The food industry is quite interested in the use of (techno)-functional bioactive compounds from by-products to develop ‘Clean label’ foods in a circular economy. The aim of this review is to evaluate knowledge, scientific evidence, and practical application on the use of green extraction technologies (ultrasound-, microwave-, and enzymatic-assisted) of bioactive compounds from pomegranate peel by-products, and their potential application via the supplementation / fortification of vegetal matrixes to improve their quality, functional properties, and safety. Most studies are mainly focused on ultrasound extraction, which has been widely developed compared to microwave or enzymatic extractions, which should be deeply studied including combinations. After extraction, pomegranate peel by-products (in powders, liquid extract, and/or encapsulated, among others) have been incorporated into several food matrixes, as a good tool to preserve ‘Clean label’ foods without altering their composition and improving their functional properties. Future studies must clearly evaluate the energy efficiency/consumption, the cost, and the environmental impact leading to sustainable extraction of the key bio-compounds. Moreover, predictive models are needed to optimize the phytochemical extraction to help taking decisions along the supply chain
1. Introduction
In accordance with Food and Agriculture Organization of the United Nations (FAO) definition, ‘food waste’ is the decrease in the quantity and/or quality of food obtaining from decisions and/or actions by retailers, food service providers and consumers, while ‘food loss’ refers to any food that is discarded along the previous food supply chain steps from harvest up to retail sale [1]. FAO indicates that around one third of the food production is globally lost or wasted at some step in the food chain. Depending on the state and the basket item, the losses level greatly varies.
In the case of fruit and vegetables (F&V) at the whole supply chain could reach up to ~50 % (Figure 1). FAO’s future challenges by 2050 is to reduce ~50 % of food waste, as one of the objectives for sustainable development (OSD). Circular economy has been considered as the principles for eco-innovation, being focused on a ‘zero waste’ society and economy, using wastes as raw materials.
Between 2016 and 2018, FAO Statistics Division developed a food loss estimation model called “The Food Loss and Waste database”, an online collection of data including food loss and food waste. Figure 1 shows the % loss of F&V (food loss + food waste) worldwide in each value chain step in the first 20 years of XXI century [2]. The boxes show where ~50 % of the collected data falls into, the mid-value of the percentage loss at every stage in the supply chain is shown by a line. In this sense, postharvest and retailing are the main steps in the food chain where the F&V losses represent the highest mean percentages. Mean percentage during processing is less than 10 %, but in some cases, it reaches ~40 %. Moreover, although the mean percentage during distribution represent less than 10 %, the range is from < 5 % to > 30 %. Therefore, several strategies of creating active packaging with encapsulated key compounds have been developed, to avoid high percentage of food waste/loss [3]. The range of each step is wide since the value depends on the type of F&V, the country, and the year.
Although, this review is focused on pomegranate by-products, percentage of food loss related to this fruit is not available in the mentioned official database. Nevertheless, knowing that the total production of pomegranate worldwide is three million tons, and its peel and seeds approximately represent the ~54 % of the fruit, these results in ~1.62 million tons of waste [4,5]. Therefore, it is a huge amount of waste produced, so it is crucial to find suitable methods to revalorize them by optimizing the bioactive compounds extraction of pomegranate residues and then, convert them into value-added products. Consequently, savings can also be made on other resources involved during production, harvesting, preservation and distribution, such as energy, water, and land, as well as contributing to the environment [5]
Parallelly, health, well-being, and sustainability are the current trend in the food market. Consumers and food producers are interested in ‘Clean label’ foods or ingredients [6,7]. It means that they are interested in foods or ingredients obtained by green-processing technologies (non-thermal, green-solvents), and bioactive compounds with health promoting properties (nutraceuticals), among others. The bioactive compounds obtained from F&V by-products present technological and functional features and can be incorporated within other food matrixes to enhance their nutritional, functional, and sensory quality [6,8]. Also, the use of bioactive compounds from F&V by-products has previously been classified as potential green-ingredients for the cosmetic and pharmaceutical industries, developing different products intended for specific populations such as sport-people [9].
In accordance with the Food and Agriculture Organization of the United Nations (FAO) definition, ‘food waste’ is the decrease in the quantity and/or quality of food obtaining from decisions and/or actions of retailers, food service providers, and consumers, while ‘food loss’ refers to any food that is discarded along the food supply chain, from harvest up to retail sale [1]. FAO indicates that around one third of global food production is lost or wasted at some step in the food chain. The degree of loss greatly varies depending on the state and the basket item.
In the case of fruit and vegetables (F&V), losses over the whole supply chain could reach up to ~50%. FAO’s future challenge is to reduce ~50% of food waste by 2050, as one of the objectives for sustainable development (OSD). The circular economy has been considered as the principle for eco-innovation, being focused on a ‘zero waste’ society and economy, using wastes as raw materials.
Between 2016 and 2018, FAO Statistics Division developed a food loss estimation model called ‘The Food Loss and Waste database’, an online collection of data including food loss and food waste. The boxes show where ~50% of the collected data falls into, and the mid-value of the percentage loss at every stage in the supply chain is shown by a line. In this sense, postharvest and retailing are the steps in the food chain where the F&V losses represent the highest mean percentages. The mean percentage during processing is less than 10%, but in some cases, it reaches ~40%. Moreover, although the mean percentage during distribution represents less than 10%, the range is from <5% to >30%. Therefore, several strategies have been developed around the creation of active packaging with encapsulated key compounds, to avoid the high percentage of food waste/loss [2]. The range of loss percentages at each step is wide since the value depends on the type of F&V, the country, and the year.
The present review aims to evaluate scientific evidence and knowledge on the use of green technologies for the extraction of phenolic compounds from pomegranate by-products, and its further incorporation techniques and potential applications via the supplementation / fortification of F&V matrixes to improve their quality and safety in a circular economy. For this purpose, a literature review was conducted, focusing on ultrasound-, microwave-, and enzymatic-assisted technology to enhance phenolic compounds extraction from pomegranate peel by-products. Moreover, different incorporation techniques and applications have been reviewed. Results may provide the scientific community with the state of the art on pomegranate peel revalorization. The study may also help scientists, and food industry to develop solutions to better suit society demands.
21. Nutritional Composition of Pomegranate Byproducts
Both primary (sugars, pectins, proteins, and fats) and secondary (polyphenols, pigments, and sulfur compounds) metabolites have been found in F&V byproducts
[3][10]. The food industry and researchers are interested in reducing the environmental impact, and then focus on the recovery of the target compounds
[4][6]. Carbohydrates (around 60%)
[5][11], pectin (yield range from 6 to 25%)
[6][7][12,13], proteins (around 3%)
[8][9][14,15], and fats (<1%)
[9][15] have been previously identified in pomegranate peel. Since this re
svie
archw is focused on the extraction of secondary metabolites from pomegranate peel, especially phenolic compounds,
Figure 1 shows the classification of the main ones found
[9][10][5,15].
Figure 1. Classification of the main phenolic compounds in pomegranate peel
[9][10][5,15]. Glu: glucoside; Cy: cyanidin; Dph: delphinidin; Pg: pelargonidin; Pt: petunidin; Gal: galactoside.
Among them, the top ten have recently been identified and quantified
[11][16], being punicalagin (28,000–104,000 µg/g) the major compound found, followed by ellagic acid (1580–4514 µg/g), and others such as punicalin (203–840 µg/g), catechin (115–613 µg/g), corilagin (71–418 µg/g), gallic acid (10–73 µg/g), gallocatechin (69–1429 µg/g), epigallocatechin (5–106 µg/g), epigallocatechin gallate (4–70 µg/g), and kaempferol-3-O-glucoside (16–99 µg/g)
[11][16].
Apart from pomegranate peel, seeds (wooden part) are generated after juice processing as a byproduct. Although this re
svie
archw is not focused on pomegranate seeds revalorization, previous studies have indicated that pomegranate seeds are rich in polyunsaturated fatty acids (88–92%), the most abundant being linolenic acid, especially punicic acid which ranges in terms of percentage of total fatty acid profile from 59.7 to 74.3%
[12][13][17,18].
32. Pomegranate Peel Byproducts Incorporation Techniques
32.1. Powders/Flours
Pomegranate peel powder/flour is commonly acquired by drying and grinding until obtaining the desired particle size. Similar drying technology applied to edible fruit and plant material could be used in F&V byproducts to avoid undesirable bioactive compound changes
[14][69]. The most common drying technologies are convective drying, sun-drying, MW drying, and freeze-drying in which key variables should be optimized (for instance, temperature and time). Moreover, spray-drying is commonly catalogued as a good tool for byproducts drying. This powder could be applied as a solid ingredient for the fortification of different products such as meat-based, F&V-based, and bakery products
(Section 6) since this material presented high dietary fiber and techno-functional properties (high water- and oil-holding capacity, and low water absorption) in previous studies
[15][70]. Similarly, powders can be obtained from liquid extracts after bioactive compounds extraction using different technologies such as freeze-drying or spray-drying
[16][71]. Such technologies are included in the section on encapsulation due to the need for different processes to be carried out
(Section 5.3).
32.2. Liquid Extracts
With pomegranate peel powders obtained as previously detailed, extraction techniques with different solvents can be used, including those reported in this re
svie
archw. These liquid extracts are not suitable for direct incorporation into the different food matrixes, except when the solvents may be classified as a food ingredient (e.g., water). Therefore, these solvents must be removed through evaporation. Once they have been evaporated, drying should be carried out (for instance convective or freeze-drying) to later redissolve it in water, as the most common liquid. In this way, the liquid extract is ready to be incorporated into the matrixes at different solid–liquid ratio
, as observed in Section 6. In addition, liquid extracts can be used to obtain coatings, and can be encapsulated by different carriers and techniques.
32.3. Encapsulation
Encapsulation is a means to protect sensitive key bioactive compounds found in the food industry byproducts against undesirable heat, oxygen, light, and pH conditions
[17][72]. The process needs a carrier agent and a technique to create the protective capsules. Different techniques may be used for the encapsulation of target compounds from F&V byproducts, such as spray-drying, freeze-drying, complex coacervation, and ion gelation
[18][73], among others. Spray-drying is the liquid food drying method and has been widely used to obtain powders from F&V juices
[14][19][20][21][69,74,75,76]. Currently, the transformation of F&V byproduct extracts (liquid) into powders using a spray-drier (the extracts are sprayed into a hot air chamber) has garnered attention because the process is complex, although this technique is one of the fastest, cheapest, and more reproducible, despite its complexity. In lyophilization as well as in spray-drying, a solution, dispersion, or emulsion is first obtained depending on the encapsulating agent and the active compound. The first step of freeze-drying-based encapsulation consists in creating an emulsion between the carriers and the target compounds, followed by a conversion into microcapsules by applying the freeze-drying technique
[22][77], which consists of water removal by sublimation (primary drying) and secondary drying.
Table 1 shows the main technologies (spray-drying, freeze-drying, double emulsion, and ion gelation) and the carriers used to encapsulate target bioactive compounds from pomegranate peel. It can be seen that there is an interest in using novel carriers such as citrus byproducts.
Table 1.
Main technologies used to encapsulate target compounds from pomegranate peel.
Technology |
Carriers |
Target Compound/Activity |
Ref. |
Spray-drying |
Maltrodextrin |
F-TPC, UPLC-TPC, Pn, EA, P, GA |
[23][24] | [78,79] |
Maltrodextrin + others: Tween 80 (99:1); Skimmed milk powder (50:50); Whey protein isolate (50:50); Gum arabic (50:50) |
NA (Yield/Stability) |
[25][26] | [28,80] |
Skimmed milk power |
NA (Yield/Stability) |
[25][26] | [28,80] |
Orange juice byproduct |
F-TPC, DPPH |
[27][28] | [81,82] |
Maltodextrin/Pectin |
TPC, Pn, EA |
[29] | [83] |
Whey protein |
Pn, EA, P, GA |
[24] | [79] |
Arabic gum |
Pn, EA |
[30] | [84] |
Chitosan |
Pn, EA |
[30][31] | [84,85] |
Pectin |
Pn, EA |
[30] | [84] |
Modified starch |
Pn, TPC, HTC, DPPH |
[32] | [86] |
Alginate |
NA (Yield/Stability) |
[31] | [85] |
Freeze-drying |
Soy phosphatidylcholine liposomes |
Pn, EA, rutin, epifallocatechin, TPC |
[33] | [87] |
Maltodextrin (5 and 10%) and b-cyclodextrin (5 and 10%). |
F-TPC, FRAP |
[34] | [38] |
Prunus armeniaca | gum exudates |
FRAP, DPPH |
[35] | [88] |
Chitosan |
FRAP, DPPH |
[35] | [88] |
Maltrodextrin |
TPC, TFC, Pn, EA, FRAP, DPPH |
[36] | [89] |
Maltodextrin and calcium alginate |
ANCs, FRAP, DPPH |
[37] | [90] |
Maltodextrin and soy lecitin |
NA (Yield/Stability) |
[38] | [91] |
Double emulsion |
Water | 1 | in Oil in Water | 2 | :
Water | 1 | (ethanolic solutions) in Oil (castor, soybean, sunflower, medium chain triglyceride and orange) in Water | 2 | (aqueous solution with Tween | 80 | ) |
NA (Yield/Stability) |
[39] | [92] |
Ion gelation |
Chitosan gel (1%):gelatin 2:1 |
F-TPC, DPPH |
[40] | [93] |
Spirulina |
TPC, DPPH |
[17] | [72] |
Microalgae |
EA |
[41] | [94] |
Chitosan + others:
Dialdehyde guar gum
Gelatin-based materials |
F-TPC, DPPH |
[42] | [95] |