preserved in small quantities by individuals or households by freezing and drying techniques, the author has not seen a comparative analysis of the effects of freezing and drying on the sensory attributes of okra. The use of unsuitable preservation techniques may adversely affect the sensory qualities of stored vegetables, and consumers are more likely to accept preserved vegetables that retain most of their sensory attributes ^{21, 22}. The main objective of this study was to examine the effects of freezing and drying on the sensory attributes of okra. The hypothesis tested was that there were no significant differences in the sensory attributes of fresh and preserved okra samples.

Food preservation is a group of methods for treating food to stop or greatly slow spoilage caused by microorganisms ^{23, 24}. These methods involve taking actions to maintain the desired properties of foods for longer periods. Consumers’ demand for healthier and more convenient foods has been found to affect how food is preserved ²². Food preservation extends seasonal availability and ensures longer shelf lives. Additionally, food preservation retains food quality in terms of color, aroma, flavor, and texture. However, the quality of preserved food may be diminished in terms of its sensory attributes, depending on the preservation method employed. Ineffective preservation may result in loss of quality ranging from minor changes, such as color loss, to severe changes that result in the food becoming unfit for consumption. Food preservation methods are categorized as heat treatment, chemical application, drying, and freezing methods ^{25, 26}.

Heat treatment involves exposing food to high temperatures, which is an effective way of preserving food because most harmful pathogens are killed at temperatures close to the boiling point of water ^{25, 26}. Blanching is a form of heat treatment that involves brief exposure to hot water or steam (for a few minutes) to inactivate enzymes. Blanching is not a primary food preservation method but rather a pretreatment method carried out before a primary preservation method. Blanching helps stabilize the color and protects food’s flavor and texture, especially green vegetables. Overblanching of fruits and vegetables can cause excessive softening and loss of flavor and color. Blanching vegetables prior to freezing or drying reduces bacterial load and helps to preserve sensory attributes such as color, aroma, taste, and texture ^{23, 24}.

Drying is one of the oldest methods of food preservation. Drying preserves food by reducing water activity sufficiently to prevent or greatly delay microbial growth. Microorganisms need moisture to grow and reproduce. Food preservation by drying decreases the water content of the food below a certain critical value and thereby depresses microorganisms’ growth. Eliminating water from food makes it unavailable to bacterial cells and prevents their multiplication ^{23, 24}. A low water content slows the rates of respiration, enzymatic action, and overall deterioration that make food products susceptible to decay. Many fruits and vegetables contain considerable amounts of water that make them susceptible to spoilage by microorganisms. Drying fruits and vegetables lower their water activity to levels that inhibit their spoilage during long-term storage. While drying is a good method of food preservation, some horticultural produce may lose some vitamins and/or develop unpleasant tastes after drying. For example, drying can decrease the contents of fat-soluble vitamins such as vitamin B (carotene) by as much as 25 percent, and vitamin C is very sensitive to light and oxygen ^{25, 26}. Enzymes in foods can be inactivated by temperature and moisture changes. Microorganisms require free water for biochemical reactions. When water is removed from food, it becomes unavailable to microbial cells.

Food can be preserved by freezing because microorganisms do not multiply below 18°C (0°F), and biochemical reactions are increasingly suppressed as temperatures fall below 0°C ²⁴. Freezing eliminates the presence of liquid water as the essential medium for microbial action. However, freezing can adversely affect the sensory quality of food, particularly texture. Freezing causes the formation of ice crystals in foods, resulting in profound effects on food texture, taste, and aroma. Fruit and vegetables with high moisture contents do not freeze well because their cellulose structures tend to be broken down by freezing, making the fruits and vegetables undesirably soft upon thawing. Blanching prior to another method of food preservation allows many vegetables to retain their natural color, flavor, and texture ^{25, 26}. The use of domestic freezers to store vegetables in smaller quantities by individuals or households can be an effective way to promote food security.

2. Materials and Methods

Fresh okra pods were purchased from a local market in Cape Coast. The selected okra samples were washed with tap water and drained to remove foreign materials. The stems and tapered tips of the cleaned pods were cut off to form cylindrical shapes approximately 50 mm long. Each trimmed pod was then cut into pieces approximately 10 mm long. The cut okra pieces were divided into three portions. Charcoal ash was sprinkled on one portion of the cut pieces of fresh okra, and the pieces were then sun-dried in a simple rectangular household basin covered with plastic netting to protect them from flies. Charcoal ash has been reported to prevent fungus from spoiling dried okra. The sun-drying was carried out for seven days until the pieces reached the desired moisture content. The sun-dried okra pieces were placed in moisture-resistant polythene bags and then stored on a shelf. One of the remaining two portions of cut okra pieces was blanched in very hot water for approximately 2 min. The blanched okra pieces and the remaining unblanched okra pieces were preserved in a domestic deep freezer for eight weeks.

The three preserved okra samples (fresh-frozen, blanched-and-frozen, and sun-dried) represented three preservation treatments compared to a control (fresh okra). The three preserved okra samples and a control sample were cooked in equal amounts of lightly salted water for approximately 9 min. An evaluation sheet based on 9-point hedonic scales was used by the judges to evaluate the treated and control samples. Nine-point hedonic scales are widely used in consumer research to assess various aspects of food acceptability. Each scale ranges from a value of 1, denoting extreme dislike, to a value of 9, denoting extremely high liking. A mean preference score of 7 or higher on a 9-point scale is usually suggestive of a highly acceptable sensory quality.

Such scales allow researchers to compute the mean values of responses and compare the mean values using parametric statistical tests, such as analysis of variance and t-tests ²⁷. Students and staff members from the School of Agriculture at the University of Cape Coast were invited to participate in the evaluation. This is a self-sponsored study and the researchers were not required by any agency to receive ethical approval for the study. Nevertheless, all the respondents agreed to participate in the research study and they were free to participate with or without any justification. The consent to publish individual data in any form was obtained from the study participants. Twenty judges who reported using okra in their diets on a regular basis were provided with prior skill training on sensory evaluation of food. Each of the judges was provided with tissue paper, spoons for tasting, and a palate cleanser (sparkling water) between tastes to avoid the transfer of sensory qualities. Pieces of each of the cooked okra samples (fresh, fresh-frozen, blanched-and-frozen, and sun-dried) were placed on plates labeled A, B, C, and D. The judges were asked to rate their preferences and acceptability of the four okra samples in terms of color, aroma, taste, and texture, using a 9-point hedonic scale for each attribute, as well as a 9-point hedonic scale for overall acceptability.

It was hypothesized that there are no significant differences in the sensory attributes of fresh okra and the three preserved okra samples. To test the hypothesis, paired-sample t-tests were performed, and eta squared values were computed to assess the magnitude of any significant effects detected. Paired-sample t-tests are suitable for evaluating two related observations per subject and determining whether a significant difference exists in the means of two treatments with respect to normally distributed variables ²⁸. The statistical significance (p-value) of a test reflects whether a significant effect exists, but it does not reveal the size of the effect. Eta squared is an appropriate effect size statistic. For example, values of 0.2, 05, and 0.8 for eta squared are considered to reflect small, moderate, and large effect sizes, respectively. By determining both p values and ta squared values, a researcher is well-positioned to know whether and how much a treatment affects a dependent variable ^{28, 29}.

3. Results and Discussion

Sensory attributes greatly influence consumers’ food preferences ^{30, 31}. However, inappropriate food preservation methods can adversely affect the quality of sensory attributes. Identifying and utilizing appropriate preservation methods that optimally retain the sensory attributes of vegetables is therefore important ^{24, 25, 26}. The statistical results of the effects of freezing and drying on the sensory attributes of preserved okra are discussed below.

The paired-sample t-test results shown in Table 1 indicate a statistically significant difference between the color of the fresh okra sample (control) (M = 7.90, SD = 1.62) and the color of the fresh-frozen okra sample (M = 5.30, SD = 1.34) (t₍₂₀₎ = 7.41, p < 0.00). The magnitude of the effect of freezing on the color of the fresh-frozen okra sample was moderate (eta sq =.59). Similarly, there was a significant difference between the color of the fresh okra sample (control) (M = 7.90, SD = 1.62) and the color of the dried okra sample (M = 4.15, SD = 1.90) (t₍₂₀₎ = 7.55, p < 0.00), and the magnitude of the effect of drying on the color of the sun-dried okra sample was also moderate (eta sq. = .60). There was no significant difference detected between the color of the fresh okra sample (control) (M = 7.90, SD = 1.62) and the color of the blanched-and-frozen okra sample (M = 7.00, SD = 1.52) (t ₍₂₀₎ = 1.96, p < 0.06), and the magnitude of the effect of blanching and freezing on the color of the blanched-and-frozen okra sample was small (eta sq. = .06). Color is an essential aspect of the human sensory experience of food. Natural bright colors give the sensory impression of high-quality, healthy, nutritious food, whereas colorlessness gives an impression of poor food quality ³². A given vegetable is known to have a specific color when fresh and ripe. The outer cell walls of edible fresh okra pods are normally green in color. The blanched-and-frozen okra sample therefore emerged as the most preferred preserved okra sample with respect to color. This can be explained by the fact that blanching and freezing cause little physical or chemical change to okra and therefore help it to retain a green color similar to that of fresh okra.

Aroma and taste are major components of flavor. Aroma is the fragrance or odor of food. Vegetables contain volatile aromatic compounds in their intact tissues that give them characteristic aromas ³¹. Sometimes, these compounds undergo changes following processing and preservation ^{8, 9} (Spence, 2015; Srivastava & Kumar, 2017). The paired-sample t-test results showed that there was no significant difference between the aroma of the fresh okra sample (M = 7.45, SD = 1.30) and the aroma of the blanched-and-frozen okra sample (M = 6.66, SD = 2.41) (t₍₂₀₎ = 1.68, p < 0.11). The magnitude of the effect of blanching and freezing on the aroma of the blanched-and-frozen okra sample was small (eta sq.= .07).

According to Ramaswamy ²⁴ (2021), blanching tends to preserve the flavor of food, especially green vegetables. There was a statistically significant difference between the aroma of the fresh okra sample (control) (M = 7.45, SD = 1.30) and the fresh-frozen okra sample (M = 5.35, SD = 1.60) (t₍₂₀₎ = 5.30, p < 0.00), and the size of the effect was near to moderate (eta sq.= .43). There was also a statistically significant difference between the aroma of the fresh okra sample (control) (M = 7.45, SD = 1.30) and the aroma of the dried fresh okra sample (M = 4.10, SD = 1.21) (t₍₂₀₎ = 11.45, p < 0.00), and the effect size was large (eta sq.= .78). The judges’ ratings of the aromas of the fresh-frozen and sun-dried samples as being quite different from the aroma of the fresh okra sample can be attributed to the undesirable effects of freezing and sun-drying on the two preserved samples ^{25, 26}.

The paired-sample t-test results shown in Table 3 indicate a statistically significant difference between the taste of the fresh okra sample (M = 7.30, SD = 1.56) and the taste of the fresh-frozen okra sample (M = 5.80, SD = 1.70) (t₍₂₀₎ = 3.94, p < 0.00). There was also a statistically significant difference between the taste of the fresh okra sample and the taste of the sun-dried okra sample (M =3.70, SD = 1.34) (t₍₂₀₎ = 9.31, p < 0.00). The eta squared values (.28 and .70) indicate small effect size in taste for the fresh-frozen and moderate effect size in taste for the sundried okra samples, respectively. The results showed that there was no significant difference between the taste of the fresh okra sample (M = 7.30, SD = 1.56) and the taste of the blanched-and-frozen okra sample (M = 7.02, SD = 1.87) (t₍₂₀₎ = 1.32, p < 0.20). Food taste components (sweet, sour, bitter, salty, umami, and fat) arise from chemical compounds associated with specific macronutrients in foods ³⁰. However, these chemical compounds can be diminished or lost by some preservation methods ^{25, 26}. Blanching and freezing might have contributed to the retention of macronutrients in the blanched-and-frozen okra sample, making its taste comparable to that of the fresh okra sample. It could be concluded that freezing (without blanching) or drying might have adversely affected the taste of the other two treated samples, resulting in diminished desirable taste sensation compounds or the development of unpleasant taste sensation chemicals ^{24, 25}.

Food texture is a physical property arising from the structural constituents of food and is often sensed by touch. Consumers have specific expectations for given food textures ³³. Consumer perceptions of diminished food texture are closely related to the disintegration and decomposition of food. Undesirable textures can turn consumers off ³³. The paired-sample t-test results shown in Table 4 indicate that the blanched-and-frozen okra sample had the most acceptable texture (gelatinous) of the preserved sample, with no significant difference in mean value (M = 7.20, SD = 1.51) from the fresh okra sample (control) (M = 7.75, SD = 1.37) (t₍₂₀₎ = 1.99, p < 0.06) as reflected by the small effect of blanching and freezing on the texture (eta sq.= .09. In contrast, there was a statistically significant difference in texture between the fresh okra sample (control) (M = 7.75, SD = 1.37) and the fresh-frozen okra sample (M = 5.95, SD = 1.15) (t₍₂₀₎ = 4.49, p < 0.00), with a small effect size (eta sq. = .35). Similarly, the judges’ assessment of the texture of the fresh okra sample (control) (M = 7.75, SD = 1.37) was significantly different from that of the dried okra sample (M = 3.9, SD = .97) (t₍₂₀₎ = 12,08, p < 0.00), and the effect size was large (eta sq. = .90). This finding could be explained by the fact that vegetables such as okra with high moisture contents do not freeze well because their cellulose tends to be broken down by enzymes, which makes the vegetables soft. Freezing causes the formation of ice crystals within food structures, with resulting profound effects on texture. Blanching prior to freezing helps the okra retain its texture ^{25, 26}.

A consumer’s preference for and opinion of the acceptability of a given vegetable depends on a combination of sensory attributes (color, aroma, taste, and texture) ³⁰. The paired-sample t-test results in Table 5 show statistically significant differences between the judges’ overall acceptability ratings of the fresh okra (control) (M = 8.00, SD = 1.17) and those of the fresh-frozen okra (M = 5.70, SD = 1.26) (t₍₂₀₎ = 7.07, p < 0.00. The magnitude of the effect of freezing on the overall acceptability of the fresh-frozen okra sample was moderate (eta sq. = .57). There was also a statistically significant difference between the overall acceptability of the fresh okra (control) (M =8.00, SD = 1.17) and the dried okra (M = 4.25, SD = 1.29) (t₍₂₀₎ = 10.81, p < 0.00) with a large effect size (eta sq.= .75). However, there was no significant difference between the overall acceptability of the fresh okra (control) (M =8.00, SD = 1.17) and that of the blanched-and-frozen okra (M = 7.55, SD = 1.32) (t₍₂₀₎ = 1.44, p < 0.00), as also reflected by the small effect of blanching and freezing on the overall acceptability (eta sq.= .05). The blanched-and-frozen okra sample emerged as the overall preferred preserved sample, with sensory attributes closest to those of fresh okra. The fresh-frozen okra was less preferred than the blanched-and-frozen okra because freezing vegetables without blanching results in the formation of ice crystals that adversely affect sensory attributes. The sun-dried okra sample was the least preferred because sun-drying caused changes in and some loss of micronutrients, resulting in a darkly colored product with less desirable sensory attributes. This finding is consistent with those of previous studies that have shown that consumers typically do not like sun-dried okra ¹⁰.

4. Conclusion

Small-scale preservation of fruits and vegetables such as okra preservation at the household level is vital to achieving food security as food wastage prevents households from obtaining the required diets. The findings of this study indicated that blanched frozen okra sample is non significantly different from the fresh okra sample in terms of sensory attributes and overall acceptability. It is recommended that blanched-freezing is employed to preserve fresh okra at the household level as these preservation techniques retain most of the sensory attributes of the fresh okra. This study has improved our understanding of what kind of preserved okra product consumers would prefer and consume.

References