In the temperate zone, apples are the most significant fruit economically. Apple production achieves approximately 10% of the world’s fruit production, with one-fourth pro- duced in Europe.
There are opportunities for the utilization of apple press cakes:
r Drying, r Feeding, r Composting, r Storing.
However, both environmentally and economically, the best technique is drying for pectin extraction.
Apple Pectin
Pectin for use in food is defined as a polymer containing galacturonic acid units (at least 65%). The acid groups may be free, combined as a methyl ester, or as sodium, potassium, calcium, or ammonium salts, and in some pectins, amide groups may also be present.
Commercial Apple Pectin Production Process details vary between different companies, but the general process is as follows:
The pectin factory receives apple residues or pomace (Car- son et al. 1994) or citrus–orange peels from a number of juice producers (El-Nawawi and Heikal 1996). In most cases, this material has been washed and dried, so it can be transported and stored without spoilage.
If the raw material is dry, it can be assessed and selected from storage when the need arises. If wet citrus peel is needed, it has to be used immediately on receipt because of rapid deterioration (Kim et al. 2004).
The raw material is added to hot water containing a pro- cessing aid, usually a mineral acid, although others such as enzymes could be used (Schieber et al. 2003). Water alone will extract only a very limited amount of pectin.
After pectin is extracted, the remaining solids are sepa- rated, and the solution clarified and concentrated by remov- ing some of the water. The solids can be separated by filter, centrifuge, or other means. The solution is then filtered again for further clarification if necessary.
Either immediately or after a holding period to modify the pectin, the concentrated liquid is mixed with an alcohol to precipitate the pectin. The pectin can be partly de-esterified at this stage, or earlier or later in the process.
The precipitate is separated, washed with more alcohol to remove impurities, and dried. The alcohol wash may contain salts or alkalis to convert the pectin to a partial salt form (sodium, potassium, calcium, and ammonium).
The alcohol (usually isopropanol) is recovered very effi- ciently and reused to precipitate further pectin.
Before or after drying, the pectin may be treated with ammonia to produce amidated pectin if required (Braddock 1999). Amidated pectins are preferred for some applications.
The dry solid is ground to a powder, tested, and blended with sugar or dextrose to a standard gelling power or a product with other functional property such as viscosity or stabilizing effect.
Pectins are also blended with other approved food additives for use in commercial applications.
The various raw materials yield different amounts of ex- tractable pectin: pomace, 10–15%; sugar beet chips, 10–20%;
sunflower infructescence, 15–25%; and citrus peels, 20–35%.
Application of Apple Pectins Apple pectin is one of the most versatile stabilizers available. Its gelling, thickening, and stabilizing properties make it an essential additive in the production of many food products.
Traditionally, pectin is primarily used in the production of jams and jellies. It produces the desired texture, limits the creation of water/juice on top of the surface as well as an even distribution of fruit in the product. Product and applica- tion development by the major pectin producers has over the years resulted in a large expansion of the opportunities and applicability of pectin. Pectin is a key stabilizer and is used in many food products as follows:
r Fruit applications in jams, jellies, and desserts.
r Bakery fillings and toppings in fruit preparations for dairy applications.
r Dairy applications in acidified milk and protein drinks, yogurts (thickening).
r Confectionery in fruit jellies, neutral jellies.
r Beverages.
r Nutritional and health products.
r Pharmaceutical and medical applications.
This wide range of applications explains the need for many different types of commercial pectin, which are sold accord- ing to their application, for example,
r Rapid set pectin traditionally used for jams and mar- malades.
r Slow set pectin used for jellies and some jams and pre- serves, especially for vacuum cooking at lower tempera- tures. It is also important for higher sugar products such as bakery and biscuit, jams, sugar confectionery.
r Stabilizing pectin used for stabilizing acidic protein products such as yogurts, whey, and soya drinks during thermal processing.
r Low methyl ester amidated pectin used in a wide range of low-sugar products, reduced sugar preserves, fruit preparations for yogurts, dessert gels and toppings, and savory applications such as sauces and marinades. It can also be used in low-acid and high-sugar products such as preserves containing low-acid fruits (figs and bananas) and confectionery.
Apple By-products: Coloring and Noncoloring Sweetener
After distilling the alcohol used for the precipitation of apple pectin and other fruit extracts, such as sugar and fruit acids, the natural flavors will remain. For example, apple extract obtained from pomace will be used as sweetening agents for the preservation of freshness and/or coloring of food. A further possibility is to ferment it to form apple ethanol.
At a further processing stage, special technologies are used to remove dark natural coloring agents, mineral substances, and fruit acids from these fruit extracts. The resulting prod- ucts will only contain the sugars of the respective raw material that has been processed. They will be used by the food indus- try as sweetening agents (Khachatourians and Arora 2001).
Apple Pectin By-product: Fodder
After apple pectin is extracted, the various residues of the original raw material are dried and pressed into pellets. Due to their high energy content and nutritive value, these products are in demand as fodder. The residual moisture and the fodder value of these products are checked continuously so as to ensure that products of uniform quality are obtained (Bennett 2002).
Apple Pomace Processing (Fiber Utilization)
After adding wine yeast to the apple pomace, remaining from fruit juice and apple pulp production, the marc is fermented at 30◦C in solid phase, resulting in a liquid with a 4–5%
ethyl alcohol content. Then it is concentrated to 10% by vacuum distillation. With further fermentation, high-quality apple vinegar can be obtained.
If we add “Aspergillus niger” mold and methyl alcohol to the apple pomace, its sugar content will decrease by 81% in 5 days. Meanwhile, from 1 kg of apple marc, we can extract 90 g of citric acid or a yield of 88%, if expressed in sugar.
If apple marc is treated with a thin alkali solution, we get two fractions: fibers comprising alpha-cellulose pentosanes (26%) and pectin (10–18%). Both fractions can be used for apple products as a thickener and a calorie-free texture modifier.
In civilized societies, there is a preference for refined and cleaned foodstuffs. However, consumers deprive themselves of many substances that are considered healthy. A lack of fiber, for example, would result in diseases and abnormali- ties, which are unknown in uncivilized societies. Nutrition scientists are researching the degree to which refined food will trigger health problems (Barta 1993). There are ongo- ing efforts to add back important substances, such as dietary fibers (Larrauri 1999; Miguel and Belloso 1999), coloring matters, aromas, volatile compounds, vitamins, etc. These substances have been removed or cleared during operations to purify the food for a convenient “end product.” They may also be the result of a negative effect from an essential pro- cessing. However, some of such “removed” substances are very important for our health. Examples include fibers or vi- tamins (Ramadan and M¨orsel 2003), which are added back, for health reasons or legal requirements, after removal during processing. Today, fiber products are very important dietary supplements. The indigestible parts of plant cell wall, such as cellulose and lignin, were considered as unnecessary parts of foodstuffs that decrease the energy, compositional, and sometimes even the sensory values.
The nutritional effect of dietary fiber components is due to their physical and chemical properties. The human body does not have enzymes to digest fibers. Fibers are resistant to digestion by gastric juices; only some bacterium can decom- pose a certain quantity. Consequently, fibers possess slight nutritive value, but they play an important role in digestion (Barta et al. 1989).
Citrus Waste Utilization
Citrus processing produces a large amount of waste materi- als, which can be divided into three categories: animal feed, raw material used for further extraction of valuable compo- nents, and food by-products. Dried citrus meal that is used for animal feed is probably the main waste-recovery prod- uct. The meal is produced by liming the slurry followed by pressing to remove moisture. The moisture is further reduced to about 8% using rotating dryers. This material is similar in feed value to beet pulp.
Citrus pulp consists mainly of the rag, peel, and seeds of oranges with minor amounts from other fruits. This waste usually collects on concrete slabs or in open pits at canneries.
Citrus pulp usually is used as a source of energy because of its composition. Fat and protein of citrus pulp vary with the seed content, which ranges from 1.0% to 17.7% depend- ing upon the variety of fruit. The citrus seeds are also used efficiently. They can be used for oil extraction and also the production of a citrus seed meal for feed rations.
Citrus molasses is a good material that can be used as a feed supplement. Some work has been done on mixing sodium carbonate with waste peel and pulp materials from some cit- rus fruit processing operations (Braddock 1999). This treat- ment raises the product’s pH and results in deesterification
of pectin, forming a gel. Waste production is decreased in some products where a fraction of the pulp is comminuted and becomes part of a fruit-drink base. The raw material that is further extracted produces peel oils, flavonoids, and seed oil. Food items produced are brined and candied peels, marmalades, syrups, and peel products used in food sea- soning. The peel juice, or press liquor, can also be utilized as a fermentable carbohydrate source for the production of feed yeast, industrial alcohol, vinegar, butylene, and lactic acid. The practical use of these products depends on the eco- nomics of the process. Waste coming from the processing of such other fruits, such as apples and pears, can also be used in the manufacture of pectin, but not as economical as from citrus (Salunkhe and Kadam 1995).
Citrus Pectin
Citrus peels and residues contain 2.5–5.5 % pectin. After the extraction of essential oil from the peel and juice from the fruit, the residue is dried. The peel is sliced and ground. The residue is washed with cold water on a sieve, and the washed material is boiled with 0.015–0.20 N hydrochloric or sulfuric acid, or with 0.025 M citric acid for 40–45 minutes. The liquid is pressed and filtered to obtain the pectin solution.
This solution is then centrifuged to remove the sediment.
The pectin solution is then treated with enzymes and with decolorizing carbon to obtain the pure product. The pectin solution is then concentrated, and finally pectin powder is prepared (Salunkhe and Kadam 1995).
Citrus Oils
Fresh orange peel yields about 0.54% oil by the cold-press methods. Citrus peel oil, extracted by the cold process, fetches a better price than distilled oil, which is of inferior quality.
By-products of the Citrus Industry
Citrus fruit production can be divided depending where it arises in the juice extraction process:
r Juice and juice cells, which form about 40–45% of the fruit.
r Peel (flavedo) and rag (albedo), which constitute about 45–60 % of the fruit: the flavedo contains the essential oils and carotenoid pigments; the albedo contains cellu- lose, pectins, and flavonoids.
Table 19.1 shows different products produced from cit- rus fruit. The main by-products from the endocarp, or inner part of the fruit, are juice cells and pulpwash (Salunkhe and Kadam 1995).
Table 19.1. Products of the Citrus Industry
Juice and Cells Peel and Rag Essential Oil Concentrated juice Pectin Cold pressed oil
Juice Cloudy concentrate Terpenes
Premium pulp Hesperidin Concentrated oil Pulpwash concentrate Naringin Distilled oil Dehydrated cells Dried peel
Water and oil phase Molasses
Volatiles Alcohol
Natural color Source: Arthey and Ashurst (2001).
Citrus Waste as Ethanol Feedstock
In the 1990s, the USDA Citrus Lab developed an ethanol production process that used the residual sugars and cellu- lose in citrus peel. A series of citrus-specific enzymes was developed to convert the cellulose to sugars for the fermen- tation process. From there, the ethanol process is similar to that used in corn-ethanol production (Anon 2008).
Peach Waste Utilization
Processing of peach results in the generation of waste in the form of peel, seeds, and trimmings, and washing water having high biological oxygen demand (BOD) and COD, see in Table 19.2.
The waste contains proteins, polysaccharides, sugar, amino acid, and pectin. Therefore, it can be processed into useful products. Nevertheless, it has to be treated to reduce BOD as per the stringent standards laid down by environmental protection agencies.
Seeds of peach also constitute waste, but due to their high protein and fat contents, they have potential for utilization, perhaps after detoxification. Agricultural residue including peach leaf litter, after pretreatment and fermentation, has proved to be a good source of methane gas. Enzymatic pre- treatment of peach solid waste for ethanol production has been investigated for the utilization of the waste. However, more research is needed before it is advocated for industrial application (Salunkhe and Kadam 1995).
Table 19.2. Characteristics of Waste Generated from Processing of Peaches
Characteristics Amount
Peach fruit processed 1100×103tons
Waste water 4400×103gal
Biological oxygen demand 60 lb/ton
Suspended solids 10 lb/ton
Solids residuals 500 lb
Source: Salunkhe and Kadam (1995).
Banana Waste Utilization
About 1000 banana plants are estimated to yield 20–25 tons of pseudostems, which contain about 5% edible starch, useful for sizing in the textile industry. The process for the manu- facture of starch from banana pseudostem has evolved and its physicochemical properties studied. Researchers studied the utilization of banana stem waste for growing food yeast.
The residual fiber portion of the stem left over after ex- traction of starch can be used for the preparation of paper pulp. The composition of different fruit waste is given in Table 19.3.
The central core of banana pseudostem constitutes 10–15%
of stem and can be candied or crystallized into a highly ac- ceptable product that resembles tender bamboo shoot candy.
The fresh material, commonly used as a vegetable, can be canned along with potato and tomato as a curried product.
After blanching and steeping in dilute citric acid solution con- taining a small amount of potassium metabisulfite, the slices can also be dehydrated into a fairly acceptable product. Green banana fruit, pseudostems, and foliage are suitable as animal feed. They mainly provide a source of energy and require supplementation with a protein source. Bananas are econom- ical as a source of animal feed only where the livestocks are nearby, because of the high cost of transport. Corns, shoots, and male buds find widespread use as an animal food in Asia and Africa (Salunkhe and Kadam 1995).