Understanding retort processing: A review - PMC

Retort processing is a food preservation technique to address the challenge posed by Clostridium botulinum for commercial sterility of a food product to get microbiologically safe and stable products by heating. This review aims to explore the journey of retort processing, starting from its early use in single&#;batch canned foods and progressing to its contemporary applications with different types of containers and heating mediums. Additionally, it will delve into the adaptability of retort equipment, including its ability to operate in stationary and various agitation states, as well as its flexibility in processing speed for both single&#;batch and continuous operations.

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This review aims to explore the journey of retort processing, starting from its early use in single&#;batch canned foods and progressing to its contemporary applications with different types of containers and heating mediums. Additionally, it will delve into the adaptability of retort equipment, including its ability to operate in stationary and various agitation states, as well as its flexibility in processing speed for both single&#;batch and continuous operations.

Thermal processing is a widely used technique for preserving food and has gained increasing interest in recent years due to the demand for longer&#;lasting high&#;quality food (Singh et al., ). Heat treatment is a commonly used thermal processing technology in the food industry because it is a safe, chemical&#;free, and cost&#;effective method for producing cooked aromas and flavors while extending shelf life. The primary aim of thermal treatment is to destroy hazardous contaminants, including C. botulinum, which can also be prevented in food products by monitoring pH levels (&#;4.6) and water activity (&#;0.85). Sterilization is necessary to ensure the absence of bacteria under living conditions, but for commercial operations, extreme temperature exposure significantly reduces food quality. Instead, foods are treated through a process known as &#;commercial sterility&#; (Awuah, Ramaswamy, & Economides, ). The history of thermal processing dates to the early 19th century, with the development of canning by Appert, and improved by Peter Durand years later with the invention of the tin can. Static retorts, heated with steam as the medium, have been used in industrial purposes for a long time, and new other versions were created such as water, steam, air, water cascades, and water spray systems. Improvements in the thermal retort process include agitation and changes in processing speed (Tucker & Featherstone, ). Numerous studies have been conducted to determine optimal methods for operating retorts to different food products. This review will review methods and technologies relevant to the retort process.

Food contamination by microorganisms is a significant public health concern, where fungi cause deterioration and bacteria foodborne illnesses (Clark et al., ). Spores, such as those from Clostridium thermosaccolyaticum, Bacillus spp., and Clostridium botulinum, can also pose health risks as they can often be highly heat&#;resistant and thrive under anaerobic conditions (Awuah, Ramaswamy, & Economides, ). Foodborne illnesses affect billions of people each year and impose a significant burden on public health globally (Seboka et al., ). To prevent this, it is crucial to implement procedures like sterilization and pasteurization to guarantee food safety.

The sterilization procedure involves a three&#;stage cycle (Figure 2 ). The first stage, CUT (come&#;up time), is the time required for a high&#;flow heating medium to reach the retort temperature of 240&#;250°F (approximately 115&#;121°C) and pressure required 15&#;20 psi (approximately 1&#;1.4 bar) above atmospheric pressure. In the second stage, P t (holding or cooking stage), the retort maintains temperature and pressure to guarantee lethality; this varies depending on the target microorganism or anticipated microbial contamination. In the third phase, CDT (come&#;down time), cooling water is added for a continued temperature decline. Excessive heat processing of food can be avoided by cooling it, which also prevents the development of thermophilic microorganisms. However, the cooling process can cause retort pouches to burst. This can be avoided by applying overpressure air during the cooling procedure, which helps to maintain package integrity and avoid container deformation (Mosna & Vignali, ).

The commercial sterilization method involves using heat to raise the temperature of the containers in a commercial closed vessel known as a retort or autoclave; principles are shown in Figure 1 . Packaged foods commercially sterilized may be stored in hermetically sealed containers at room temperature for up to 2 years (Featherstone, ).

The process of continuous retorting does not require breaks and offers several advantages over batch processing, including cost savings, reduced labor, energy consumption, and equipment downtime. This is particularly beneficial for high&#;volume production lines that do not require frequent adjustments to processing conditions or container sizes. In the conventional method, canned products move on a container conveyor through a retort, where they are exposed to different temperatures based on their position. Hydrostatic retort is another design used for continuous retort processing. This method involves using steam pressure that is controlled by the height of the water leg. These retorts can be of various sizes, and sometimes, the legs can be held outside the retorting facility. The hydrostatic retort typically includes four chambers: a preheat water leg, a sterilizing section, a precool water leg, and a cooling section (Figure 5 ). Hydrostatic retorts can operate at different temperatures and pressures, making them versatile thermal processors (Chen et al., ).

Crateless retorting is a batch retort in which time is not taken to fill or empty crates as compared to regular batch operation, and the process is managed automatically. The vessels are initially half&#;filled with hot water, and the cans are loaded from the top. When the cans are loaded, the water in the vessels acts as a cushion for them. There is a pause between thermal processing cans (Kou et al., ; Teixeira, ). The retort is filled with steam and pushed out through a drainpipe due to the air in the equipment expanding. After that, the device functions similarly to conventional steam&#;air processing. There are two methods for removing cans from a retort of this type. In the first technique, the water level in the cushion canal must be kept just below the bottom door of the retort. For larger can sizes, submerged or vacuum emptying is recommended. When utilizing this method, remember that the cushion canal's water level must be maintained above the retort's bottom door (Berk, b ).

The crates used in batch processing can also impact the process. They hold the various retorted goods and allow for faster filling and removal (Teixeira, ). Separators between the container layers must minimize resistance to flow because heat is transferred via a combination of forced water circulation and container mechanical movement (Featherstone, ). Separators are utilized to create separation between container layers, limit the expansion of semi&#;rigid and flexible containers, and provide support and circulation channels in thin containers (Llosa Sanz, ).

The venting step consumes the most energy in the first few minutes of the processing cycle. Large&#;scale manufacturers that utilize retorts may have issues with energy consumption because retorts must run on a staggered timetable to minimize peak energy demand. To optimize energy usage and processing in batch procedures, researchers have investigated utilizing time&#;variable retorting temperature processing (TVRT). According to studies, nutrient retention at a constant retort temperature is comparable to that of a TVRT process (Simpson et al., ). However, the processing time is significantly reduced with the use of a TVRT process.

They generally come equipped with a pressure gauge, a thermometer, and automatic control systems. Batch retorts are adaptable with containers such as cans, flexible pouches, and other containers with only minor changes to the processing conditions (Figure 4 ). However, batch retorts have some limitations, such as peak energy and labor demand, underutilization of plant capacity, and underutilization of individual retorts (Peesel et al., ).

The conventional method for sterilizing canned food in steam batch retorts over the past 75 years has been through a batch process (Alonso et al., ). The product is placed into the retort and thermally treated with a time gap between processing runs. Batch retorts can be motionless or agitated and can be horizontal or vertical (Figure 3 ).

The classification of retorts can be either batch or continuous (as shown in Flow Chart 1 ). The selection of the type of retort to be used depends on various factors, including the type and size of containers being processed and the quantity of the product. In general, small&#; to medium&#;sized facilities may need help to justify the cost of a continuous retort, while large canning factories with higher production volumes may find it more cost&#;effective.

According to a study conducted by McNaughton in , the use of oscillating motion during thermal processing can significantly reduce the average calculated process time by 10%&#;27% compared to static mode with the same thermal process parameters. The study utilized a two&#;basket water spray retort and calculated process times using Ball's formula method. The oscillating method utilized a speed of 10.5 RPM with an oscillating angle of 15°. The study also observed that depending on the amount of residual air in the pouch, there was a significant difference in the average slope value of the heating curves within the static motion, within the oscillating motion, and between the static and oscillating motions ( Flow Chart 2 ) (Holland, ; Singh & Ramaswamy, ).

The Shaka process is used only for liquid products with fast agitation rates greater than 1 Hz, whereas the Gentle Motion method is used for liquid&#;particulate meals with slower agitation rates. Many recent studies on this technique have been conducted, including investigations into reciprocation intensity, amplitude, frequency, container placement, headspace, and particle size. According to one study, the most significant effect on thermal transfer was reciprocation speed, followed by amplitude and frequency. Another study discovered that thermal processing canned shrimp with reciprocal agitation resulted in superior product quality, improved process parameters, and potential energy savings. At all agitation speeds, reciprocal agitation technology resulted in a shorter process time than static retorts for achieving the target F0 value and improved product quality. This is due to the combination of gravity and horizontal acceleration in the processing (Walden & Emanuel, ).

Reciprocation agitation, which involves rapid back&#;and&#;forth motion of containers, is a promising type of agitation. Walden invented the first reciprocating steam cooker in , but Gerber invented it in . Reciprocal agitation has grown in popularity since the invention of the Shaka and Gentle Motion retorts. The optimal shaking rate for a reciprocal agitation sterilization system is determined by factors such as the food product, container size and shape, and the desired level of microbial inactivation.

Intermittent agitation involves intermittently stopping the agitation during continuous processing. This technique is used commercially in the FMC Sterilmatic, which processes cans using intermittent axial rotation (Tattiyakul et al., ). This method is appropriate for products that may be harmed by the shearing force inherent in constant agitation. When starch granules are heated, for example, they expand and form a thickened dispersion. The dissolution of granule components in the matrix could be caused by the heating and shearing of starch granules in the matrix (Tattiyakul et al., ). This results in hydrogen bond breakdown, depolymerization, and decreased viscosity.

Another method for agitation is biaxial agitation, which is commonly used in continuous retorts. In this method, metal cans change their direction of rotation twice during the revolution of the cage, which cancels out the centrifugal issues found in EOE rotation and improves heat transfer (Dwivedi & Ramaswamy, ; Singh et al., ). The use of horizontal axial rotation for agitation dates back to the s, when cans were rolled to produce agitation. In the s, vertical rotation was suggested as a means to improve heat transfer in canned products.

End&#;over&#;end (EOE) or axial mode pack rotation is a popular technique that has gained popularity in recent years, where packages are rotated in limited&#;rotation baskets or crates. This batch method allows for greater manufacturing adaptability and is not restricted to cylindrical metal cans (Zhu et al., ). Commercial rotary retorts such as Sterilmatic, Steristar, and Rotomat operate on EOE rotation. However, this technique has several drawbacks. Researchers have discovered that particles accumulate at the edge of the container after a specific speed, which interferes with the headspace bubble movement within the can due to the equal centrifugal and gravitational forces within the can. The EOE rotation technique involves rotating cans in a circular motion, which requires headspace in the container (Singh & Ramaswamy, ).

An alternative to overcome this issue is through agitation. Agitation can help decrease heat damage and speed up heat penetration, leading to improved heat transfer and fewer cold spots, which ultimately results in more efficient thermal processing. Different agitation techniques (as shown in Flow Chart 2 ) include end&#;over&#;end, fixed axial, biaxial, and reciprocal agitation. When the container is agitated, air bubbles move around it, leading to a more uniform distribution of heat.

The first type of retort used for canning, known as static or still retorts, is commonly used for liquid food products and does not produce any agitation in the containers. However, this approach has some limitations, such as slow heat penetration creating different temperature zones within the package, which can lead to overcooking, uneven texture, and flavor.

In , Surdry of Spain patented the atomizing steam and water method, which is a relatively new batch&#;retorting technique. This method utilizes atomized air to provide excellent heat transfer to rigid containers, and a fan is not used to circulate the air. Instead, water is drawn from a pump and combined with condensate from the retort's center and recirculated condensate before being supplied directly into the chamber via atomizing nozzles located around it. While the atomizing nozzles allow for rapid heating, they tend to obstruct water flow during cooling, resulting in longer processing times compared to cascading water, immersion, or water spray retorts (Holland, ).

The type of airflow is determined by the retort's design. Positive&#;flow retorts create upward flow and are intended for vertical retorts. Horizontal flows are used in Lagarde retorts, which are intended for horizontal retorts. A study comparing these two styles found that the overall mean heating rate index for positive flow was only slightly higher than that of the Lagarde retort (Ramaswamy & Tung, ).

Early studies of steam&#;air processing media revealed the possibility of producing a non&#;homogeneous mixture of steam (Ramaswamy et al., ). Europe and Japan used the method for commercialization for a long time before North America adopted it. Later research revealed that the heat transfer pattern would be adequate as long as there was enough mixing (Ramaswamy et al., ).

The use of steam and air is another popular medium for thermal processing. Lagarde Autoclaves patented this process in , and it is highly efficient. The process differs significantly from the steam retort, with a horizontal vessel that has quick&#;opening doors for easy loading and unloading of baskets, forced steam circulation, and, most crucially, independent control of temperature and pressure (Holland, ). Steam and air are continuously supplied to the retort vessel to create a homogeneous mixture. When water and steam are combined, the retort is pressurized, resulting in an overpressure situation that causes continual venting. This continuous flow of heated steam past the containers prevents the formation of cold spots (Adepoju et al., ). The technique was initially designed for flexible and semi&#;rigid containers, such as military rations in aluminum foil packs, but it has since been adopted for pouches and ready&#;to&#;eat food (Holland, ).

Controlling the float of packs can be a challenge, and pouches and trays have often impeded this process, increasing basket manufacturing expenses and reducing adaptability. Half&#;immersion occurs when the vessel is half&#;filled with water, and part of the rotation occurs in and out of the water. This method is beneficial for high rotational speeds because the cage creates less turbulence. Manufacturers such as Stock Inc., FMC, Lagarde, and Lubeck produce this system (Holland, ).

Water is recirculated during the heating process to ensure uniform heat distribution throughout the retort. Researchers have found that the position of products in a tray and the tray's height can impact heat transfer coefficients (Ramaswamy et al., ). Poor circulation can result in insufficient heat transfer.

Water immersion is a commonly used retort process (Adepoju et al., ), where water is first heated and then pumped into the retort for processing. The container is typically fully submerged in water during processing, with overpressure created by blowing air or steam for improved heat transfer patterns. However, in certain situations, such as with half&#;immersion, packages are only partially submerged in water (less than half). This can be advantageous for high rotational speeds, as the cage creates less turbulence (Featherstone, ; Holland, ).

The cascading water technique is a type of indirect steam heating where water is sprayed under pressure onto the top trays of retort carts. This method evenly distributes heat across a large volume of water, allowing heat to pass through the sidewalls of the container as water passes through the containers. Studies have shown that concerns about insufficient temperature dispersion are unfounded, as the variation in processing lethality throughout the retort is minor, and there are significant temperature variations during the cooking period. Furthermore, atomizing nozzles positioned around the chamber can be used to enhance the retorting process by providing excellent heat transmission and quick heating without requiring a fan to circulate air.

A saturated steam retort is a simple type of autoclave that is usually vertical and uses steam as a heating medium. It is crucial to remove air during the ventilation stage using venting, an injected steam method, to prevent cold zones from forming. The use of saturated steam in retort processing has been found to be cost&#;effective and energy&#;efficient compared to other heating methods. However, this type of retort poses several challenges to processors, such as pressure fluctuations, which make it challenging to process pouches, semi&#;rigid containers, and trays without package distortion or cold spot formation.

The type of heating used can affect the rate of heat transfer and processing time. For example, Zhu et al. ( ) found that heating medium had no significant effect on processing time but did impact on heat transfer rate. Additionally, Ramaswamy and Grabowski ( ) reported that the type of heating medium used significantly affected the heating rate index in samples of Pacific salmon. These findings suggest that selecting the type of heating medium is crucial for improving processing efficiency and reducing production costs while maintaining high&#;quality canned food.

The three primary retort processes are steam, falling water, and full water immersion. Each of these categories has subcategories, such as steam&#;air, steam spray, water spray, and half&#;immersion (Holland, ). In each case, a form of water is used, sometimes in combination with air, to transfer thermal energy to the product (as shown in Table 1 ). Pressure is applied in all retort methods to increase the boiling point of water and allow for higher temperatures. Some methods, such as those using air, also create an overpressure to prevent container deformation (Mosna & Vignali, ).

Designing the sterilization process requires information on the time&#;temperature distribution profile within the mass of the food product, as well as the kinetics of thermal inactivation, such as thermal death and thermal destruction. However, the difficulty of heat transfer phenomena during retorting is due to the fact that foods can be a mixture of solid and agitated liquids (Figure 8 ). Additionally, certain types of foods, such as starch or protein&#;based foods, can significantly change their rheological characteristics during processing (Zhu et al., ).

Thermal non&#;uniformity is a concern that can result in cold spots (Figure 7 ), which lower the microbiological safety of the product. To address this, heat distribution and heat penetration are two important variables that need to be considered during thermal processing. Heat distribution refers to the delivery of heat by retort equipment to the product area. In contrast, heat penetration refers to the alteration in the supply of heat from the product area to its coldest point (areas within the product receive less heat relative to its adjacent regions).

The IFTPS ( ) states using thermocouples, wireless data loggers, and other comparable devices to monitor temperatures during thermal processing. All instruments used must be of high precision and size, as well as be available in sufficient numbers, to ensure proper and safe observation of the process environment. Before conducting experiments, all TMDs (temperature&#;measuring devices) should be carefully calibrated and tested under the same retort conditions desired for the process (IFTPS, ; Llosa Sanz, ).

Thermal processing is typically carried out with the help of an onboard control system or a computer. The LOG&#;TEC Process Management System was the first commercial system introduced, and it is still in use today. The HP&#;85 desktop computer with an HP&#; datalogger was the first computer used for this purpose (Gill et al., ). Regulatory agencies such as the FDA/USDA in the United States and the FSA in the United Kingdom recommend additional data collection to improve thermal processing control and protect customers. A host computer with customized product recipes is used to store data and meet the regulators' requirements for data sent electronically in a specific format and by a PC. This document can then be filed, either directly from the computer or via a remote PC, in an easily readable form that requires no extra software (Mosna & Vignali, ).

Modern data loggers are equipped with sensors to collect data and can be either connected or wireless. They are multi&#;channel systems with digital responses, allowing readings to be sent directly to a laptop for collection and storage (Awuah, Khurana, et al., ; Berrie, ).

Temperature monitoring systems use various sensors throughout the retort to collect data for temperature distribution and heat penetration studies. Thermocouples (TMD) are the most widely used instruments made from two dissimilar metals connected at two junctions. The T&#;type (copper constantan) and K&#;type (chromel constantan) are the most popular types (Forney & Fralick, ). They are widely accepted due to their low cost, precision within the desired temperature range, responsiveness, and ability to be assembled on different types of containers, such as jars and pouches, as shown in Figure 6 (Berrie, ).

6. PROCESS CALCULATIONS TO ENSURE FOOD SAFETY

According to Powers et al. (), public health concerns are caused by &#;improper application of the technique&#; rather than the procedure itself. Lúquez et al. () reported that from to , 34 instances of foodborne botulism associated with commercially processed foods were reported by the Centers for Disease Control and Prevention (CDC), and only four of these occurrences resulted from poor canning techniques. However, in , ten cases of botulism were linked to commercially marketed canned hot dog chili, indicating inadequate safety measures at manufacturing plants that allow botulism spores to remain under harmful conditions. Home&#;canned foods are more commonly associated with botulism than commercially canned goods, which are usually prepared following proper procedures (Juliao et al., ). Despite the implementation of retort technology, there have been significant safety concerns due to bacterial and spore contamination (Gill et al., ).

Thermal process calculations are crucial to ensuring the proper process and safety of food products. Bigelow's general approach, which he developed in the early twentieth century, and Ball's semi&#;analytical approach are commonly used to calculate the lethality value of thermal processes. The effectiveness of thermal processing depends on several factors, such as processing temperature, environmental conditions, microbial properties, and product characteristics.

The F0 value, introduced by Ball (), is essential in thermal processing, as it describes the log reduction of bacteria during the retort process at a predefined location. The heat transfer coefficient (h) and overall heat transfer coefficient (U) are also critical parameters to consider during thermal processing. The thermal death rate of bacteria follows a first&#;order semilogarithmic rate, which means that a product cannot be entirely sterilized but can be commercially sterile. The mathematical values used to express thermal resistance during processing, such as the decimal reduction era (D&#;value) and the (Z&#;value), are uniform across different types of thermal processing.

Dr. Robert Bigelow created the first method to calculate the basis of minimal safe sterilization, known as Bigelow's general approach (Bigelow & Esty, ), which has been widely used in practice since then (Awuah, Ramaswamy, & Economides, ; Simpson et al., ). This technique relies on real&#;time monitoring of the coldest point in thermal processes using computer programs to obtain the lethality value of a process. Although this method was found to be extremely time&#;consuming, difficult, and unworkable due to the lack of programmable calculators or personal computers at the time (Simpson et al., ), it is still useful today.

In response to the need for a more efficient method, in , scientist Ball (General method) unveiled a semi&#;analytical approach for thermal process calculation to the scientific community. This alternative technique employs theoretical models that predict thermal survival and heat penetration equations based on physical theories (Simpson et al., ).

The determination of the heat transfer coefficient (h) and overall heat transfer coefficient (U) are crucial parameters in heat transfer analysis. Empirical equations that use dimensionless quantities are often used to calculate these values (Nelluri et al., ). Broken&#;line heat penetration curves, which resemble straight&#;line heating curves, are characterized by a heating lag factor (jh), two heating rate factors (fh1 and fh2 for the first and second linear segments), and a breakpoint time (xbh) (Bigelow & Esty, ; Zhu et al., ). These parameters can be estimated through a graphical analysis of a heat penetration curve or a computer program developed by (Denys et al., ).

6.1. Bacterial inactivation

One of the primary aims of thermal processing is to inactivate bacteria. To produce safer and more shelf&#;stable products. Retorting has been demonstrated to be an effective and cost&#;efficient technique for producing food (Verheyen et al., ). However, the excessive microbiological safety margins of the thermal processes employed often lead to nutrient loss and sensory dulling due to higher temperatures and times. The rate of thermal death of bacteria is a time/temperature process with first&#;order semilogarithmic rates. This implies that a sterile product cannot be manufactured, but commercially sterile products can. The mathematical value definitions are the same for each type of thermal processing. Decimal reduction time (D&#;value) and the Z&#;value express thermal resistance in processing. The D&#;value is the duration of heat treatment at a specific temperature necessary to kill 90% of the microbial population. The Z&#;value is the temperature change needed to shift the D&#;value by 1 log unit (Ates et al., ; Zhu et al., ). The efficacy of thermal treatment is determined by the processing temperature, pressure, target microorganisms, and product characteristics (Tavman et al., ).

The Fo value, established by Ball (), is one of the most crucial values in thermal processing. It describes the log reduction of bacteria that would occur during the retort process at a specific spot (Awuah, Ramaswamy, & Economides, ). Stumbo established the D121.1 value of 0.21&#;min at 121.1°C (250°F) as equivalent to 2.52&#;min, establishing a Fo value of 2.52, based on the acceptable probability of survival is not more than 1 in containers. Since , a minimum &#;botulinum cook&#; Fo value of 3&#;min has been determined and is still used today for low&#;acid canned items (Bean et al., ).

Testing and validation of new thermal processing technologies require time and replication. As a result, many researchers have utilized food substitutes rather than real food samples to evaluate their procedures (Verheyen et al., ). However, the current outbreak demonstrates that this method must be analyzed. In other words, there is a lack of experimentally confirmed microbial inactivation with actual food matrixes via reciprocal agitation. Only Ates et al. () have studied this agitation method using a fish soup model to analyze Listeria innocua inactivation, and the effectiveness of reciprocity agitation in microbial inactivation compared to a static retort process using sausage and chicken soup samples treated with L. innocua and then thermally processed. The shaking process provided equivalent lethality in a significantly shorter time than the static retorting technique. Other researchers investigated the thermal inactivation rates of Listeria monocytogenes under shaka agitation. This study also looked at the impact of viscosity and fat content (Verheyen et al., ). It was found that at the 20% fat emulsion, the fat created a protective environment for bacterial cells. This only led to local changes in heat transfer but did not affect the final L. monocytogenes reductions.

6.2. Spore inactivation

The presence of spores in a food product can pose a significant risk to consumers; therefore, their elimination is crucial. Spores are known to be resistant to heat, radiation, and various chemicals, making their elimination challenging (Ates et al., ). However, thermal processing can weaken spores, and some bacterial spores can be eliminated using high&#;pressure conditions (>&#;MPa). Unfortunately, such high&#;pressure conditions are not feasible in the food sector due to practical limitations (Ling et al., ).

Research has shown that spore inactivation can vary based on the retort processing technique employed. Different processing techniques, such as reciprocation, static, and high&#;pressure&#;temperature treatments, have been investigated for their effectiveness in spore inactivation (Ates et al., ). The results indicate that spores of B. subtilis can be inactivated at significantly lower temperatures with reciprocation than with static processing. For instance, the agitation method was able to achieve a 7&#;log reduction of spores after 17&#;min, whereas the static processing technique required 53&#;min to achieve the same result.

6.3. Critical factors

In retort processing, various parameters must be examined and controlled to ensure the safety and quality of the final product. These parameters include process variables such as the initial temperature of the product, formulation, variance in ingredient weights, packing density (which should be 5%&#;10% more than the nominal weight of the product), drained weight (maximum expected under production circumstances), and viscosity in semi&#;liquid or liquid products. Container parameters such as dimensions, headspace (when using rotary retorts), vacuum, number of residual gases, and maximum thickness are also important to examine and control (IFTPS, ; Llosa Sanz, ). Other variables such as the rotary axis and time from filling to processing are also necessary to monitor and control (MacNaughton et al., ; May & Chappell, ; Meng & Ramaswamy, ).

Furan formation in products can be reduced by using variable retort temperature profiles (VRTPs) and retortable pouches. VRTPs enable more precise thermal processing condition control, reducing processing time by 20%&#;30% and improving surface quality by 5%&#;15%. Thermally processed foods' quality and safety can be improved by minimizing surface overcooking and using retortable pouches that allow for rapid heat penetration. Furan formation can be reduced further by reducing overprocessing, adjusting process parameters, and employing kinetic models. The VRTP is an effective tool for reducing processing time, increasing quality retention, and reducing furan formation in foods (Fardella et al., ).

The headspace fingerprint was investigated by Grauwet and Shpigelman () as a potential multivariate intrinsic indicator to monitor temperature variation during thermal in&#;pack processes. The authors used broccoli puree as a case study to demonstrate the potential of this approach to provide real&#;time monitoring of temperature variation in packaged foods.