Artificial Ripening Technologies for Dates

Abstract

Date palm fruits have essential importance due to their high economic value, nutritional benefits, and contribution to food security in arid and semi-arid regions. The unfavorable climatic conditions, drought or water scarcity, inconsistent pollination, genetic factors, and nutrient deficiencies cause date fruits to remain unripe for a long time. Artificial ripening is hastening fruit ripening using various techniques and chemicals. Artificial ripening techniques are employed to ripen date palm fruits to reduce their spoilage and waste, enhance their quality, and extend their shelf life. Therefore, artificial ripening has an economic benefit by supplying high-quality fruit, potentially increasing farmers’ profits. However, using safe and approved techniques for artificial ripening is essential, as some processes can have negative health influences if misused. This chapter aims to discuss the concept of artificial ripening for date palm fruits and its benefits, explore various chemical and physical methods, analyze their effects on fruit quality, and examine the regulatory and safety considerations associated with artificial ripening. Additionally, the chapter examines the advantages and disadvantages of different ripening methods and their corresponding effects on the dates’ nutritional value and sensory quality. The chapter highlights the need for sustainable and safe artificial ripening practices to meet consumer demand and ensure the high quality and availability of date palm fruits.

1. Introduction

The date palm (Phoenix dactylifera L.), a cross-pollinated fruit tree, is cultivated in arid and semi-arid regions for its sweet fruit and a staple food in dry regions. With over 120 million trees globally, it is primarily grown in the Arab world, with the majority in Egypt, Iraq, Saudi Arabia, Algeria, Morocco, Tunisia, and the UAE. Arab countries have 70% of the world’s date palm trees, contributing 67% to global production. It is native to North Africa and the Middle East. It is one of the most valuable crops in these regions, and people use its fruits, leaves, and sap for medicine, food, and shelter [1]. It is cultivated on 1.31 million hectares and produces 9.82 million tons of fruit annually worldwide [2]. Its importance can be observed in several domains, including agriculture, nutrition, economic nutrition, traditional medicine, and culture. Date palm cultivation has significantly influenced the growth of oasis agriculture systems, providing shade and microclimates for crop growth. These trees also prevent soil erosion due to their extensive root systems. The date palm fruit is a sweet, fleshy berry rich in calories, sugars, fiber, vitamins A, B1, B2, and C, and minerals such as potassium, calcium, and iron. The fruit is also a good energy source and can be consumed fresh, dried, or processed into syrup, paste, and vinegar. Date palm sap, a sweet, milky liquid extracted from the tree trunk, makes date honey syrup, alcoholic beverages, and vinegar. The global date market, worth billions of dollars annually, includes fresh dates, processed products like date syrup, date paste, and date-based sweets, contributing to the economy [3, 4].

The cultivation of date palms is significant for several reasons, including its economic importance. The date palm industry is labor-intensive and helps both males and females by generating revenue and jobs. Due to increased employment prospects in rural areas, widespread migration to cities is lessened. Women have a significant role, especially during the palm propagation and post-harvest stages, including packaging and marketing [5]. Date production and trading help local economies and serve as a source of revenue for farmers and exporters. Over the past few decades, the Kingdom of Saudi Arabia’s agricultural sector in general and the date palm sector, in particular, have experienced tremendous growth and support [6]. Many countries in the Middle East and North Africa, such as the Kingdom of Saudi Arabia, Iran, Iraq, Egypt, and Tunisia, rely substantially on the export of dates. In 2021, the Kingdom of Saudi Arabia was the leading global exporter of fresh or dried dates, with an export value of about 322.84 million USD followed by Israel (317.07 million USD), Iran (305.23 million USD), Tunisia (255.90 million USD), United Arab Emirates (209.43 million USD), Algeria (140.79 million USD), and United States of America (116.79 million USD) [7].

The cultural value of date palm cultivation contributes to its significance. For thousands of years, dates have been integral to the Middle East and North Africa’s food and culture. They are frequently offered at special occasions and festivals and are mentioned in many religious writings. In many civilizations, date palms are regarded as symbols of hospitality and prosperity [5, 8]. Date fruits are believed to have laxative properties, aiding in constipation relief. Seeds are used in poultices for skin conditions like burns and wounds. The leaves and sap of the tree are used in traditional remedies for fever, diarrhea, and respiratory disorders. The cultivation of date palms is also important for the ecosystem. They are one of the valuable crops in areas with scarce water resources. Stabilizing sand dunes with their extensive roots can also help prevent soil erosion and desertification [9, 10]. In addition to their economic, cultural, and environmental significance, dates also have nutritional benefits. They are rich in carbohydrates, vitamins, fiber, and minerals. In traditional medicine, dates have been used for medicinal purposes [4, 11].

The unfavorable environmental conditions, climate change, pest and disease infestation, inconsistent pollination, genetic factors, nutrient deficiencies, and droughts affect the natural ripening process of date palm fruits. These unfavorable parameters can result in some fruits remaining unripe while others ripen or disrupt the natural ripening process. Therefore, date palm fruits do not naturally ripen simultaneously, leading to a prolonged ripening process and causing uneven ripening within a bunch [12, 13]. Artificial ripening techniques are employed to hasten fruit ripening using various techniques and chemicals. Artificial ripening techniques for date palm fruits offer benefits such as consistent supply, increased marketability, extended shelf life, control over the ripening process, reduction in post-harvest losses, access to distant markets, and environmental sustainability. This chapter aims to highlight the applications of artificial ripening in ensuring a consistent supply of ripe fruits, improving fruit quality and shelf life, and reducing post-harvest losses. Additionally, it discusses the potential impact of artificial ripening on the nutritional value of date palm fruits. It provides an overview of the global regulations and standards governing the artificial ripening process.

The rest of this chapter is structured as follows: First, we introduce the impact of environmental change on date ripening in Section 2; Section 3 describes artificial ripening and its benefits; Section 4 provides an overview of chemical methods of artificial ripening using ethylene (C₂H₄), calcium carbide (CaC₂), ethanol (C₂H₆O), and ethephon (C₂H₆ClO₃P); Section 5 provides benefits and application of physical methods of artificial ripening using hot air circulation, steam treatment, radiation exposure, and application of solar energy for artificial fruit ripening; Section 6 details effects of artificial ripening on date quality. Section 7 indicates the regulatory and safety considerations; Section 8 concludes the work.

2. Impact of environmental change on dates ripening

Environmental conditions such as temperature, humidity, rainfall, and other factors can significantly influence plant growth and development. Climate change is a significant concern for socioeconomic activities, including agriculture. Over the past 30 years, its effects on plant growth and development have become evident. Global evidence suggests climate change is occurring, with land surface temperatures increased by 0.6 ± 0.2°C over the twentieth century and predicted to rise by 1.4–5.8°C by 2100 [14, 15]. Climate change is altering global seasonal rainfall patterns, affecting crop responses due to rising temperatures, changing water resources, and rising carbon dioxide concentrations. It is being monitored in various physical and biological systems, with plant phenology being a crucial bio-indicator due to its ability to provide significant temporal and spatial information regarding ongoing changes [16]. This negatively impacts plant flowering and fruiting patterns, leading to potential shifts in climate suitability for date palm farming. The changing climate is expected to alter the suitability of certain regions for date palm farming, while others that are currently unsuitable may become suitable in the future [17].

Temperature is crucial in controlling plant reproductive phenology, particularly in tree species. Climate change has significantly impacted global weather patterns, causing increased temperatures and decreased rainfall. This impact is particularly severe in arid and semi-arid dry regions where date palm is commonly grown as a staple food. Date palm requires a long summer with high temperatures, a mild winter without frost, no rain during flowering and fruit set, low humidity, and plenty of sunshine [17, 18]. Temperature, humidity, and precipitation are some environmental factors affecting the ripening process of date palm fruit. Date palm fruit ripening can be affected by climate change directly or indirectly, depending on how these variables change. Climate change impacts date palm fruit ripening through temperature patterns. High temperatures during the growing season can accelerate ripening, causing premature fruit drops and reduced quality, while low temperatures can delay ripening and prolong fruit maturity. These temperature conditions are crucial for optimal growth and fruit development in date palms. Precipitation patterns significantly impact date palm fruit ripening. Despite their arid adaptation, date palms require water for growth and development [13, 19]. Climate change-related changes, like droughts or heavy rainfall, can negatively affect date palm fruit ripening. Drought conditions can cause water stress, affecting fruit size and quality. Excessive rainfall can damage fruits, promote fungal diseases, and reduce insect activity, affecting pollination and fruit set. Humidity levels significantly impact date palm fruit ripening, with high humidity increasing the risk of fungal diseases and pollination, while low humidity can lead to excessive fruit moisture loss, resulting in shriveled and poor-quality dates [20, 21]. Climate change can disrupt the natural ripening process of date palm fruits by altering day length (degree days) through changes in cloud cover or atmospheric conditions, as date palm is sensitive to these changes.

Studies suggest that the number of heating degree days impacts fruit softening. The finest date cultivars require 3300 heat units for full maturity; however, this varies by country, region, and cultivar [18]. In addition, climate change can indirectly affect date palm fruit ripening by altering pest and disease dynamics. Rising temperatures and altered precipitation patterns can create more favorable conditions for pests and diseases, leading to fruit damage and reduced fruit quality. Climate change has caused a delay in the in situ fruit ripening of date palms in Saudi Arabia over the past few years. The outer spikelets of fruits remained unripe, while the inner spikelets ripened. The irregular ripening is believed to be caused by a temperature and humidity imbalance (per. Comm.).

3. Artificial ripening and its benefits

Fruits supply essential nutrients that the body needs to maintain health. They are mostly consumed at mature or ripening stages. Because of their irregular in-situ ripening, climacteric fruits are harvested at a mature stage and artificially ripened afterward. Many date palm cultivars are not edible due to soluble tannins at the full coloration stage (Khalal/Bisr). Date growers face prolonged Rutab development, especially when ripening is uneven, exposing fruits to abscission. The beginning of the Rutab stage is defined as the initiation of ripening in date fruits, which proceed to the Tamr or full ripe stage. Date growers must wait for 50% of the bunch to ripen, and a large amount must be harvested quickly, necessitating the development of an artificial ripening method at the Khalal/Bisr stage. Artificial ripening is the process of hastening fruit ripening with various techniques and chemicals. It is commonly used in the agricultural industry to ensure a steady supply of ripe fruits year-round despite seasonal variations. It is a controlled ripening process to enhance consumer acceptance and sales by achieving desired characteristics [22, 23]. Artificial fruit ripening techniques such as using chemicals (calcium carbide, ethylene, ethanol, ethephon, acetylene gas, and lauryl alcohol) as fruit ripening agents can harm health. Studies indicate that chemicals alter the organic composition of vitamins and micronutrients while only changing the fruit’s skin color, ensuring it remains raw inside [24, 25].

The date palm is a climacteric fruit that undergoes five distinct stages after pollination and fertilization: Hababouk (light green color), Kimri (immature green color), Khalal or Bisir (mature, red, or yellow color), Rutab (ripe, half flesh color brown), and Tamer (fully ripe, flesh completely brown color). Every stage is distinguished by variations in physical and biochemical characteristics [26]. Date palm trees produce fruits that do not ripen simultaneously, necessitating multiple pickings over several weeks. Harvesting ripe fruits at the right time is crucial for growers to maximize investment returns and obtain the highest quantity and quality of dates from each cultivar. The time taken to fruit ripening varies with each cultivar [27].

Moreover, unfavorable environmental conditions can cause some fruits not to ripen and remain red or yellow. These unripe fruits are vulnerable to insect pests and disease infestation, which cause further damage. When harvesting unripe fruits, most farmers either waste them or feed them to their animals. Therefore, artificial ripening methods are becoming simple, cost-effective, and environment friendly, reducing fruit production’s ecological footprint. Unripe fruits of date palms are artificially ripened using ozone, table salt, vinegar, ultraviolet light, microwave, ultrasound, heat, and humidity [13, 28].

These artificial ripening techniques have some benefits, such as they emit less greenhouse gas emissions than traditional methods, thus reducing the environmental impact of the fruit industry. They can significantly reduce water consumption in the fruit industry by allowing fruits and vegetables to ripen without requiring water. Environmentally friendly ripening techniques can be utilized to ripen fruits without the need for plant growth regulators and chemical substances. These methods help to improve food safety and security. In addition, environmentally friendly artificial fruit ripening can improve quality, increase shelf life, and increase availability [29]. Artificial ripening ensures a consistent supply of fruits throughout the year, extending their shelf life and allowing farmers to distribute them to markets even during off-seasons, thus reducing dependence on specific seasons or geographical locations. It can reduce post-harvest losses by ensuring fruits reach their optimal ripeness before market transport, especially for perishable fruits with short shelf life. This helps farmers prevent premature spoilage and increase the chances of selling produce before wastage. Artificial ripening offers greater control over natural ripening processes, resulting in more uniform and consistent fruits, which is crucial for commercial purposes where consumers expect standardized products with predictable characteristics. Artificially ripened fruits enhance marketability by enhancing their attractive appearance and desirable qualities like color, texture, and flavor, as consumers are more likely to purchase visually appealing, uniformly ripe fruits. Artificially ripening fruits allow for earlier harvesting and transportation while maintaining their firmness, reducing damage risks, and allowing for longer shelf life. This allows farmers to reach distant markets and expand their customer base, as ripe fruits are more susceptible to bruising and spoilage. Artificial ripening techniques use controlled environments such as temperature and humidity, enabling better storage and distribution management. This helps farmers extend fruit shelf life and ensure optimal fruit delivery to consumers [23, 30].

4. Artificial ripening methods

Artificial ripening methods for dates can be categorized into chemical and physical methods. Among the chemical methods, commonly used substances include ethylene (C₂H₄) gas, which triggers ripening by stimulating ethylene production within the fruit; calcium carbide (CaC₂), releasing acetylene gas when in contact with moisture but raising safety concerns; ethanol (C₂H₆O) vapor initiates ripening, and ethephon (C₂H₆ClO₃P) that decomposes to release ethylene gas. Physical methods encompass hot air circulation, employing heat to expedite metabolic processes, steam treatment, steam for ripening and microbial control, radiation exposure through ionizing radiation, and solar energy in solar dryers or greenhouses to harness sunlight for faster ripening. Figure 1 shows the chemical and physical methods of artificial ripening.

Figure 1.

Artificial ripening methods.

4.1 Chemical methods of artificial ripening

4.1.1 Artificial fruit ripening using ethylene (C₂H₄)

Climacteric and non-climacteric fruits are the main groups into which fruits are usually categorized. In general, climacteric fruits can ripen after harvest, while non-climacteric ones cannot. An elevated respiration rate and a subsequent burst of ethylene production are characteristics of climacteric fruit ripening. Fruit ripening is a complex, genetically programmed process that involves physiological, biochemical, and organoleptic changes, resulting in a soft, edible fruit with desirable quality attributes, possibly independent of each other. Ethylene, a naturally occurring hormone, is responsible for accelerating the ripening of fruits. It is synthesis from the activity of 1-aminocyclopropane-1-carboxylic acid (ACC) synthase (ACS) and 1-aminocyclopropane-1-carboxylic acid oxidase (ACO), which alter S-adenosyl-l-Met (SAM) into ACC and convert ACC into ethylene, respectively. The transmembrane receptor proteins found in the endoplasmic reticulum of cells sense ethylene [31, 32]. It can either inhibit or promote cell division, sometimes inhibiting cell expansion and sometimes stimulating lateral cell expansion. It triggers physiological, biochemical, and molecular changes in the fruit. It regulates fruit ripening by coordinating gene expression, causing respiration increase, autocatalytic ethylene production, and changes in color, texture, aroma, and flavor. The autocatalytic production of ethylene in climacteric fruits occurs when an initial concentration of ethylene increases, causing the fruit to naturally increase in signal production, accelerating the ripening process [33].

Globally, it is commercially produced through the steam cracking of petroleum hydrocarbons using various feedstocks such as ethane, propane, butanes, naphthas, and gas oils. Chemical treatment of ethylene for artificial fruit ripening involves exposing fruits to ethylene gas or using ethylene-releasing compounds such as ethephon, glyoxime, etacelasil, etc., used in the food industry to maintain a consistent supply of ripe fruits and meet consumer demands. One of the most popular techniques for artificial fruit ripening is ethylene gas treatment. Ethylene gas is introduced into a controlled atmosphere where fruits are stored. The type of fruit and the desired level of ripeness determine the ethylene gas concentration and exposure time. The gas penetrates through the fruit’s skin and triggers a series of biochemical reactions that lead the fruit to ripen [34].

Ethylene-based artificial fruit ripening enhances fruit quality, appearance, and marketability by accelerating the ripening process and improving overall fruit characteristics. Artificial fruit ripening using ethylene ensures consistent ripeness across batches, giving consumers access to ripe fruits year-round. It also prevents variations in fruit taste, texture, and appearance. Exposing fruits to controlled amounts of ethylene gas can synchronize the ripening process, resulting in a more consistent product. This uniformity is crucial for commercial purposes, allowing better planning of harvesting, storage, transportation, and distribution [35]. Ethylene-induced artificial fruit ripening enhances flavor and aroma by promoting the synthesis of volatile compounds responsible for the characteristic flavors and aromas of ripe fruits. This process is particularly beneficial for fruits harvested prematurely or unripe for logistical reasons or to extend their shelf life [8, 36]. Ethylene-based artificial fruit ripening enhances visual appeal by stimulating color changes, resulting in vibrant hue angles and attractive pigmentation, which indicate fruit ripeness for consumers [37]. Artificial fruit ripening can reduce post-harvest losses by minimizing spoilage and extending fruit shelf life. Ethylene treatment accelerates ripening, allowing fruits to reach optimal consumption stages quickly. This is especially beneficial for short shelf life or perishable fruits. By reducing harvest-consumption time, ethylene treatment minimizes spoilage losses and increases fresh fruit availability in the market [35, 38]. Ethylene treatment has gained popularity because of consumer demand for fruits regardless of the time of year. Many consumers have developed tastes for fruits, which may not always be available in their area or at times of the year. These fruits can be harvested before they are completely ripe and artificially ripened with ethylene gas while being transported or stored. It enables vendors to satisfy customer requests and offer a steady supply of requested fruits all year round [39, 40].

It is important to highlight that while ethylene-induced artificial fruit ripening has many advantages, its application has potential drawbacks and concerns. Fruit quality and flavor can be negatively impacted by over-ripening or uneven ripening caused by incorrect application of ethylene or ethylene-releasing compounds. Fruits over-ripened, softened, or deteriorated due to improper application or overexposure to ethylene may have these effects. Food safety is of concern when artificial fruit ripening involves chemicals. Adherence to regulations and guidelines is imperative to ensure that the chemicals utilized are suitable for consumption and do not pose any health risks. To protect consumers and ensure food safety, ethylene use may also be subject to regulatory restrictions in certain countries or regions. Ethylene gas—a volatile organic compound (VOC)—can contribute to air pollution if not appropriately managed. Ethylene gas should be handled and disposed of safely to reduce any negative environmental effects [41, 42].

4.1.2 Artificial fruit ripening using calcium carbide (CaC₂)

Calcium carbide, a grayish-white crystalline solid produced by heating lime and coke in an electric furnace, is used in industries like steelmaking and welding due to its ability to generate acetylene gas (C₂H₂) when heated with water, a highly flammable fuel source. It is a chemical compound that reacts with water to produce acetylene gas and is used in artificial fruit ripening to accelerate the process; however, concerns have been raised about its safety and potential health risks. This controversial practice has been used in some regions globally. It is used for artificial fruit ripening to produce acetylene gas that triggers physiological changes such as fruit flesh softening, color development, and flavor enhancement. This mimics the effects of ethylene, a crucial plant hormone, allowing for the accelerated ripening process by exposing fruits to acetylene gas [43, 44]. Calcium carbide has potential risks, including extremely hazardous chemicals such as arsenic and phosphorus. These toxic chemicals contaminate fruits and pose serious health risks when consumed by humans, particularly arsenic, which can cause cancer and kidney diseases.

Similarly, the acetylene gas produced affects the neurological system by causing prolonged hypoxia [41]. The use of calcium carbide can cause irregular fruit ripening, causing unnatural appearance and imbalanced quality and potentially affecting the taste and nutritional content of fruits due to the accelerated ripening process. Therefore, due to these concerns, many countries have banned or strictly regulated the use of calcium carbide for artificial fruit ripening. Nonetheless, it is still in use in many countries, especially underdeveloped countries, where there is less awareness of the hazard risks. Alternative methods for artificial fruit ripening have been developed instead of relying on calcium carbide [45, 46].

4.1.3 Artificial fruit ripening using ethanol (C₂H₆O)

Ethanol, or ethyl alcohol, is a colorless, volatile liquid used in food and beverage production. Artificial fruit ripening is a practice that uses ethanol to expedite fruit ripening. Ethanol ripening is a promising technology for extending fruit shelf life, reducing food waste, improving flavor and quality, offering a sustainable alternative to traditional methods involving synthetic chemicals, and minimal equipment required. It is primarily used in fruit ripening as it stimulates ethylene production, a natural plant hormone responsible for regulating physiological processes in plants, including fruit ripening, senescence, and abscission [47, 48]. This method is beneficial for fruits harvested prematurely or require ripening. It allows growers to regulate fruit ripening timing to ensure optimal maturity before sale or consumption. It can potentially prevent cell wall degradation, resulting in firmer and more flavorful fruits. It decreases the respiration rate in fruits, potentially extending their shelf life and improving food security. Ethanol is commonly used for artificial fruit ripening in the commercial production of certain fruits. Unripen fruits are exposed to ethanol vapor or ethylene gas in specialized rooms, triggering a cascade of biochemical reactions that result in softening, color change, and flavor development. This process is commonly used when fruits are still green and firm for transportation [49, 50].

Ethanol can be used to ripen fruits in two ways: by exposing them to ethanol vapor in a sealed container, applying ethanol directly to the fruits through a solution, or spraying with an ethanol mist. This process allows the ethanol to evaporate and fill the container, allowing the fruits to ripen more quickly. Additionally, it is a simple and low-cost process, requiring only a sealed container or sprayer [51]. The ripening rate of fruits depends on the type of fruit and desired ethanol concentration. Higher concentrations and longer exposure times lead to faster ripening, but excessive ethanol can damage fruits and inhibit ripening [52]. Although the use of ethanol for artificial fruit ripening accelerates the process, it has disadvantages, such as residual ethanol on the surface of fruit can affect taste or pose health risks if consumed excessively. Therefore, producers must follow appropriate guidelines and regulations regarding ethanol use for fruit ripening [48, 53].

4.1.4 Artificial fruit ripening using ethephon (C₂H₆ClO₃P)

Ethephon, 2-chloroethylphosphonic acid, is the most popular synthetic plant growth regulator commonly used in the agricultural industry to accelerate fruit ripening, fruit coloring, fruit yield, germination, and flower induction by releasing ethylene gas when it comes into contact with moisture. Ethylene gas from ethephon stimulates fruit ripening by triggering physiological changes such as cell wall softening, starch breakdown, sugar production, and color development [54]. The method of applying ethephon for the artificial ripening of fruits is dipping or spraying the fruits in a diluted ethephon solution. Fruits are also artificially ripened in a sealed chamber with an ethephon generator. The type of fruit and its desired ripening stage determine the concentration and application method. Ethephon exposure is crucial for fruit ripening, with higher concentrations and longer exposure times resulting in faster ripening. However, excessive ethephon can damage fruits and inhibit ripening, so it is essential to balance these two factors [55, 56].

The ripening process starts when the fruit tissues absorb the ethephon solution and transform it into ethylene gas [57, 58]. Ethephon ensures uniform ripening of fruits, allowing consistent quality and appearance in commercial production. This is especially important for fruits that ripen unevenly. Fruits treated with ethephon have an extended shelf life due to delaying senescence and slowing decay, resulting in longer storage and transportation periods. Artificial fruit ripening using ethephon also meets market demands by supplying ripe fruits throughout the year, allowing better planning and management of production cycles [59, 60].

Using ethephon to ripen fruit artificially has raised concerns about potential health risks. Regulatory authorities set maximum residue limits for ethephon in various fruits, but proper application practices and adherence to these guidelines are crucial for consumer safety. Moreover, attention should be given to the optimal ethephon concentration and exposure time, which depend on the type of fruit and desired ripening rate. Ensuring uniform ripening is challenging due to uneven distribution of the ethephon solution or gas and uneven stacking of fruits. Safety concerns include the chemical’s potential for skin and eye contact. Despite its safety, it is crucial to store and use ethephon in a well-ventilated area and avoid any body contact. Therefore, carefully considering and optimizing these parameters are essential for successful ethephon use [61, 62].

4.2 Physical methods of artificial ripening

A controlled application of heat and moisture for artificial fruit ripening is a widely used technique in the agricultural industry to ensure a consistent supply of ripe fruits throughout the year despite seasonal variations. In order to simulate the natural ripening process, artificial fruit ripening usually entails exposing the fruits to particular temperature and humidity levels. The main objective is to accelerate fruit ripening by increasing ethylene gas production, a naturally occurring plant hormone. Fruits undergo various physiological changes from ethylene, including softening, color development, and flavor enhancement [30, 63].

Artificial fruit ripening is regulated by food safety authorities in several countries to protect consumers. These regulations frequently outline acceptable ranges for temperature and humidity as well as the usage of ripening compounds that have been approved. Sometimes, humidity and heat are combined with ethylene gas to accelerate ripening. Its use is, nevertheless, closely controlled to avoid overexposure that can pose health hazards [64].

Heating is a crucial step in the artificial ripening of fruit because it speeds up ethylene production. Fruits are frequently exposed to warm temperatures for a specific duration, usually between 20°C and 50°C [13, 65]. The type of fruit and desired level of ripeness determine the precise temperature and duration; for example, a temperature of 50°C is recommended for artificial date palm fruit ripening [13]. Humidity control is also crucial for artificial fruit ripening, as it maintains firmness and prevents moisture loss. Low humidity can cause fruit desiccation and shriveling. The ideal humidity range for artificial fruit ripening varies depending on the fruit type and cultivar but typically ranges between 85–95% [13, 66].

Several techniques, such as hot air circulation, steam treatment, or radiation exposure, can achieve heating.

4.2.1 Hot air circulation

It is a widely used technique for artificial fruit ripening in agricultural and horticultural practices. It involves the controlled application of warm air to accelerate the process of fruits, maintaining consistent temperature and humidity levels. This method is primarily used in commercial settings such as fruit storage facilities, warehouses, and packing houses [67]. The process involves setting the desired temperature and humidity range, which vary with the type of fruit being ripened. Hot air circulation systems, consisting of fans or blowers, distribute warm air evenly throughout the ripening room, ensuring uniform conditions and equal exposure to heat, whereas humidity is provided using a humidifier. Regular monitoring and adjusting temperature and humidity levels is essential during the fruit ripening, allowing operators to adjust the hot air circulation system. Automated systems equipped with sensors and controllers can help streamline this monitoring process. This method ensures uniform ripening, accelerates the ripening process, and extends the shelf life of fruits [13, 68]. Mohammed and Alqahtani [30] designed a sensor-based artificial ripening system (S-BARS) combined with ultrasound pretreatment, an efficient approach for improving the quality of date fruits. Figure 2 shows an image of the experimental setup of the sensor-based artificial ripening system. The system effectively controlled temperature and relative humidity, resulting in improved color and density of the artificially ripened fruits. The ultrasound pretreatment reduced the required time for ripening, decreased the percentage of damaged fruits, and increased the percentage of ripened fruits. The optimal treatment combination for ultrasound pretreatment and ripening parameters resulted in high-quality date fruits with attributes such as fruit weight, density, color, firmness, total soluble solids, pH, and sugars [30].

Figure 2.

Experimental setup of the sensor-based artificial ripening system.

4.2.2 Steam treatment

The Steam treatment method involves exposing fruits to high temperatures and humidity to stimulate the ripening process. Steam treatment is a method used to accelerate fruit ripening by inactivating enzymes that inhibit it and activating those that promote it. It also increases the permeability of the fruit’s cell walls, allowing ethylene gas to enter. A well-insulated steam chamber is used for this process, with the temperature and humidity set to optimal levels for the ripened fruit type. The steam circulates around the fruit, accelerating the ripening process. The ripening time varies depending on the fruit type but is typically shorter than at room temperature [69, 70].

4.2.3 Radiation exposure

Infrared radiation, a type of electromagnetic radiation, is widely used in various applications, including fruit ripening, accelerating the process, and enhancing the quality of fruits by using specific wavelengths. It also increases the permeability of the cell wall of the fruits, making it easier for ethylene gas to enter. It has longer wavelengths than visible light but shorter than radio waves. It is categorized into near-infrared (NIR), mid-infrared (MIR), and far-infrared (FIR) categories with distinct properties and applications. NIR radiation, with wavelengths ranging from 700 to 2500 nm, is primarily used for fruit ripening [71]. It penetrates the outer layers of fruits, absorbing energy and causing physiological changes that promote ripening in the fruit’s tissues. The absorbed energy increases the fruit temperature, accelerating metabolic processes such as respiration and ethylene production. NIR also enhances enzymatic activity in fruits, causing color, flavor, aroma, and texture changes. It also influences the breakdown of complex carbohydrates into simpler sugars, making fruits sweeter [72].

Infrared radiation is a non-destructive and non-contact method for accelerating fruit ripening, unlike traditional methods that cause damage to the surface of fruit or structure. It offers precise control over the ripening process by adjusting the intensity and duration of radiation exposure. It benefits commercial fruit producers who must ensure consistent ripening across large quantities of fruits. It also enhances fruit quality by increasing antioxidant and vitamin levels and improving sensory attributes [73]. An IR ripening chamber must be well-insulated and equipped with Infrared emitters to use Infrared radiation for fruit ripening. The wavelength and intensity of the radiation should be set to the optimal ripening conditions for the fruit type. The Infrared radiation heats the fruit inside the chamber, accelerating the ripening process. The ripening time typically is shorter than the fruits ripened using hot air circulation or steam treatment methods [74]. Infrared radiation offers several advantages for fruit ripening, including faster ripening times. It helps to reduce transportation and storage costs, improve fruit quality by promoting uniform ripening, reduce food waste by increasing fruit shelf life, and increase profits for farmers and retailers. It also has a few disadvantages, including high installation and running costs, complex monitoring and control systems, and potential health risks to the eyes and skin. It is, therefore, essential to take appropriate safety precautions when using this technology, especially for more significant operations [75].

4.2.4 Application of solar energy for artificial fruit ripening

Solar energy is a renewable and sustainable energy source that can be used for heating, cooling, lighting, and powering appliances. It is also utilized for fruit ripening, a crucial process in the agricultural industry that impacts fruits’ quality and shelf life. Solar energy can significantly reduce costs associated with fruit ripening by eliminating the need for fossil fuels. This clean and renewable energy source also minimizes the environmental impact of fruit ripening. Furthermore, solar energy can enhance the sustainability of the fruit industry by decreasing its reliance on nonrenewable fuel sources [76]. Solar energy can be utilized in various ways:

• The fruits can be dried using solar radiation to remove moisture, extending their shelf life and helping preserve them. This technique works especially well for drying heat-sensitive fruits. A solar dryer, a tray or basket with a clear plastic or glass cover over it to hold the fruits, is used for sun drying. The moisture content of fruit evaporates as heat from the sun’s rays enters through the cover [77].

• In order to slow down the ripening process, solar storage involves storing fruits at a constant temperature using sun energy. This technique performs especially well for fruits that are sensitive to temperature changes. A solar-powered refrigeration system, which uses the sun’s energy to cool the fruits, is also used for solar storage [78].

• Using sun energy to hasten fruit ripening is known as solar ripening. This technique works especially well for crops like bell peppers and tomatoes plucked before they fully mature. The process of solar ripening involves exposing the fruits to direct sunshine while they are covered with transparent plastic or glass. The fruits will mature more rapidly because the sun’s rays penetrate the cover and heat them up. Unripe date palm fruits at the Khalal stage of development are harvested and artificially ripened using ambient solar energy [79].

• Solar-powered ventilation uses solar energy to maintain a controlled atmosphere for fruit ripening. Most fruits that require specific temperature and humidity levels benefit most from this strategy. To assist in keeping the fruits’ environment stable, solar-powered ventilation also uses a dehumidifier or fan [80].

• The process of producing ethylene gas with solar energy is known as solar-powered ethylene production, which is used to ripen fruits. Sunlight is converted into ethylene gas using a solar-powered ethylene generator, which can be used to produce ethylene using solar power. Climacteric fruits that need a high ethylene concentration can benefit most from this strategy [81].

5. Effects of artificial ripening on data quality

Date palm fruit development undergoes a phase change due to ripening, which is influenced by primary and secondary metabolism changes. This process forms carotenoids, flavor compounds, antioxidants, and sugars, enhancing the nutritional quality of fruits [82]. Enzymes trigger these responses, causing changes in date palm fruit color, taste, and texture. Chlorophyll degradation leads to new pigments, while the amylase enzyme breaks down starch into sugar, making the fruit sweeter [83]. Due to climatic variability, date fruits in the Rutab stage may take a long time to reach the Tamar stage. Thus, in order to expedite the ripening process, date palm farmers commonly use artificial ripening techniques [57]. Artificial ripening can significantly impact the nutritional value of dates. According to certain studies, artificially ripened dates could have less antioxidants and vitamin C than naturally ripened dates. However, the nutritional value of naturally ripened and artificially ripened dates seems to be similar, according to other studies. Although the effects of artificial ripening using chemicals on the nutritional value of date palm fruit are not entirely known, some research has shown that it may be detrimental.

Artificial ripening by chemicals requires ethylene, calcium carbide, ethanol, ethephon, etc. These chemicals produce ethylene gas, which triggers fruit ripening in controlled environments. The dry weights of pulp and seed, titratable acidity, soluble solids, and respiration rates increased. In contrast, pH, firmness, and astringency decreased when the Shahani date palm cultivar fruits were treated with ethephon [84]. The application of ethephon reduced the fruit transition time from Kimri to Rutab. Also, it enhanced biochemical properties such as ascorbic acid, glucose, fructose, sucrose, total phenolics, total flavonoids, and total antioxidants of date palm cultivars Hillawi and Khadrawi [58]. El-Kafrawy and Abdel-Hamid [85] applied three techniques (sun drying, oven heat, and calcium carbide) for the artificial date palm cultivar Sewy ripening. They reported that although all three ripening methods improved fruit quality and reduced the decayed fruits and weight loss, sun drying artificial ripening was the best compared to others.

In addition to chemical artificial techniques, other non-chemical techniques that improved fruit texture, color, flavor, aroma, shelf life, etc., were also applied, including hot air circulation, steam treatment, and radiation exposure. In a study conducted on unripe date palm fruits of cultivar Dhakki, the application of sodium chloride proved to be more successful, leading to a 75% increase in ripening and enhanced fruit quality [86]. Similarly, when utilizing a 2% brine solution, Khalal fruits of the Dakkai cultivar can be artificially ripened to a level of up to 75% [87]. The artificial ripening of unripe Biser date palm fruits of cultivar Khalas was found to be significantly improved by a combination of temperature (50°C) and humidity (50%). As a result, weight loss and artificial ripening time were decreased while the fruit’s marketable size, color, firmness, total soluble solids, pH, and sugars were improved [13]. Another study revealed that microwave pretreatment (80 W for 50 seconds) and controlled temperature (50°C) treatments significantly improve the nutritional profile and structural characteristics of date fruit cultivar Khupra, with microwave-processed samples being more acceptable than sun-dried ones [28]. The artificially ripened date fruits’ color and density were improved by the ultrasound pretreatment, which also reduced the amount of time and electricity needed for fruit ripening and increased the percentage of ripened fruits without having an adverse effect on the fruit quality characteristics [30].

6. Regulatory and safety considerations

Food safety is a significant concern due to potential adverse impacts on human nutrition and health, including food-borne diseases, toxicants, zoonotic infections, agrochemical use, pesticide exposure, and antimicrobial resistance. Despite growing concerns in industrialized, high-income countries, developing countries experience the most damaging effects of unsafe foods, which bear the greatest burden of food-borne diseases. These diseases disproportionately affect malnourished individuals, children, pregnant women, and the elderly with weak immune systems, leading to a vicious cycle of morbidity and mortality [88]. Poor food safety issues hinder developing nations’ agricultural development and access to export markets regulated by the World Trade Organization, including Sanitary and Phytosanitary Measures and Technical Barriers to Trade Agreements. Food safety significantly impacts developing countries’ economies, although quantifying it in monetary terms is challenging [89].

Date palm growers use artificial ripening techniques in order to meet customer demand since natural ripening is a slow process. Artificial ripening, a method of accelerating the ripening of dates using chemicals and other physical methods, can enhance yields and reduce costs; however, it also raises concerns about food safety and consumer protection. Farmers and vendors generally use artificial fruit ripening agents, but their potential health hazards have led to global debate over the effectiveness of these methods. As such, strict regulations are in place to regulate the artificial ripening of dates to guarantee consumer safety and product quality. The regulatory requirements for the artificial ripening of dates vary from country to country [41]. Consuming acetylene directly can reduce brain oxygen supply and cause hypoxia. Calcium carbide, an alkaline compound, can cause stomach disorders after consuming artificially ripened fruits. Industrial-grade calcium carbide contains impurities like arsenic and phosphorus, leading to health risks like dizziness, frequent thirst, mouth and nose irritation, weakness, skin damage, difficulty swallowing, vomiting, and skin ulcers. Ethylene glycol consumption can cause kidney failure. These potential health risks highlight the need to carefully handle these products [24, 90, 91].

In most countries, the use of calcium carbide for artificial ripening is prohibited. The US Food and Drug Administration (FDA) and the Canadian Food Inspection Agency (CFIA) prohibit the use of calcium carbide for artificial ripening, deeming date palm fruits ripened with calcium carbide unsafe for human consumption and prohibiting their import or sale. In a few other countries, the use of calcium carbide is regulated but allows for artificial ripening. However, it is allowed only to be used by licensed facilities and following their guidelines. The artificially ripened fruits must be labeled for the consumers’ information to make an informed choice [64, 92]. The FDA in the US regulates the artificial ripening of dates under the FD&C Act, ensuring they are safe and not adulterated, misbranded, or violating the Act. The agency has set guidelines for using artificial ripening agents in dates, including the types and maximum levels of use. California, one of the largest date-producing states in the United States, has regulations governing the use of artificial ripening agents in dates. The state prohibits the use of certain chemicals, such as ethylene gas, in the process, in addition to federal regulations [92].

Many countries have banned harmful chemicals like calcium carbide and potassium nitrate in the artificial ripening of dates due to potential health risks, including cancer and reproductive diseases. These chemicals have been linked to health problems, making their use in date production strictly regulated. The concerned food authorities mandate that all dates, whether naturally or artificially ripened, be labeled as “ripened” or “mature”, indicating that the date has been treated with an artificial ripening agent, ensuring consumers understand the product’s nature and potential benefits. Internationally, the guidelines for the artificial ripening of dates have been created by the Food and Agriculture Organization (FAO) and the World Health Organization (WHO) in collaboration with the Codex Alimentarius Commission, a joint food standard-setting authority. Countries may utilize the Codex principles as a foundation to create their own laws pertaining to the use of artificial ripening agents in dates [93].

The Gulf Co-operation Council (GCC) imports fresh fruits from other countries, comprising Bahrain, Kuwait, Oman, Qatar, Saudi Arabia, and the UAE. To export fruits to GCC countries, producers and exporters must comply with regulations set by these countries, ensuring fruit safety and quality. The Gulf Standards Organization (GSO), comprising six GCC countries and Yemen, aims to promote scientific and technical advancement in agricultural and food industries. Despite the development of around 1000 food-related legislations and standards, there are still differences between proposed standards and existing international guidelines. The GSO Food Standard Act mandates that fruits should be provided fresh to consumers upon preparation and packaging. The GSO Food Standards Committee harmonizes GCC standards with guidelines from Codex Alimentarius, ISO, and other international organizations, with most acts following the Codex Alimentarius. Gulf countries adhere to GSO standards collectively, but some countries have imposed additional acts under national acts for food safety [94].

7. Conclusion

Ripening is a process in fruits that enhances their edible qualities, resulting in a sweeter, less green, and softer fruit. The ripening process of many fruits, including date palm, can be very slow, uneven, and unpredictable, leading to the use of chemicals or physical methods to ripen fruits artificially. Ethylene and ethylene-generating chemicals like ethephon and calcium carbide accelerate ripening and improve peel color. However, these chemicals have some disadvantages in post-harvest shelf life and can harm product quality and human health. Ethylene is an explosive gas and expensive, whereas calcium carbide is banned in many countries, posing serious health hazards to humans. However, due to several health-related concerns, the effectiveness of chemically induced artificial ripening has come under criticism. The date palm fruits are also artificially ripened with a few non-hazardous chemicals, such as vinegar and salt, although this affects the flavor of the fruits. In order to ripe the unripe date fruits, alternative fruit ripening methods—which are easy to use, eco-friendly, and reasonably priced—have gained interest. Artificial heat and humidity treatments can be used to ripen date palm fruits when the tree is still not completed or when early rains threaten to damage the harvesting process. Among these physical artificial ripening methods are hot air circulation, steam treatment, and infrared radiation exposure.

Additionally, the oven-drying method can enhance the quality and sensory qualities of unripe date palm fruits. Similarly, in various horticulture-based enterprises, unripe date palm fruits are ripened in controlled conditions with different humidity and temperature ranges depending on the cultivar. Recently, date palm fruits subjected to microwave radiation shortened the ripening time of unripe fruits. It is expected that solar energy will be used to supply the energy needed to implement physical artificial ripening techniques. Because it is a renewable and sustainable energy source used for heating, cooling, lighting, and powering appliances, it will reduce costs, minimize environmental impact, and enhance the sustainability of the date palm fruit industry by decreasing its reliance on nonrenewable fuel sources. Moreover, throughout the artificial ripening process, real-time monitoring of the quality and sensory characteristics of date palm fruit will be performed via the use of artificial intelligence (AI) and Internet of Things (IoT) technologies. Additionally, these technologies will be used to automate a range of operations related to artificial ripening settings.

Acknowledgments

The authors gratefully acknowledge the financial support from Date Palm Research Center of Excellence, King Faisal University, Saudi Arabia.

Conflict of interest

The authors declare no conflict of interest.