Peeling of fruits and vegetable: A review

Introduction

Fruit and vegetable peels play a significant role in human nutrition, as they are rich in dietary fiber that contributes to satiety and hunger reduction. In fact, many fresh fruits and vegetables may contain up to one-third more fiber when consumed with their peels. However, in food processing, peeling is often an essential preliminary step, since the removal of the outer layers is required for most agricultural commodities. The effectiveness of peeling largely depends on the physical and mechanical properties of the produce, and continuous research has been directed towards developing improved methods that ensure efficiency and sustainability. The primary objectives of an optimal peeling operation are to minimize product losses, reduce energy and chemical consumption, and limit environmental impact. Peeling methods can be broadly categorized into manual peeling (using knives or blades), mechanical peeling (using abrasive drums, rollers, or cutting devices), chemical peeling, enzymatic peeling, and thermal peeling. Among these, abrasive peeling has been successfully applied to a range of vegetables using abrasive peelers (Fouda et al., 2019) [7] .

Methods of fruits and vegetable peeling Manual peeling

Manual peeling is one of the oldest and most common methods of removing the skin from fruits and vegetables. It is usually performed with hand tools such as knives or stationary/rotary peelers. This technique often produces fresh-cut products with acceptable microbiological quality. Klaiber et al., (2005) [12] noted that using knives for peeling carrots caused less tissue injury compared to abrasion peeling, which may lower the chances of microbial contamination during subsequent handling. In contrast, O'Beirne et al., (2014) [22] found no major difference between coarse abrasion and hand peeling regarding the attachment of E. coli O157:H7 on carrot surfaces. While manual peeling ensures careful handling and can help retain product quality, the method is slow, labor-demanding, and therefore better suited for small-scale or householdlevel operations. One advantage of this approach is the ability to peel produce of irregular shapes and sizes with minimal waste, making it especially useful for delicate fruits and vegetables (Garcia and Barrett, 2002) [39] . However, in large-scale food industries, its dependence on human labor reduces productivity and raises operational costs (Rico et al., 2007) [26] . In addition, differences in operator skill and hygiene practices can lead to inconsistent quality and potential cross-contamination risks (Nguyen and Carlin, 1994) [21] . For these reasons, the food industry is increasingly moving toward mechanical, chemical, or enzymatic peeling methods, which offer greater efficiency, uniformity, and food safety assurance.

Mechanical Peeling

Mechanical peeling involves processes that act directly on the surface of fruits and vegetables to detach and remove the skin. Commercially this is achieved through abrasive devices, rotating drums, rollers, knives, and milling cutters. These systems are valued for producing high-quality fresh end products while being environmentally safe and nontoxic. The effectiveness of mechanical peeling largely depends on the mechanical and physical characteristics of the produce including skin thickness, firmness, toughness, variety, rupture force, cutting force, shear strength, tensile strength, and rupture stress (Shrimohammadi et al., 2012) [30] . In addition to conventional applications efforts have been made to design specialized peeling machines. For example, a sugarcane peeling machine was developed to address challenges in the commercial processing of sugarcane. The canes were classified into small, medium, and large sizes, and the prototype equipped with an abrasive peeling tool achieved a peeling efficiency of approximately 59.67% (Gadekar et al., 2018) [9] . This development provides a foundation for further improvements in the design and industrial production of sugarcane peeling equipment. Evaluated the performance of a new small-scale sugarcane peeler machine. The maximum machine production efficiency (88.85%) and the minimum electrical power consumption (5.56kW) were achieved with no. of feeding canes per minute of 3 canes respectively. Also, the minimum machinery unit cost was 67.49 LE/Mg and 9 cans/min feeding rate (Yamini et al., 2016) [37] . Developed a power operated batch type mechanical peeler for potato peeling. The machine consists a peeling drum with protrusions on the inside surface and the drum rotates and then detaches peel from potatoes by abrasion. Additionally, the peeler have a water spraying unit that washes the potatoes and simultaneously peels are removed from the drum. Singh and Shukla (1995) [31] .

Abrasive devices

Abrasive peeling is a simple yet effective technique in which the outer skin of vegetables is removed by mechanical friction. Traditionally, this has been achieved using gloves or pads with rough surfaces that scrape off the peel when the produce is rubbed against them. The method is particularly suited for small root crops such as potatoes, where the skin can be loosened and detached with minimal effort (Somsen et al., 2004) [32] . Recent developments have led to the introduction of specialized abrasive gloves and tools that provide a safer alternative to knives or sharp peelers. These products feature textured surfaces designed to improve efficiency while reducing the risk of injury during handling. Their growing popularity in both households and small-scale commercial settings is attributed to their ease of use, cost-effectiveness, and ability to peel a variety of vegetables without significant product loss (Pereira and Vicente, 2010) [24] . In industrial food processing, abrasive peeling is also applied through rotating drums lined with abrasive materials, where continuous friction removes the peel. This approach enables faster processing and uniform peeling, though it can lead to higher product losses compared to more precise methods such as steam or enzymatic peeling (Mizrahi, 2015) [16] . Another limitation is the potential for microbial contamination due to the increased surface area exposed after abrasion, requiring careful sanitation and handling (Rico et al., 2007) [26] . Despite these drawbacks, abrasive peeling remains a widely adopted method because of its simplicity, low equipment cost, and adaptability for different scales of operation.

Knife and Blade

Blade peeling machines are commonly designed with cutting edges that physically separate the skin from the underlying edible tissue of fruits and vegetables. In a typical setup, the produce is placed on a holding base and rotated to ensure that all surfaces come into contact with the blade. The peeling action is often supported by mechanical systems such as threaded rods, springs, or adjustable arms, which allow the blade to maintain consistent pressure against the surface of the rotating produce (Tardif and He, 1999) [35] . This arrangement enables uniform and controlled removal of the peel while minimizing unnecessary loss of edible material. Modern designs of blade-based peelers have incorporated features such as automated rotation, adjustable blade angles, and sensors that adapt to different shapes and sizes of produce, thereby improving efficiency and reducing waste (Sankat and Maharaj). Compared to abrasive methods, blade peeling generally produces smoother surfaces and better preserves the structural integrity of the product (Kumari et al., 2018) [15] . However, the technique may require frequent blade maintenance and can be less suitable for very small or irregularly shaped items. In industrial applications, high-speed mechanical peelers are widely employed for root vegetables like carrots, beets, and potatoes. These machines offer advantages in terms of precision, throughput, and quality, although their initial cost and energy requirements are higher than simpler methods such as abrasive peeling (Mizrahi, 2015) [16] . Despite these challenges, blade-based peeling continues to be an important technology in both small-scale and large-scale vegetable processing, balancing efficiency with product quality.

Lye or Chemical Peeling

Lye peeling is among the oldest and most widely practiced chemical methods for removing the skin from fruits and vegetables. In this technique, the raw material is briefly immersed in a hot alkaline solution, usually sodium hydroxide, maintained at temperatures around 90-100 °C (Di Matteo et al., 2012) [5] . The alkali penetrates the outer layers and hydrolyzes structural polysaccharides such as pectins and hemicelluloses. Specifically, the alkaline treatment cleaves the α-(1→4) glycosidic linkages between galacturonic acid residues in the pectin chain, disrupting the middle lamella and weakening the structural integrity of the skin (Barreiro et al., 2007) [1] . As a result, the peel loses adhesion to the flesh and can be easily removed during subsequent washing. One of the main advantages of lye peeling is its high efficiency, which makes it suitable for large-scale processing of produce such as peaches, apricots, citrus fruits, and tomatoes (Castro-Giraldez et al., 2014) [4] . The method ensures rapid peeling, relatively smooth surfaces, and low mechanical damage compared to abrasive or bladebased peeling. However, lye peeling also generates significant quantities of alkaline wastewater, which can present environmental and disposal challenges if not properly treated (Sapers, 1995) [27] . In addition, prolonged exposure or excessively high concentrations of alkali may cause product softening or loss of sensory quality. To reduce these limitations, modern processing facilities often combine lye peeling with steam or hot water rinsing systems to neutralize residual alkali and improve peel removal efficiency (Ribeiro et al., 2010) [25] . Recent research has also explored the replacement of traditional sodium hydroxide solutions with alternative, more eco-friendly chemical formulations, as well as enzymatic or combined treatments, to minimize chemical load and enhance sustainability (Mir and Behera, 2016) [18] . Despite its drawbacks, lye peeling remains an important technique in food processing due to its effectiveness, speed, and adaptability to a wide range of fruit and vegetable commodities.

Enzymatic Peeling

Enzymatic peeling is a biotechnological fruit-processing technique that involves the use of high-activity enzymatic solutions containing polysaccharide-hydrolyzing enzymes such as pectinases, cellulases, and hemicellulases. These enzymes specifically target the structural polysaccharides pectin, cellulose, and hemicellulose which are primarily responsible for the tight adherence of the peel to the underlying fruit tissues. By breaking down these complex polysaccharides the enzymatic treatment weakens the integrity of the middle lamella and primary cell wall allowing the peel to be removed more efficiently and with minimal damage to the fruit flesh (Suutarinen et al., 2003) [33] . The enzymatic preparations used in peeling are generally obtained through the controlled fermentation of genetically modified fungal microorganisms developed and optimized in modern biotechnological industries to produce enzymes with high yield and activity. Compared to conventional chemical peeling methods which often employ alkaline or acidic solutions, enzymatic peeling is regarded as a safer, more environmentally friendly, and nutritionally preserving alternative (Seminario et al., 2016) [40] . It not only reduces chemical waste and prevents undesirable alterations in flavor and nutrient content but also improves surface quality and uniformity of peeled fruits such as citrus fruits, peaches, and tomatoes (Lee et al., 2013) [41] . In addition, enzymatic peeling demonstrates a significant potential for sustainable industrial applications, as it minimizes energy consumption and reduces effluent load compared to thermal or chemical peeling methods (Ramesh & Kumar, 2018) [15] . With advancements in enzyme technology and microbial fermentation, this process continues to evolve as a clean-label peeling technique, aligning with consumer demand for minimally processed foods and environmentally responsible manufacturing practices.

Thermal Peeling

Thermal peeling is widely employed in the processing of vegetables with relatively thick skins, such as potatoes, carrots, beets, and onions. This method relies on the application of heat, which can be provided either through wet systems (e.g., steam) or dry systems (e.g., flame, infrared radiation, or hot gases) (Schlüter & Knorr, 2004) [28] . Among these, steam peeling has become the most commonly used industrial approach because it enables high levels of automation, accurate regulation of pressure, time, and temperature, and results in lower chemical waste compared to lye or other chemical peeling methods (Garrote et al., 2000) [42] . The mechanism of steam peeling involves a combination of physical and biochemical changes. When the produce is exposed to high-pressure steam, rapid heating generates internal vapor pressure within the tissues. This sudden increase in pressure weakens the epidermal layers and disrupts the adhesion between peel and flesh, causing the skin to loosen and detach (Lisinska and Leszczynski, 1989) [20] . In addition, elevated temperatures modify the tissue structure by softening cell walls, degrading pectin, and reducing turgor pressure, which further facilitates peel removal (Garrote et al., 2000) [42] . Once the steam chamber is depressurized, the loosened peel can be easily removed by mechanical brushes or water sprays. Compared to other methods, thermal peeling is considered more environmentally friendly since it avoids the use of alkaline solutions, thereby reducing wastewater treatment requirements (Barreiro et al., 2007) [1] . It also minimizes product loss, provides smooth peeled surfaces, and allows continuous large-scale operation. However, prolonged exposure to heat can sometimes cause partial cooking, loss of volatile compounds, or changes in texture, which may not be desirable for certain products (Brodnitz, 1990) [2] . To address this, modern steam peeling systems employ precise time-pressure controls, rapid depressurization, and integration with mechanical finishing devices to optimize peeling efficiency while preserving quality (Kita and Lisinska, 2005) [11] .

Emerging Methods

Infrared (IR) radiation, located between visible light and microwaves on the electromagnetic spectrum, has gained attention as an innovative non-contact heating method in food processing. When IR waves strike a material, part of the radiation is reflected or transmitted, while the absorbed portion is transformed into heat energy. This process increases the temperature of the material, with the outer layers heating more rapidly than the inner core (Li et al., 2014) [19] . In the context of peeling, this localized heating softens or loosens the peel, creating a separation between the skin and underlying tissue without excessive damage to the edible portion. The primary advantage of IR peeling lies in its ability to provide uniform and controlled surface heating. By precisely adjusting radiation intensity, wavelength, and exposure time, processors can achieve effective peel removal while preserving the nutritional compounds, texture, and sensory properties of fruits and vegetables (Nowak et al., 2012) [17] . Compared to conventional thermal methods such as steam or flame peeling, IR systems typically require shorter processing times and lower energy input, making them a promising alternative for sustainable food production (Zhu et al., 2013) [43] . Another important aspect of IR peeling is its potential to reduce resource consumption. Unlike lye peeling, which generates alkaline wastewater, or steam peeling, which demands large amounts of water and energy, IR peeling is a dry process with minimal effluent. This not only lowers environmental impact but also decreases post-peeling washing and wastewater treatment requirements (Chaudhry and James, 2010) [3] . Furthermore, IR radiation has inherent antimicrobial effects, which can enhance food safety by reducing microbial loads on the produce surface (Krishnamurthy et al., 2008) [14] . Despite these benefits, IR peeling is still at a developmental stage and faces challenges in scaling up for industrial applications. Non-uniform heating of irregularly shaped products, high equipment costs, and the need for optimized process parameters remain barriers to widespread adoption. Nevertheless, ongoing research and advancements in IR emitter design, automation, and hybrid systems suggest strong potential for the integration of IR peeling in future fruit and vegetable processing lines (Wang and Raghavan, 2017) [36] .

Conclusion

Peeling is a critical step in the post-harvest processing of fruits and vegetables, significantly affecting yield, quality and safety. Various methods manual, mechanical, abrasive, knife/blade-based, chemical, enzymatic, thermal, and emerging technologies like infrared peeling offer specific advantages and limitations. Manual peeling ensures minimal tissue damage but is labour intensive whereas mechanical and specialized machines provide high-quality output and environmental safety. Chemical and enzymatic peeling effectively weaken peel adhesion while thermal and infrared methods improve efficiency and preserve nutrients. Emerging techniques aim to reduce water and energy usage, minimize waste and enhance food safety reflecting a trend toward more sustainable and efficient processing. Future research should focus on optimizing peeling technologies for industrial and small-scale applications. Key areas include automation and sensor-based control to adapt peeling to fruit and vegetable characteristics energy and water-efficient systems integration of non-thermal methods such as pulsed electric fields or ultrasound and strategies for improved microbial safety. Additionally sustainable design of peelers and utilization of peel waste as dietary fiber bioactive compounds or bioenergy can contribute to circular and eco-friendly processing. These developments will enhance both the efficiency and quality of fruit and vegetable processing while reducing environmental impact.

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