Fruit processing (Mango, Grapes, Papaya etc.): waste characteristics, management, value addition

Mango

          Mango is one of the most delicious and widely cultivated fruits of the tropical world. It is processed extensively, thereby generating huge quantity of solid and liquid waste. Solid waste is comprised of mango peel, stones, stalk, trimmings, fibrous material and rotten fruits. This constitute about 40-50 % of total waste. Utilization of waste is both a necessity and challenge. This not only economizes the cost of finished product and reduces the pollution level, but also leads to more complete utilization of the raw material. In fruit processing industries various enzymes such as cellulases, pectinases and amylases are invariably used for pulp liquefaction, juice clarification etc. There is now increased interest in enzyme production from food processing wastes. Cellulases and pectinases could be produced from mango peel using microorganisms like fungi, actinomycetes and bacteria. Amylases from mango kernel could be produced by microbial fermentation.

It can be estimated that mango processing yields between 150,000 and 400,000 t of wastes worldwide, which may cause environmental problems in the vicinity of the processing plants.

The edible pulp makes up 33–85 per cent of the fresh fruit, while the peel and the kernel amount to 7–24 percent and 9–40 percent, respectively, on a fresh weight basis. The by-products/wastes available after processing of mango includes cull fruits (fresh fruits unsuitable for human consumption), mango kernel meal (containing 6−6 per cent mango oil on DM basis), deoiled mango kernel meal and mango peels.

Mango processing yields about 40-50% of by-products, which can be used to feed livestock. These by-products are also potential sources of pectins and phenolic compounds (antioxydants). The mango kernel contains 7-12% of an oil rich in stearic (24-57%) and oleic (34-56%) acids that can be fractionated to give an olein with excellent emollient properties and a stearin that is one of the few fats that can replace cocoa butter in chocolate in certain countries (including the European Union).

Mango sees kernel

Anti-nutritional factors: Mango seed kernels are rich in tannins, which progressively lead to depression in growth rates and efficiency of feed utilization, when included as a major component in diets of pigs and poultry. They also contain 64 mg/kg DM cyanogenic glucosides, 42 mg/kg DM oxalates and 20 TIU (trypsin inhibiting unit)/g DM trypsin inhibitors. Soaking in water was most effective, and it removed 61 per cent of the tannins and 84 per cent of hydrocyanic acid (HCN).

Ruminants: Mango seed kernels can be incorporated in the concentrate mixture up to 50 percent without any adverse effects. In sheep, DM digestibility of dried seed kernels was 70 percent but intake was low (1.2 percent of body weight), mainly due to the tannin content.

Non-ruminants: Raw mango seed kernel meal included at 5−10 per cent in the diet depressed feed intake and growth in broiler chicks. The recommendation for optimum growth is to use boiled mango seed kernels allow efficient utilization of the energy in the diet.

Mango peels

Mango peel is the major by-product of mango processing, and compromises 7-24% of the total mango weight.

Mango peel waste is either used as cattle feed or is dumped in open areas where it adds to pollution to the environment. Pectin is an extremely versatile organic resource in mango peels. The amount of pectin can be very high, for example lemon peels contain 15-20 %, apple peels about 10% and mango peels had about 12.9%.

The mango peel can be used as a source of pectin. Pectin is used to improve the quality of many food and pharmaceutical products. Mango peel fiber has been found to have better in vitro properties than lemon fibres due to higher values for antioxidative activity. It could also be used as a substrate for mushroom cultivation after supplementing it with rice straw. Pectins are major constituents of the cell walls of edible parts of fruits and vegetables. The middle lamellae which are situated between the cell walls are mainly built up from protopectin which is the insoluble form of pectin.

Pectins are considered as intercellular adhesives and due to their colloid nature they also have an important function in the water-regulation of plants. Water binding capacity is greatly increased by the amount of hydrophilic hydroxyl and carboxyl groups. Pectin was first isolated in 1820 and shown to be the key substances in making jams and jellies. Pectin is a polymeric material having carbohydrate group esterifies with methanol. These are negatively charged acidic glycosidic macromolecules and have high molecular weight. Pectic substances are classified into four main types based on the type of modifications of the backbone chain which are; protopectin, pectic acid, pectinic acid and pectin.  

In order to produce good silage, mango peels were mixed with rice straw and legume to facilitate fermentation. Ensiled mango peels and rice straw had 60 per cent DM digestibility, which increased when Leucaena leaves were included in the diet.

Non-ruminants: Dried mango peels up to 10 percent in the diet of finishing pigs had no deleterious effect on feed conversion ratio or performance and economized feeding cost.

 

The use of mango wastes in livestock feeding is a way of reducing environmental concerns

Grapes

 The main by-products of a vineyard are the grape stalks rich in lignin, cellulose, nitrogen and potassium (vine prunings ~5 tonnes/ha/year). These can be used for producing compost. Grape stalk compost has a high agronomic value and is particularly suitable for the soils of the vineyards which have very low organic matter content. The use of grape stalks in the form of single cell protein, as ruminant feed or a component in the feed has also been proposed after solid state fermentation using fungal strains. After removal of lignin through the fungal treatment, the cellulose is better accessible to rumen micro-organisms. Since the fermented product has good protein value and low lignin content, it has DM digestibility similar to forages (54−60%). The grape pomace contains 9−12 percent Crude protein  and 5−7 percent Ethyl extract and has very low metabolisable energy (ME) (1.06 Mcal/kg DM) and Net energy (NE) (0.69 Mcal/kg DM) for lactating dairy cows.

During the wine making process, there are produced several by-products that are rich in biodegradable organic matter that can cause, if not treated correctly, potential environmental hazards. More precisely, inappropriate grape pomace disposal can attract flies and pests and can easily create unwanted hazards. Moreover, utilization of wine by-products in combination with the production of sustainable wine can provide enough advantages. Vineyard and winemaking by-products contain valuable chemical compounds that are not extracted during wine making process. The nutritional concentrations vary on the different type of each by-product (marc, pomace, grape seeds) and also on the different wine making process (white or red vinification).

Single Cell Protein (SCP)/microbial protein is one of the alternative protein sources that can supplement the conventional protein sources. SCP refers to dead, dry microbial cells or total proteins that are extracted from pure microbial cell culture and is produced using a number of different microorganisms (fungus, algae and bacterium). SCP contains high protein, fats, carbohydrates, nucleic acids, vitamins and minerals and also essential amino acids like lysine and methionine that are not found in most plants and animals.

Grape by-products

According to FAO, 67.1 million tons of grapes were utilized in wine production in 2013. 20% of the weight of processed grapes is not found in the final product. There are three main by-products derived from the wine making process, which are the stalks (2.5 to 7.5%), grape pomace/marc (15%), grape seeds (3 to 6%) and wine/yeast lees (3.5−8.5). Stalks and grape pomace are the left-over from pressing grapes and wine lees (dead yeasts, yeast residue and other particles) is a material derived throughout the whole wine making process.

Grape by-products contain a respectful amount of organic components (sugars, phenolics, lipids, pectins) and are recognized as environmental pollutant. In the recent years, there are various attempts to use those by-products not only as a way to protect the environment by minimizing their unwanted effects arising from their disposal but also as a way to provide an extra income to the wine industry. Grape by-products can be used as nutraceuticals, antioxidants, colorants, antimicrobials, animal feed, fertilizers, in combustion process, biomass for biofuels, medical remedies, cosmetics and natural ingredients to improve nutritional value of food Grape pomace.

Grape pomace (GP) is a major by-product of wine making, as it is equivalent to about 20% of the grapes used. It consists of pressed skins and pulp (10-12% of grapes), grape seeds (3-6%) and stems (5-7%,) and contributes to approximately 62% of the organic waste. GP is a rich source of high value products including ethanol, tartates, malates, citric acid, grape seed oil, hydrocolloids and dietary fiber. Furthermore, GP has been found to be an excellent source of low-ash carbon whose extensive porosity. The overall properties of GP has widened their utilization and supported the sustainable agricultural production as long as the existence of phenolic phytochemicals, that are found in abundance in grapes, wines and wine by-products.

GP provides phenolic compounds as they share characteristics with anthocyanins in terms of solubility in waster and stability to temperature and oxygen. GP is a good source of soluble fiber, with high antioxidant activity due to the natural presence of polyphenols and other bioactive compounds. Thus, GP and grape seeds extracts can be used in healthy food products such as yoghurt and salad dressing. The GP had low moisture content and pH (lower than 4) that created unfavorable conditions for microbial growth. Hence, it do not have microbial contamination and toxicity. Moreover, GP could be used as medicine to reduce the ill effects of mycotoxins/affalotoxins.

Fermentation process

Grape and wine by-products are a good source of carbon and have been used to generate various high-value products like as citric acid, lactic acid, gluconic acid, ethanol through submerged and solid state fermentation. Trichoderma harzianum, Aspergillus niger, Penicillium chrysogenum and
P. citrinum have been used in order to degrade winery biomass.

Fungi belonging to the division Ascomycota, such as Trichoderma spp., Aspergillus spp. and Penicillium spp., are known for their biomass degrading ability and should prove useful in grape biomass degradation. The fungi have been studied also for their ability to produce high levels of cellulose and hemicellulase degrading enzymes. Protein rich products can be used as feedstock for animals. The protein content of grape marc has been managed to be increased from 7% to up to 27% in five days using solid state fermentation process, certain fungal strains and specific conditions like temperature and moisture content.

Submerged fermentation of grape waste using the species of mould, Monascus purpureus produced a red pigment that can be used in food industry The usage of bacteria, Lactococcus lactus and Lactobacillus pentosus led to the production of lactic acid. The bracket fungus, Trametes pubescens  can be used to produce enzyme, laccase. Pleurotus ostreatus complex is the third most important edible mushroom cultivated worldwide to decompose lignocellulose without chemical or biological pretreatment (as it possesses an enzymatic complex system that includes phenol oxidases and peroxidases). Therefore, they can be utilized and recycled by solid state fermentation (SSF) using various strains of mushroom.

Solid-state fermentation (SSF) is defined as the fermentation involving solids in absence of free water. However, substrate must possess enough moisture to support growth and metabolism of micro-organism. All the fermentation processes used in ancient time were based on the principles of SSF. Nowadays, SSF offers numerous opportunities in processing of agro-industrial residues. It stimulates the growth of micro-organisms in nature on moist solids, it has lower energy requirements, it produces lesser wastewater and it is environmental-friendly as the problem of solid wastes disposal is solved. The bioconversion of vineyard prunings and grape pomace by Pleurotus spp. with SSF was evaluated by measuring the fruiting body as alternative attempt for recycling of winery agro-industrial wastes.

Composition of winery wastewater

One of the biggest issues for the wine industry is the management of large volumes of wastewater. While wine production does not have a reputation as a polluting industry, the wastewater volumes worldwide are increasing and the wastewater has a high organic load, low pH, variable salinity and nutrient levels, all of which indicate that the wastewater has the potential to pose an environmental threat.

The four biggest components contributing to wastewater pollution in a winery are:

1. Sub-product residues: stems, skins, sludge, lees, tartar.

2. Lost brut production: must and wine occurred by spillage during winemaking activities.

3. Products used for wine treatment: fining agents and filtration earths

4. Cleaning and disinfection products (e.g. sodium hydrox-ide (NaOH) and potassium hydroxide(KOH) used to wash materials and equipment.

Most of the wastes generated in a cellar (80 to 85%) are organic wastes. The organic material in winery wastewater is generated from the grapes and wine. COD levels for grape marc can range from 15,000 to 44,900 mg/L, for lees from 27 200 to 36 100 mg/L and for wine from 26,200 mg/L.

The composition and components portions of winery wastewater has  90% of the organic component is ethanol, except during harvest, when it is mainly sugars. Ethanol concentrations of 4900 mg/L and sugar (glucose and fructose) of 870 mg/L were detected in winery wastewater with a dissolved COD of 12700 mg/L.

Contributing to the difference in the composition of the organic material in wastewater are the uncontrolled chemical reactions that take place in the wastewater. Organic acids (acetic, tartaric, malic, lactic and propionic), alcohols, esters and polyphenols play an important role in the composition of winery wastewater

The composition of the inorganic compounds in winery wastewaters is dependent mainly (up to 76%) on the components of the cleaning agents used in wineries, except for potassium, which is present in high concentrations in grape juice. Strong alkaline-based cleaning agents that are good for tartrate removal include caustic soda (NaOH) and caustic potash (KOH). Wineries that uses sodium-based cleaning agents have problems with the salinity of the wastewater if it is used for irrigation. The inorganic ions present are predominantly potassium and sodium, with low levels of calcium and magnesium, although the concentrations of both organic and inorganic constituents vary with differences in winemaking operations over time, as well as between individual wineries.

          Moderate quantities of winery waste and wastewater that are exposed to soils can increase the organic material due to the high concentration of soluble organic carbon in winery wastewater, which, in turn, will enhance the fertility of the soils.

Unfortunately, continuous exposure to the organic material can lead to organic overload that blocks the pores and lowers the quality of the soils immensely. The continuous addition of winery wastewater to soils can also contribute to high soil salinity, which can lead to dispersion.

The disposal of grape pomace/marc, a complex lignocellulose material made up of the skin, stalks and seeds, has also been a problem for wineries. In total, more than 20% of wine production is waste, comprising thousands of tons. Untreated grape marc can lead to several environmental threats, including foul odours and ground water pollution. Decomposing grape marc provides the perfect environment for flies and pest to flourish. Leachate from the marc contains tannins and other chemical compounds that infiltrate the surface soil and ground water, leading to oxygen depletion. It is possible to use the marc in other industries, however, this can be expensive.

The average COD of wine lees is 76,000 mg/L, therefore a small volume (of spillage when the tank is washed out after use) could have a vast influence on the raw wastewater of the winery.

Elimination of salt (K, Ca, Na & Mg) usage in the winery should be promoted to reduce the EC and sodium absorption rate (SAR); consequently no treatment would be necessary before irrigation with the wastewater.

The use of non-sodium-based cleaning chemicals is advised by Chapman. Replacing disinfectants and cleaning agents with ozone will result in lowering the EC and COD. The initial cleaning with caustic can also be substituted with a high-pressure rinse or with heat/steam. When caustic is used for cleaning, the aim should be to re-use it.

          Papaya is a tropical fruit grown more prominently in recent years, production next to fruits such as mango, banana, citrus and pineapple at the global level. The fruit is characterize for its active pectinolytic enzymes during ripening. Papain, the proteolytic enzyme is of industrial importance. The fruit is known for its carotenoid content and retinol activity, to counteract vitamin A deficiency.

Ripe fruits, Pepain, latex extracted fruits,

Animal feed: Leaf stem skin and waste materials of raw and ripe fruits are pulverized and fed to animals, poultry and pigs

Unripe green fruits considered as unsuitable for product making or fruits after obtaining latex used for pectin extraction. Green papaya is rich in pectin containing 10% of dry weight basis. Pectin can also be extracted from peel waste of green papaya.

Worldwide production of papaya has exceeded 12.6 million metric tons in 2014 as reported by the Food and Agriculture Organization of the United Nations. With such a large amount of papaya produced, and an estimated 30–50% cull rate, there is a large amount of agricultural waste produced. Thus, in order to not only alleviate papaya waste but also produce value-added products, microbial utilization of papaya waste has attracted considerable interests.

The seeds represent a significant portion of the waste (up to 30% of the papaya by volume) and contain a high percentage of lipids (25–30% by dry weight). One possible valorization of papaya seed lipids is conversion to biodiesel but papaya lipid-derived biodiesel has low fuel quality with high cloud and pour points. The papaya seed lipid is an untapped waste feedstock to provide carbon and energy sources for growing certain microorganisms to produce high-value products. 

Case study

Around 52 mango fruit processing units are established in Chittoor district of Andhra Pradesh. These units process the mangos of 60,000 tonnes per annum, of which 40,000 tonnes after processing are exported. Typical factory will process 5 tonnes of Totapuri mangoes (a popular variety of this region) per hour and peel would be accumulating as waste.