Utilization of Vegetable and Fruit By-products as Functional Ingredient and Food

With the constant growth of the human population, the global demand for food is increasing annually. Food security is an arising issue due to decreased resources and massive waste production from the agricultural sector. For example, not all parts of fruits and vegetables are consumed by consumers, and this phenomenon can lead to huge amounts of food wastes that are produced globally. Moreover, non-utilized agriculture by-products, including seed coat, hull, husk, peels, seeds, and pomace, can cause environmental issues. Hence, efficiently utilizing food wastes, such as vegetable and fruit by-products, could be a way to increase food sustainability, and in line with the United Nations Sustainable Development Goal (SDG) to ensure sustainable consumption and production patterns. Moreover, certain agriculture by-products are reported to have a high nutritional value and could be potentially used as functional ingredient and food in the food industry. This review article summarizes findings on the development of new functional foods by utilizing different types of agriculture by-products, that is, vegetable and fruit by-products as ingredients. Furthermore, the nutritional values, processing methods, product acceptability, and potential uses of these vegetable and fruit by-products are also discussed. These by-products can be an alternative source of nutrients to support the global demand for functional foods and as one of the strategies to cope with food insecurity. Studies have shown that different types of fruit and vegetable by-products were well-incorporated in the development of functional foods, such as bakery products and dairy products. Of great importance, this review article provides an insight of the nutritional value, health benefits, and utilization of fruit and vegetable by-products.

Introduction

In the agriculture field, postharvesting, processing, distribution, and consumption sectors could generate agriculture by-products that are wasted in huge amounts, which can contribute to the food waste problem (1). These food wastes are part of unintentional and intentional losses, which can lead to wastage of agriculture by-products (2). The unintentional losses include inadequate farming technologies and lack of proper transportation, whereas the intentional losses include due to human eating habits. Approximately 45% of fruits and vegetables are wasted worldwide, which is one of the categories with the highest wastage rate (1). The highest agricultural wastage is found in Europe, Latin America, North America, and Oceania, and it accounted for ~10% higher than that of industrialized Asia (1). Moreover, the highest production wastage is found in Sub-Saharan Africa, which constitutes ~20% (1).

It is of great concern that a massive amount of agricultural by-products contributes to the food waste problem, subsequently, it can further lead to many environmental and economic issues (1). According to FAO, food loss and waste is the second highest cause of greenhouse gas emission. Statistically, 1.3 billion tons of wasted foods caused the emission of around 4.4 gigatons of greenhouse gas (1). Greenhouse gases are mostly originated from landfill emissions of decaying food, on-farm agriculture emission, electricity and heat in the manufacturing process, and the energy used for agriculture by-products that are lost or wasted. Clearly, these events can cause the greenhouse effect and lead to global warming and climate change, which might further affect the extinction of some animal species (3).

From the economic perspective, agricultural by-products and wastages can affect both the income of farmers and the expenses of consumers. For example, the cost of food waste is very high, especially in countries with a high population such as in China and the United States of America (4). It is evident as foods worth $32 billion are thrown away yearly in China. Moreover, on average $1,600 worth of food are wasted annually from a family of four in the United States of America. These large amounts of food wastes have a big impact on the income of farmers (1). In the sub-Saharan Africa, farmers earn only <$2 in a day due to a large amount of postharvest losses. In fact, $940 billion worth of foods are estimated to be lost or wasted annually, and the World Bank estimated $40 million of economic gains could be achieved with just 1% reduction in postharvest losses, where this can benefit the small farmers (3). The extremely huge amount of wastage has drawn the attention of researchers to look for an alternative of utilizing fruit and vegetable by-products. Therefore, utilizing agricultural by-products not only helps the farmers and economics of customers, but also it can increase food sustainability and reduce food insecurity, especially for the underdeveloped countries. This review summarizes different types of food developed using fruit and vegetable by-products. Through this review article, the potential use of agricultural by-products is explored and discussed.

Agricultural By-products

Agricultural by-products or wastes are the residues from the growing and processing of raw agricultural products, such as fruits, vegetables, meat, poultry, dairy products, and crops (5). Moreover, the wastes include animal wastes, such as manure and animal carcasses, food processing waste, crop waste, and dangerous and toxic agricultural waste, such as pesticides, insecticides, and herbicides. These agricultural wastes can exist in the form of liquid, slurry, or solid.

It is estimated that about 998 million tons of agricultural wastes are produced annually due to the expanding agricultural production. With the drastic demand, there is a significant increase in livestock waste, agricultural crop residues, and agro-industrial wastes. Hence, the expansion of the farming systems in developing countries increases the global agricultural wastes. It is evident as organic wastes generate up to 80% of the total solid wastes in any form, whereas the manure production can reach up to 5.27 kg/day/1,000 kg live weight, on a wet weight basis (5). Agriculture by-products are also largely contributed by the improper utilization of fruits and vegetables that are commonly overproduced seasonally (6). Vegetable and fruit by-products represent 44% of global food wastes by commodity, whereas roots and tubers contribute that of 20%, and cereal constitutes that of 19% (1, 7). In this review article, two types of agricultural by-products, that is, vegetable and fruit by-products are further discussed on their usefulness as functional ingredient and/or foods.

Vegetable By-products

Vegetable by-products are the secondary products that are often discarded or wasted during manufacturing or other stages of food processing. Up to one-third of vegetables could be wasted in the preparation process. Interestingly, certain parts of the vegetables are knowingly wasted due to their unfavorable taste or texture. For example, vegetable parts, such as hulls, bagasse, and seeds, are mostly discarded in the production line. For certain types of vegetables, such as broccoli, cauliflower, and pumpkin, the stem and leaves are not consumed and thrown away.

The seed coat, which is the outer layer that covers the seeds or beans, functions to protect them from external damages (8) and is one of the vegetable by-products. Seed coats are usually not consumed due to the texture and taste, except for some thin seed coats, such as peanut seed coats. In the canned food industry, seed coats are discarded in a large quantity as ~22% of oilseeds and pulses worldwide are lost or wasted annually (1). The wastages of oilseeds and pulses are highest in the North Africa, West Asia, and Central Asia regions, and these by-products are mostly lost during the agriculture stage (1).

Unlike the seed coat, the hull is the hard-protecting cover of the seeds or grains that protects them during the growing period. Due to their hardness of texture, hulls are removed before cooking or manufacturing. Globally, hulls contribute to a large amount of food waste, especially rice hulls due to the high consumption of rice among the Asian countries (9). It is reported that 30% of the whole cereals are lost, and most of the wastages are due to human consumption and postharvest activities (1). To overcome this problem, hulls are used as a building material, fuel, and fertilizer (10) as one of the strategies to reduce wastage. Due to its high content of fiber and protein (11–13), the hull can be an alternative source of functional ingredient.

Not all vegetables have peels, unlike fruits. Tubers, gourd, and allium families contain peels, yet they are not normally consumed or used in food preparation. Therefore, vegetable peels are often discarded as kitchen or production wastes in the manufacturing line. Annually, large amounts of tuber and roots are wasted, and up to 5,814,000 tons of tubers and roots are wasted during the consumption stage in the North America and Oceania regions. In addition, Europe, North America, Oceania and industrialized Asia have the highest wastage of roots and tubers at the agriculture stage, where ~45% of tubers and roots are lost globally (1).

Fruit By-products

Fruit by-product is one of the principal roots of municipal solid waste, which has been the unresolved environmental menace (14). Municipal solid waste is defined as any substances discarded from residential, commercial, and industrial activity (15). Fruit peels or rinds are one of the by-products, which are commonly discarded during the consumption or manufacturing stage. It is the outer layer of fruits that functions to protect the fruits from environmental factors. Some fruits have thick and rough skin, such as pomegranate, whereas some fruits have a thin peel, such as mango. Most of the fruit skins are not consumed by consumers due to the rough texture and bitter aftertaste.

Other fruit by-products, such as seeds, are mostly consumed together with the fruits, such as watermelon and kiwi. In contrast, larger size seeds are not consumed, and they are mostly discarded after the flesh is consumed. Fruit seeds are rich in nutritional sources and have been used to develop functional foods (16). Fruit pomace is the remaining solid matter after juice extraction of fruits and might include seeds, skin, pulps, and stems for certain fruits. For example, apple pomace accounts ~25% of apples, where wastes from the apple juice and cider industry can generate a massive amount of pomace (17). The wastage from apple pomace is especially higher in the Western countries, such as in Germany and New Zealand (18).

Nutritional Value of Vegetable and Fruit By-products

Findings in the literature showed that vegetable and fruit by-products have high nutritional value (19, 20). Seed coats were reported to contain high dietary fiber. For instance, legumes seed coats are high in dietary fiber, ranging from 65 to 86% (19). Moreover, high content of dietary fiber has been reported in lemon, orange, bambangan, and grapefruit peels are also a rich source of dietary fiber (21, 22).

Apple pomace contains high dietary fiber compared to other fruit by-products, such as citrus peel (22). There are studies in the literature showed that apple pomace has been used as a functional ingredient and source of additional fiber in developing biscuits, cakes, and bread (23–26). Furthermore, pectin extracted from apple pomace has been used as an alternative source of gelatinizing agent that is used mainly to produce jam or jelly (27). Tomato pomace, which is high in dietary fiber and minerals, has been used as a functional ingredient to develop several functional foods, such as bread, muffin, and low-calories jam (28) Fuentes et al. (29) showed that the jam added with tomato pomace had 15–20% higher dietary fiber as compared to the commercial apricot jam. Other pomaces, such as grape, contain high protein and dietary fiber (30) and also a rich source of unsaturated fatty acids (31, 32).

Apricot kernel is also a potential functional food ingredient with high nutritional value. Studies in the literature have shown that apricot kernel has a high content of protein. Moreover, the apricot kernel has been used as a fat replacer in cookies and is incorporated into the development of noodles, bread, and cookies as reported in several studies (33, 34).

Moreover, the seed of fruits has been studied extensively. For example, fat extracted from rambutan seed showed high thermal stability, which suggested its possible uses in the food industry. The fatty acids of rambutan seed fats are mainly composed of oleic and arachidonic acid, which are omega-9 and omega-6 fatty acid, respectively (35). Furthermore, rambutan seed fats had a similar composition of fatty acids to cocoa fats and can be used to replace cocoa butter in chocolate production (36).

Health Benefits of Vegetable and Fruit By-products

Antioxidant Activity

A study showed that seed coats from nuts and beans contain a high level of phenolic compound, which can contribute to strong antioxidant activity (20). The removal of seed coats of nuts has been associated with the reduction of antioxidant activity up to 90% (20). Further study showed that flavonoids and saponins extracted from the black bean seed coats are well-retained, even after the baking process (37, 38), and these flavanoids and saponins from the black bean seed coat are suitable for food development due to the thermostability properties (39).

Moreover, several types of vegetable peels have shown to exhibit strong antioxidant properties. For example, potato peels were fortified into vegetable oil as antioxidant source in a study by Mohdaly et al. (40). The addition of potato peels improved the hydrolytic stability of vegetable oil and slowed thermal deterioration and stabilized the vegetable oil. In addition, the addition up to 200 ppm of potato peels extract had comparable stabilization efficiency with the synthetic antioxidants. Furthermore, potato peels have been used as a functional ingredient in making cookies, wheat bread, and bakery goods in order to improve the nutritional properties and health benefits of the products (41–44).

Many types of fruit peels and pomace contain high phenolic compounds, which possessed strong radical scavenging activity. Some fruit peels, such as rambutan, mango, bambangan, and mangosteen peels, are reported to have high antioxidant activity and suitable to be used as natural antioxidants in the development of functional food (21, 45, 46). Apple pomace is a good source of polyphenols and exhibits strong antioxidant activity (47–51). Polyphenols from apple pomace showed stronger radical scavenging activity than vitamin C and E, which indicated that apple pomace could be a potential natural source of antioxidants (49). Apart from that, tomato pomace is a rich source of phenolic compounds and tocopherols (31, 32) for strong antioxidant activity (50, 51). Moreover, grape pomace has been used as a functional ingredient in many types of food development including breads, yogurt, cheese, muffins, salad dressing, cookies, and brownies (52–57). For example, the fortification of grape pomace has enhanced the health benefits of the products, especially in terms of antioxidant activity and increased the nutritional value of products.

Antimicrobial and Antifungal Activity

There are studies in the literature that found several types of vegetable peels with high antimicrobial and antifungal activities. In the study of Rakholiya et al. (58), phytochemicals of vegetable peels were analyzed, and the peels of Moringa oleifera showed the highest antimicrobial activity toward Gram-positive bacteria among all the fruits and vegetable peels studied. Furthermore, compatible results were shown in a study by Chanda et al. (59) in terms of the antimicrobial activity of fruits and vegetable peels. Moreover, lemon, papaya, and sapodilla peels showed high antimicrobial activity, especially toward Gram-negative bacteria, whereas papaya peels showed high inhibition toward fungi (58). Other peels, such as pomegranate peels, contain high antioxidant activity and antimicrobial activity against food-borne pathogens, such as Listeria monocytogenes, Escherichia coli, Yersinia enterocolitica, and Staphylococcus aureus (46, 60, 61).

Besides its high antioxidant profile, mangosteen seed has antibacterial properties, where its antibacterial activity was highest among all the mangosteen parts (62). Indeed, the mangosteen seed also showed a better inhibition toward Gram-positive bacteria (62).

Anticancer Activity

Citrus peels with high flavonoids levels possessed anticancer activity. A study lead by Lai et al. (63) showed that citrus peels have protective effects against cancer. Citrus peels significantly reduced the size of tumors of prostate cancer and showed strong anti-inflammatory, antiproliferative, and antiangiogenic activities, and enhanced apoptosis-inducing effects on prostate cancer. The high level of polymethoxyflavones in citrus peel could also effectively suppress azoxymethane-induced colonic tumorigenesis.

Antidiabetic and Antihypocholesterolemic Activity

Some fruits and vegetables by-products are found to have antidiabetic properties. The antidiabetic effects of mango by-products were investigated in a study by Sarahí et al. (64). Mango by-products from the juice manufacturing industry including mango peels and pulps have shown insulin mimetic effect mainly due to the high level of soluble fiber, polyphenols, and carotenoids. Moreover, onion by-products with high dietary fiber showed hypoglycemic activities by effectively decreasing the starch digestibility and inhibiting the activity of alpha-amylase (65).

In a study by Mildner-Szkudlar and Bajerska (66), bread fortified with grape by-product has shown a positive effect in reducing cholesterol levels in an animal model, where further study is warranted to evaluate its use among humans. In addition, extracts of flavonoid and saponins from black bean seed coats could inhibit cholesterol micellization up to 55.4 ± 1.9%, where the effect was superior compared to stigmasterol (39). Of great interest, the authors added black bean seed coats that can be incorporated to develop cholesterol-lowering functional food and to explore more possibilities of utilization of black bean and other seed coats (39).

Many types of fruit and vegetable by-products have shown to be rich bioactive compounds, which are closely related to the health benefits, such as antioxidant and antimicrobial activity. More future studies could be done to incorporate the vegetable and fruit by-products for the development of new functional food to increase the nutritional security of food in the future market.

Functional Foods

As people are getting more health conscience, the demand for functional food has increased over the past years. Functional food is defined as “food that has a positive impact on an individual's health, physical performance, or state of mind, in addition to its nutritious value” (67). The American Dietetic Association has defined functional food as “whole, fortified, enriched, or enhanced” food which is consumed as “… part of a varied diet on a regular basis, at effective levels” (68).

In the current market, the Asian Pacific and North American have the largest functional food market, which contributed to 34 and 25%, respectively (69). The United States of America, Europe, and Japan are the top three countries, which contributed to ~82% of the total sales for functional foods and supplements in the year 2003 (70). Functional food started with the fortification of certain vitamins or minerals into the food developments. In recent years, the development of functional foods has further proceeded to the fortification of one or more compounds, which could provide multiple health benefits (71).

Vegetable and Fruit By-product as Functional Ingredients

Bread

Bread is a common confectionery product, which is widely consumed worldwide. Based on a published report (72), $358 billion worth of breads were consumed in the year 2016, a constantly increasing trend since 2007. Although breads are originated from Egypt, they are also consumed in Middle Eastern countries. Globally, the United States of America, China, and Russia are the top three countries with the highest bread consumption. These countries accounted for 32.7 million tons of bread consumption, equivalent to ~25% of the total consumption of bread globally in the year 2016. Due to the high consumption of bread worldwide, many studies have incorporated vegetable and fruit by-products to develop functional bread, and these findings are summarized in Table 1.

TABLE 1

Table 1. Study and main findings of vegetable and fruits by-products utilization as functional ingredients in development of bread.

It is found that the incorporation of vegetable and fruit by-products increased the nutritional values of the bread. However, the physical and organoleptic acceptability are slightly affected with such incorporation in most of the studies. The color of bread and the texture profile are affected as the bread turns darker or harder, which subsequently affects the overall acceptability of consumers.

Cookies and Biscuits

Biscuits and cookies are popular confectionery products and are commonly consumed as a sweet dessert instead of savory food. Due to the low water activity and long shelf life, biscuits and cookies are also used as emergency food. However, biscuits and cookies are mostly low in nutritional values, such as fiber, protein, and minerals (76). The incorporation of vegetable and fruit by-products increased the nutritional values of the product (Table 2).

TABLE 2

Table 2. Study and main findings of vegetable and fruit by-products utilization as functional ingredients in development of cookies and biscuits.

As can be seen in Table 2, cookies and biscuits that were incorporated with vegetable and fruit by-products had improved nutritional properties, such as dietary fiber and minerals. However, high-level incorporation could affect the texture and sensory results of cookies and biscuits. Based on the findings (Table 2), the incorporations of vegetable and fruit by-products up to 10% produced high acceptability and high functional values including emulsifying capacities and bulk density. Despite the slight changes in the color and texture, a low-level incorporation of vegetable and fruit by-products into the development of cookies and biscuits could be a good alternative for healthy snacks.

Noodles

Noodles have a history of more than 4,000 years, and it is one of the most popular staple foods, especially in the Asian countries. China, Indonesia, and India are the three countries with the highest consumption of noodles across the world. Wheat flour is the main ingredient in making noodles. On average, 20–50% of the usage of wheat flour is in the form of noodles (79). In fact, up to 103,620 million servings of instant noodles were consumed worldwide in 2018 (80). Noodles are served as savory dishes globally with the presence of soup or gravy, and instant noodles were also used as space foods (80). Due to the large market of noodles consumption worldwide, researchers have developed noodles with functional properties. Vegetables and fruits by-products have been incorporated into noodles to increase the nutritional value of noodles (Table 3).

TABLE 3

Table 3. Study and main findings of vegetable and fruit by-products utilization as functional ingredients in development of noodles.

The incorporation of vegetable and fruit by-products in noodles significantly increased the content of fiber and protein. It is shown that the addition of fruits and vegetables by-products decreased the glycemic index or increased antioxidant activity of noodles as the by-products contained high fiber and phenolic compounds. As the market is lacking a variety of low glycemic food, vegetable and fruit by-products with rich source of dietary fiber could be a useful functional ingredient in exploring more low-glycemic food development.

Dairy Products

Dairy products such as yogurt, cheese, and butter are produced using milk and are widely consumed across the world. In 2019, 513 million metric tons of milk were produced globally, where the European Union is the highest dairy producer (84). In addition, the European Union has contributed to more than 30% of the global milk production (84). Dairy production is expected to reach 880 million tons in the year 2021 (85). Due to the constantly increasing worldwide trend of dairy products and functional dairy products, more developments of functional dairy products are deemed important.

Vegetable and fruit by-products are well-incorporated into dairy products, such as salad dressing, cheese, and ice cream, and the findings are summarized in Table 4. They are also used as a fat replacer and natural colorant, which showed their wide potential as functional ingredients. As the natural colorant, the incorporation in ice cream showed high acceptability, but more studies could be done on new inventions, which involve a thermal process to indicate the stability of the colorant. In another study, the addition of grape pomace and tomato peels significantly enhanced the antioxidant activities of dairy products, without affecting the organoleptic acceptability of consumers (57, 86).

TABLE 4

Table 4. Study and main findings of vegetable and fruit by-products utilization as functional ingredients in development of dairy products.

Other Products

As can be seen in Table 5, other types of food have been developed by adding vegetable and fruit by-products, and the addition improved the dietary fiber content. For example, the addition of mango seed kernel (88) and cauliflower by-products (89) improved the nutritional values of traditional Indian cuisines. Other studies have used pomace (26, 28) to improve the product. For the development of jam (28), tomato pomace addition increased the content of dietary fiber up to 20 times.

TABLE 5

Table 5. Study and main findings of vegetable and fruit by-products utilization as functional ingredients in development of other products.

Conclusions: A Way Forward in Utilizing Vegetable and Fruit By-products

Vegetable and fruit by-products are wastes from the agriculture, postharvest, processing, distribution, or consumption stages. Some of the by-products have been used as burning material, constructional material, or animal feed, whereas a large amount is being discarded. There are many types of agriculture by-products from the food industry and have been shown to have high nutritional qualities. By utilizing these by-products as functional food and ingredient, the cost of production can be lowered. Importantly, it can increase food sustainability and is one of the strategies to tackle the arising food security problem. Of great significance, findings from the literature showed that agriculture by-products, including vegetable and fruit by-products can be used to increase and enhance the nutritional value of functional foods. This review illustrates the potential of fruit and vegetable by-product as ingredients of value-added for food interventions. With proper hygiene protocol and processing technologies, utilizing these by-products in the functional food industry will be a good alternative to increase the choices of low-cost functional food in the market for consumers. In addition, more research can be done on the underutilized agriculture by-products to seek more possibilities.

Author Contributions

KL: writing—original draft. KL, MS, and SS: writing—review and editing. MS and SS: supervision. MS: project administration and resources. All authors reviewed and approved the final manuscript.

Funding

This research was funded by Putra Research Grant (Project No. GP-IPS/2018/9615800), Universiti Putra Malaysia, Selangor, Malaysia.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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