Study of sorption properties of selected fruit fibres and pomace
Pomace, which are waste product generate a huge problem in fruit and vegetable industry. Numerous studies prove that dried pomace and fibre powder are valuable source of many nutrients. Due to their properties, their popularity is growing in many industries.
There were determined water vapour isotherms and kinetics for selected fruit fibres and pomace. Additionally, the activity and water content, colour, apparent and bulk density, and material structure were investigated. Fruit pomace and fruit fibres were used in the research (aronia fibre, apple fibre, blackcurrant fibre). The determined kinetic curves proved that apple fibre absorbed higher amount of water. It was found that the isotherms had a shape characteristic for type III of sorption isotherms. The GAB model described well experimental data for sorption isotherms.
Keywords: fibre, pomace, water vapour sorption kinetic, water vapour sorption isotherm.
Fruits are a valuable source of many vitamins and minerals, therefore they are a key element in the human diet. One component of the fruit that deserves special attention is fibre. Dietary fibre, due to its health-promoting properties, should be part of a balanced diet of every human being. Additionally, the functional properties of fibre make it very popular in food technology.
The growing popularity of fruit preserves, including juices, purees or smoothies results in an increase in the amount of production waste, which is pomace. Waste products are used, among others, in the food industry as ingredients of fruit teas, an additive to confectionery and bakery products. In the cosmetics industry, pomace extracts are ingredients of cosmetics, while in the textile industry, pomace is a raw material for the production of an alternative to animal skin.
The sorption phenomenon relates to the transfer of one or more components from one phase to another. Water affects the properties of food and its quality, therefore this phenomenon in food technology is considered in terms of water vapor. Food products can absorb water in a humid environment, which is called adsorption. The opposite phenomenon is desorption, i.e. the release of water through the material. Determination of the kinetics and sorption isotherms allows us to learn about the sorption properties of the product. On the other hand, these properties are helpful in determining changes that may occur in the product under the influence of changing ambient humidity and facilitate the determination of the storage period, as well as in determining the packaging and storage conditions.
The conducted research proved that the content of total polyphenols in high-sugar jams is higher than in low-sugar jams produced from the same species of fruit, which may be due to the increased stability of anthocyanins in high sucrose concentrations. Changes in the content of vitamin C, anthocyanins and polyphenols were also studied by Martinsen et al. . The researchers showed that the high temperature required in the technological process reduces the content of the above-mentioned compounds in strawberry jam, while it has no significant effect on raspberry jam.
Fibre is a structural element of plants. The fibre is composed of elements of plant cell walls, mucilages, resistant starch, polydextrose, oligosaccharides and vegetable gums [Godula et al. 2019]. Dietary fibre is present in the form of water-soluble fractions, which include: pectins, β-glucans, mucilages and gums, and insoluble fractions, which include lignins, cellulose and hemicelluloses. The source of origin and the mutual proportions of the fractions determine its properties [Kowalska et al. 2019].
Plant-based foods are the main source of dietary fibre. Cereal products rich in dietary fibre include various types of flakes, wholemeal rye bread and mixed bread with grains. Other products containing significant amounts of valuable fibre are dried fruits and nuts. The amount of dietary fibre in vegetables varies depending on the type and is in the range of 0.5-5.8 g / 100 g of the product, while the content in fruits is about 2 g / 100 g of the product [Bienkiewicz et al. 2015].
Fibre has many physiologically important functions in the body. One of them is the prevention of overweight and obesity by the ability to swell in the stomach, which results in its slower emptying and, consequently, a longer feeling of fullness. Dietary fibre also improves intestinal peristalsis and reduces the risk of colorectal cancer [Szczepańska et al. 2010]. The individual fibre fractions are fermented by the bacteria of the intestinal flora, found mainly in the large intestine. By obtaining the energy necessary for development through the hydrolysis of fibre, microorganisms regulate the functioning of the intestines, stimulate the immune system and support the production of hormones, biotin and vitamin K [Bienkiewicz et al. 2015].
Dietary fibre is used in the production of fibre preparations, which are increasingly used in many food products. Preparations contribute to the improvement of the texture of finished products. It is influenced by the functional properties of fibre, which include the ability to bind water, which prevents syneresis. In addition, fibre preparations increase the volume of the product and reduce caloric content by replacing fat. The advantage of using them is that they do not introduce foreign tastes and smells into the product [Godula et al. 2019].
FRUIT POMACE AND THEIR MANAGEMENT
Obtaining fruit pomace
In fruit and vegetable processing, a significant problem is the management or utilization of waste products – parts not used in the technological process. Their amount is in the range of 10-35% of the weight of the processed raw material [Tarko et al. 2012]. Fruit processing by-products are produced during the production of juices, pulps and jams. They include skins, seeds and pomace. Population growth and nutritional changes are factors that contribute to higher consumption of fruit and vegetables, which results in an increase in waste related to their processing [Kringel et al. 2020]. In Europe, fruit processing by-products account for 8% of all food waste. This industry is the fifth largest producer of waste. The situation is much worse in developing countries, where fruit processing by-products are treated as irrelevant and redundant compared to processed fruit [Ningrum et al. 2018].
Possibilities of managing fruit pomace
A wet pomace is a material characterized by low durability and stability. Fruit pomace, due to the high water content, which is up to 73% in apple fruit, is exposed to a rapid increase in microbiological contamination. The most popular methods of preserving pomace include drying and ensiling, which inhibit unfavourable changes leading to a reduction in quality. Thanks to these treatments, they are suitable for further use [Tarko et al. 2012].
Depending on the variety and processing method, the dried pomace consists of peels, dried seeds and pulp remnants. Due to the content of many pro-health ingredients, in recent years there has been an increase interest in the possibility of reusing these by-products [Pawlak and Sielicka 2016].
The dried pomace is rich in vegetable fibre. Kwaśniewska and Nawirska  determined the composition of individual fibre fractions in the blackcurrant pomace, aronia pomace, apple pomace, cherry pomace and pear pomace. The conducted research has shown that apple pomace, compared to other analysed waste, has the most beneficial effect on our health. Due to the high content of pectins that have the ability to bind heavy metal ions and cellulose, which have a positive effect on intestinal peristalsis. The ability to bind heavy metal ions by fruit pomace was also observed by Król and Nawirska . Apple, cherry and blackcurrant pomace and their combinations were tested to remove lead, copper, zinc and cadmium ions from aqueous solutions in dynamical systems. The cherry pomace retained the most cadmium and lead ions, while the mixture of apple and cherry pomace retained the greatest amount of zinc and copper ions. Blackcurrant pomace was the least effective in this comparison. The adsorption properties of fruit pomace were studied by Dudczak et al. . Gooseberry pomace for the removal of copper ions in model systems was tested. The conducted experiment showed that dried gooseberry pomace at pH 4-5 is able to adsorb about 58% of copper ions present in an aqueous solution. On the basis of the obtained results, the researchers conclude that bagasse can be used as a sorption material for removing metal ions from wastewater as a new direction of fruit waste management.
Pectins, the best source of which are citrus and apple fruits, apart from their ability to bind ions, are also characterized by gelling, thickening and stabilizing properties. Thanks to this, they are used in industry as hydrocolloids [Kowalska et al. 2019]. The high content of fibre also helps to counteract obesity, diabetes, heart disease, atherosclerosis and some types of cancer [Tarko et al. 2012]. Thermal, chemical or enzymatic modified citrus pectin is registered in the United States as a dietary supplement. Clinical trials have confirmed the anti-tumor potential of modified pectin, which prevents tumor progression and inhibits metastasis. Citrus pectin also affects cells of the immune system to regulate the inflammatory response [Ben-Ohman et al. 2020].
Moreover, dried pomace is a source of many other valuable nutrients, such as: minerals, saccharides, proteins, organic acids, lipids, vitamins, aldehydes and alcohols. They are also rich in aromatic substances and dyes, including anthocyanins that can give a color from yellow to blue. Berry pomace is characterized by a high content of coloring compounds. This enables these compounds to be extracted from the pomace and the dyes to be produced. Phenolic compounds, apart from giving color, show strong health-promoting properties. Along with tocopherols, they belong to the antioxidants responsible for protection against the harmful effects of free radicals, which in turn enables the prevention of heart disease, cataracts, cancer and inhibiting the aging process [Tarko et al. 2012].
The pomace is also a rich source of biopolymers, proteins and carbohydrates that can be recovered, concentrated, transformed into precursors and then used in the cosmetics, pharmaceutical and food industries. Researchers have made attempts to obtain enzymes, oligosaccharides, biosurfactants, lactic acid and furfural from tropical fruit pomace [Ningrum et al. 2018]. The pomace can also be used for the growth of microorganisms, which allows to obtain pectinolytic enzymes from pineapple, orange or lemon pomace, mango or banana amylases or lemon peel lipases. The pomace is an alternative carbon source for glucose and sucrose commonly used in cultivation. This allows for a significant reduction in production costs and obtaining polysaccharides valuable in the industry, such as xanthan gum, pullulan or curdlan. Mohsin  synthesized kurdlan using orange pomace, which, thanks to its gelling properties, is widely used in various industries. The polysaccharide, belonging to β-glucans, is a component of biodegradable films, functional products, e.g. yoghurts, and may also be a fat substitute in meat products, reducing their caloric content. Kurdlan is also characterized by immunostimulatory properties, which increases the interest in this compound in medicine. Alcoholic fermentation, in which the carbon source is the pomace, also allows for the production of biofuel. The most efficient substrate is apple pomace, which allows for obtaining 190 g of ethanol from 1 kg of waste [Esparza et al. 2020]. Piwowarek et al.  used apple pomace as a microbiological substrate in the propionic-acetic fermentation process. The use of apple pomace as a carbon source allowed the cultivation of the wild strain of Propionibacterium freudenreichii T82 and obtaining acetic and propionic acid.
Apple peels are characterized by a high content of phenols, which means a greater concentration of these bioactive compounds in the pomace than in the whole fruit. Phenolic compounds have strong antimicrobial, anti-inflammatory and antioxidant properties. The isolation of these substances allows for the introduction of pomace into cosmetic preparations [Barriera et al. 2019], where fruit seeds, rich in valuable oils, have also been used. Commonly used in this industry is raspberry seed oil, which is appreciated for its content of minerals, provitamin A, vitamins B2, C and organic acids, including salicylic acid and apple. The chemical composition of the oil in question makes it possible be a component of sunscreen preparations, creams or toothpastes [Kalinowska et al. 2017].
Currently, the most popular way of using pomace is to use it as feed, which allows for a significant reduction in the cost of animal feeding. In addition, they contain important ingredients such as: organic acids, sugars, fatty substances, nitrogen-free compounds and vitamins. The content of the latter in the pomace after the juice pressing process may be as high as 2 g / kg of dry matter. This value depends on the conditions of the process [Tarko et al. 2012].
The pomace can also be used as an ingredient in fruit teas, improving the organoleptic qualities [Wichrowska and Żary-Sikorska, 2015]. Bober and Oszmiański  pointed to the possibility of using chokeberry marc as a fruit substitute in the production of fruit teas. Water extracts obtained from fruit and pomace dried with different methods were compared. The infusions obtained from air-dried chokeberry pomace resulted in a 2-fold reduction in the content of glucose, sorbitol and fructose compared to infusions of fruit dried using the same method. However, there was an 11.5-fold increase in anthocyanin monomers. Regardless of the drying method, the pomace infusions had a redder and more saturated color than the fruit infusions. An additional advantage of the pomace-based teas was their anti-free radical activity. The highest ability to absorb free radicals (8383 mg / L) was observed in the water extract from freeze-dried aronia pomace. For comparison, the water extract from freeze-dried aronia fruit removed about 600 mg / L of free radicals.
Sensory assessments carried out by Baranowski et al.  showed the possibility of using concentrated extracts of chokeberry and blackcurrant pomace in “ice” type fruit teas, jelly and jellies. The addition of blackcurrant pomace extract made jellies with the synthetic aroma of the described fruit better assessed in terms of taste and aroma. The products enriched with extracts had a specific, slightly sour taste. They also gained a more intense red color, which is desirable for this type of product.
Waste from the fruit industry can be successfully used in some confectionery and bakery products. Kidoń et al.  modified the recipe of dry wafers by replacing flour with dried apple pomace in the amount of 10 and 12.5%. On the basis of the conducted research, it was found that the 10% addition of pomace allowed to obtain a product with an increased content of pro-health ingredients with minimal changes in sensory characteristics. The addition of dried fruit waste caused a significant increase in polyphenolic compounds. The obtained wafers had a darker color, while the sensory evaluation showed differences in texture. Siemianowska et al.  used strawberry, apple, blackcurrant and raspberry pomace to bake shortbread cookies. The research showed that the 30% share of the pomace allowed for the preservation of appropriate sensory features without deteriorating the textural and storage parameters. Additionally, the antioxidant activity of all types of cookies was determined, which was 52.2-66.5%. Similar studies were carried out by Maner et al. , who replaced a part of wheat flour in cookies with dried grape pomace. The results of the study, as in the previous ones, allowed to state that the addition of pomace allowed to enrich the products with health-promoting substances while maintaining or imparting appropriate sensory features. Salehia and Aghajanzadeh  also described the effect of dried and powdered fruits and vegetables on dough quality. The presence of the powder increased the water absorption as a result of which the dough was characterized by higher humidity. The reduction in gluten contributed to a reduction in volume and improved firmness. The addition of the powder allowed to increase the nutritional value. The biscuit dough, in which part of the wheat flour was replaced with powdered skins, and powdered mango pulp contained more fibre, less fat and calories. Additionally, the dough has been enriched with nutritious polyphenols and carotenoids. Fruit pomace micronized into fibre powder is used in the production of gluten-free bread. The enrichment of the recipe of rice flour and potato starch with orange pomace allowed to obtain a product with sensory properties similar to the control sample [Quiles et al. 2016].
Grasso  in its review presented the possibility of using bagasse for the production of extruded snacks. These products can be an alternative to cereal snacks available on the market, which are usually characterized by low nutritional value, too much salt, sugar and fat. An example of this type of product is a snack consisting of corn, sorghum flour and apple pomace. In order to increase the content and compensate for the losses of phenolic compounds and antioxidants incurred in the extrusion process, fermentation and hydrodynamic cavitation were previously carried out. These processes also allowed to increase the fibre content in the final product. The author of the review also cited research, which proved that cherry, blackcurrant, pineapple or blueberry pomace can be successfully used in the production of snacks. The pomace can also be used in food preservation. Numerous studies have shown that extracts obtained from pomace have the effect of extending the shelf life of products. Research has been carried out to add grape marc, olives, tomatoes and pomegranate extracts to lamb chops. After 7 days of refrigerated storage, the product preserved in this way had 10-21% less mesophilic bacteria compared to the test without additives. Tomato and pomegranate pomace reduced the number of psychrophiles and bacteria from the Enterobacteriaceae family. Antimicrobial activity was also noted by enriching beef products with pomegranate peels and dipping the shrimp in a solution containing pomace. The pomace also has a positive effect on dairy products. The test subjects were cheese immersed in a solution with pomegranate peels. After 28 days of storage at 4 ⁰C, a smaller number of microorganisms reducing the quality of the product was observed compared to the control sample. On the other hand, enriching yogurt with pineapple peel powder contributed to its increased probioticity [Trigo et al. 2020].
In industry, starch is commonly obtained from tubers and cereals. An alternative to these raw materials may be the pomace of some fruits, which are characterized by a high content of this polysaccharide. Additionally, the starch obtained from the pomace has other functional properties, which increases the scope of its industrial applications. Kringel et al.  pointed to the possibility of obtaining starch, among others from pineapple, apple, mango, lychee, avocado or loaf tree fruit. The seeds of the latter are the best source of the discussed polysaccharide, the content of which is 60-80%. The starch obtained from mango pomace has unique properties, because it has a low glycemic index, thanks to which it is used in products with reduced caloric value. The production of textiles is an innovative application of bagasse. Bio2materials is a Polish brand that deals with the creation of an alternative to animal skin and synthetic apple pomace (Figure 1). The product is completely biodegradable and free of substances harmful to health [Internet 2].
Water vapor adsorption isotherms
Water vapor sorption isotherms determine the relationship between water activity and water content in food at constant temperature and constant total pressure. The shape of sorption isotherms shows the mechanism of water binding in the product. The ability to absorb water and the sensitivity to moisture of dried products is determined by the shape of sorption isotherms. Sorption isotherms are used in various fields of food technology. In theoretical aspect, they allow the study of the thermodynamic properties of water sorption and the structure of the material. On the other hand, the practical application of sorption isotherms includes mixing of ingredients, moistening, drying, packaging and storage [Domian and Lenart 2000]. Knowledge of sorption isotherms allows us to design the drying process, determine the optimal water content in the material and predict the nature of product deterioration. Water vapor sorption may lead to the critical water content being exceeded. The result of this process are unfavorable changes in the qualitative characteristics of the product of an enzymatic, physicochemical, physical or microbiological nature [Lewicki and Marzec 2004]. Water vapor sorption isotherms are also useful in determining the optimal storage conditions for dried food and in the selection of packaging [Lenart and Poszytek 2006].
Master’s thesis written under the supervision of dr inż. Sabina Galus
Institute of Food Sciences
Department of Food Engineering and Production Organization
Warsaw University of Life Sciences, Poland, 2021
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