2.1 Maillard reaction

The Maillard reaction, occurring between reducing sugars and amino acids during food processing, is crucial for flavor, browning, and aroma development. However, it can produce toxic by-products like acrylamide, a known carcinogen [7].

The primary pathway for acrylamide formation is the asparagine route. Asparagine and reducing sugars form Schiff bases, which decarboxylate to produce intermediates. The carboxylate ion in the Schiff base facilitates decarboxylation, leading to the removal of hydrogen from the hydroxy group and forming an alpha-hydroxy anion. This anion then removes the acidic alpha-hydrogen, resulting in acrylamide, as shown in Figure 1. The critical step is the decarboxylation of the Schiff base, leading to Maillard intermediates that release acrylamide [18].

Two main hypotheses explain this pathway. Mottram et al. [19] identified Strecker aldehyde as a key precursor, facilitated by alpha-dicarbonyls. Asparagine, abundant in potatoes and cereals, significantly influences acrylamide production, as shown in Figure 2. Stadler et al. [20] suggested that asparagine with alpha-dicarbonyls produces lower acrylamide levels compared to sugar/asparagine mixtures, but reactions with acetol yield the highest levels.

Stadler et al. [20] also proposed that early-stage Maillard reaction glycoconjugates, including N-glycosides, are pivotal in acrylamide formation. This hypothesis is supported by Yaylayan et al. [21] and Zyzak et al. [22]. Thus, while the asparagine route is primary, it involves various intermediates.

Alternative pathways exist. Figures 3 and 4 illustrate three:

Figure 1 The reaction mechanism of the Maillard reaction between asparagine and reducing sugar, showing three intermediates during the reaction [27].

Figure 2 Possible pathway for acrylamide formation after Strecker degradation of the amino acids asparagine. In asparagine, the side chain Z is -$C{H}_{2}C{H}_{2}SN{H}_3$, while in methionine, Z means -$C{H}_{2}C{H}_{2}SN{H}_3$ [19].

Figure 3 Synthetic pathway to the Amadori compounds of asparagine [20].

2.2 Fat oxidation

Besides the Maillard reaction, fat oxidation is another pathway for acrylamide formation, usually occurring in fatty acid-rich foods as shown in Figure 4. Fatty acids first change to acrolein through oxidative degradation, and then secrete acrylic acid to achieve acrylamide formation [20, 26]. Acrolein is a common aldehyde detected as a cytotoxic component [28]. In food, it is always present in oil and fat. There is also a new pathway showing that acrolein can react with asparagine to generate high levels of acrylamide under suitable conditions, indicating a critical role of acrolein in lipid-rich foods for acrylamide formation [29, 30]. Fat, together with nitrogen compounds in food, produces acrylamide after acrolein transfers to acrylic acid. Then, there is a reaction between acrylic acid and ammonia, which can be achieved by the pyrolysis of nitrogen-containing compounds.

Figure 4 Alternative formation routes of acrylamide via the Maillard reaction system [27].

3.1 pH

In industrial processes, pH plays a crucial role in many processing steps and in inhibiting the formation of certain compounds. For acrylamide (AM) formation, it is also a pH-dependent compound. Different types of food have different formation mechanisms, resulting in varying pH ranges to inhibit acrylamide.

Charoenprasert et al. [31] illustrated that pH value can influence the acrylamide content in California-style black ripe olives (CBROs). Commercial olives contain high levels of acrylamide, so [31] tried to find a method to reduce these levels while preserving the olives' sensory texture. As mentioned in the formation section, acrylamide easily forms in potatoes and grains, which contain high levels of sugar. These food materials have high levels of acrylamide precursors. However, olives do not have these components, being a fatty acid-rich fruit. The exact formation mechanism in olives is still unknown, but studies suggest it is correlated with the processing step—sterilization [32]. Charoenprasert et al. [31] demonstrated that glucosamine levels before sterilization and acrylamide concentration after processing are significantly positively correlated. In their research, [31] added sodium hydroxide ($NaOH$, pH over 12, 0.25 N) to facilitate the base-catalyzed degradation of glucosamine. According to Table 1, the higher the pH value of the solution treating the olives, the lower the acrylamide content in the CBROs, especially at pH values above 12, with results of 3.91b ± 0.40 and 2.17c ± 0.20 at pH 12 and pH 13, respectively. Additionally, the longer the olives were soaked in sodium hydroxide, the lower the acrylamide content, provided the soaking time was within 5 days. The details are shown in Figure 5.

Figure 5 Effect of $NaOH$ treatment time on concentration acrylamide in California-style black ripe olives.

In the biscuit industry, pH also plays a significant role. Suman et al. [33] conducted research on factors influencing acrylamide content in wholegrain biscuits and cocoa biscuits. Biscuits are a common snack made by heating dough, which contains high levels of reducing sugars such as glucose and sucrose. Based on the mechanism, it is easy to form acrylamide in biscuits due to their carbohydrate-rich materials and high levels of precursors. In this case, pH value, heating time, and temperature are relevant factors affecting acrylamide levels. When the pH increases, the reaction between reducing sugars and asparagine is enhanced. Therefore, Suman et al. [33] emphasized the importance of adding acidification agents to biscuit dough. In the food industry, citrate and tartaric acid are commonly used as acidification agents, while biscuits need to be fluffy. This is why the authors added raising agents like sodium bicarbonate or ammonium hydrogen carbonate. Both raising agents and acidification agents help decrease acrylamide levels. Commercially, biscuits should taste good. Ensuring an acceptable range from a sensory perspective, there is a drop in acrylamide levels by more than 50% as the pH value and sodium bicarbonate concentration change. Suman et al. [33] illustrated that $NaHC{O}_3$ is more effective in limiting acrylamide formation compared to other raising agents. Graf et al. [34] determined that acrylamide levels reduce by almost 70% when sodium bicarbonate is applied.

Yuan et al. [35] showed the effects of pH value on acrylamide formation in potato chips. Although this two thermal methods (conventional heating and microwave) and the type of sugar (fructose and glucose) reacting with asparagine, their research also demonstrated higher levels of acrylamide when the pH value is high, treated by these two methods. The graph shows no acrylamide content detected when pH is equal to 3. This may relate to the form of asparagine and reducing sugars, where they present in a protonated form [36]. During the pH range from 4 to 8, acrylamide content increases in the chips. The formation rate reaches its highest when the pH value climbs to 8 in both fructose and glucose systems. This matches previous studies by Brown et al. [37] and Rydberg et al. [38], which report the maximum formation rate at pH 7 to 8. Surprisingly, acrylamide content falls after the pH rises to 10. This trend is similar to acrylamide in biscuits, as both contain high concentrations of reducing sugars.

3.2 Time for Heating

The time of thermal treatment is critical in industrial processes, including acrylamide formation. Heating time influences acrylamide levels until a maximum point is reached [18]. The type of food also affects the required heating time.

Suman et al. [33] found that baking time significantly affects acrylamide levels in biscuits, with longer times promoting its formation, though pH is more critical than time and temperature. Deoxynivalenol (DON), a common contaminant in wholegrain bakery products, is also influenced by baking time. The greatest effect was observed when baking biscuits at 200℃ for 8 minutes. Increasing baking time effectively reduces DON contamination [39]. Reducing baking time to 5 minutes resulted in a 77-100% reduction in acrylamide levels while maintaining good sensory qualities. Thus, shorter baking times result in lower acrylamide levels in biscuits. To control both DON and acrylamide, the time-temperature parameters of baking are crucial. Baking at 180℃ for 5 minutes is optimal for cocoa and wholegrain biscuits, achieving acrylamide contents of 43±13 $\mu$g/kg d.m. and 16±3 $\mu$g/kg d.m., respectively, while minimizing DON content.

For potato chips, Tareke et al. [40] and Becalski et al. [41] demonstrated that longer frying times increase acrylamide levels. Yuan et al. [35] found that acrylamide content generally increased with longer treatment times, regardless of pH and heating methods. Acrylamide levels rose significantly after 20 minutes at pH 8.0, reaching 3.0 $\mu$g/ml at 30 minutes [42]. Esposito et al. [43] recommand that control the frying time in 0-4mins to minimize acrylamide formation as shown in Figure 6.

Figure 6 Change in acrylamide content of potato chip during frying at 170 degrees.

In roasted coffee, acrylamide levels are high due to precursors in coffee beans. In Figure 7, Santos et al. [44] reported that roasting at 200℃ for less than 20 minutes is optimal. Acrylamide levels peak early in the roasting process and then decrease due to degradation or depletion of precursors. Light coffee, especially Robusta, has the highest acrylamide levels ( 500 $\mu$g/kg), while Arabica has the lowest ( 150 $\mu$g/kg) [81, 4] .

Figure 7 Distribution of occurrence of acrylamide in Arabica and Robusta coffee.

Time also affects acrylamide levels in starch-based and cereal systems, such as bread and flatbread. Acrylamide levels peak at approximately 200℃, depending on the system and baking time [48]. Longer baking times reduce acrylamide levels, similar to dark coffee. For example, flatbread baked at 260℃ for 17.5 minutes had the lowest acrylamide levels under 20 mg/kg. Different conditions for bread crusts achieved similar acrylamide content, such as 195℃ for 19.4 or 40.6 minutes, and 230℃ for 15 minutes.

3.3 Temperature applied

Temperature is crucial in processing, significantly influencing acrylamide levels in food. Higher temperatures in starch-based systems result in higher acrylamide content [48]. Heating temperature combined with time is critical.

In the biscuit industry, Suman et al. [33] found that baking at 200℃ increased acrylamide in wholegrain and cocoa biscuits. Reducing the temperature to 180℃ dropped acrylamide levels to below 125 ± 14 µg/kg d.m. and 156 ± 15 $\mu$g/kg d.m., compared to 306 ± 16 $\mu$g/kg d.m. and 400 ± 27 $\mu$g/kg d.m. at 200℃. The optimal temperature for baking biscuits is 180℃ with a 5-minute baking time, achieving a 77-100% reduction while maintaining sensory quality.

For coffee roasting, Santos et al. [44] reported that roasting is usually done at around 200℃. Optimal final temperatures for different roasting levels and coffee types vary between 215-238℃ [43]. Acrylamide forms during frying, roasting, and baking above 120℃ but degrades at temperatures as low as 180℃. The effect of temperature on acrylamide levels is closely related to heating time [46, 45].

For potato chips, high temperatures lead to acrylamide formation, with reducing sugars as limiting precursors [50, 51] . Gokmen et al. [52] found acrylamide levels rise substantially with frying temperature, reaching almost 6000 ppb at 190℃ compared to 1000 ppb at 150℃. Knol et al. [47] showed that glucose converts to fructose above 160℃, and the reaction between fructose and asparagine is preferred over glucose above 140℃. High temperature and carbohydrate conversion significantly influence acrylamide formation. During frying, oil temperature above 170℃ allows temperature increase and water evaporation, favoring acrylamide formation [52].

Temperature also affects acrylamide formation in starch-based and cereal systems. Tareke et al. [40] showed acrylamide levels increase with baking temperature. Bråthen et al. [49] indicated that baked starch gel changes color from white to dark as temperature increases. Acrylamide content in freeze-dried flatbread peaks at 180℃ with a 5-minute baking time, reaching almost 3600 mg/kg. Ordinary flatbread baked at 200-220℃ achieves the highest acrylamide content. At temperatures above 260℃, acrylamide concentration peaks at nearly 500 mg/kg in bread crusts. Acrylamide reduction in starch model systems is controlled by time-temperature combinations.

4.1 Different Cooking Methods

Different cooking methods are crucial in mitigating acrylamide in food, as they can produce food products that are sensory acceptable to consumers. This section introduces conventional heating methods, microwave cooking, and air-frying. Conventional heating methods, such as boiling, frying, steaming, and baking, have been used for many years. However, they do not significantly reduce acrylamide levels in food. For instance, the acrylamide content in roasted coffee powder ranges from 135 $\mu$g/kg to 1139 $\mu$g/kg, while French fries contain 36 $\mu$g/kg to 1441 $\mu$g/kg, and potato chips range from 211 $\mu$g/kg to 3515 $\mu$g/kg [53]. Boiling provides the lowest acrylamide levels in food. At a pH of 8.0, the optimal condition for acrylamide formation, the acrylamide content in potato chips ranges from 0.5 $\mu$g/ml to 0.9 $\mu$g/ml, which is lower than when using microwaves.

4.2 Enzyme

Asparaginase is an enzyme present in living organisms, plants, and animals [62]. It hydrolyzes asparagine to aspartic acid and ammonia [63], reducing acrylamide formation by decreasing asparagine levels. The enzyme is derived from sources like E. coli, A. oryzae, and A. niger. Researchers have used asparaginase to reduce acrylamide in French fries, bread, potato chips, and biscuits. Zyzak et al. [22] used E. coli asparaginase, achieving a 99% reduction in acrylamide in potatoes. Ciesarová et al. [64] treated potatoes at 180°C for 20 minutes, resulting in a 50-90% reduction. Mahajan et al. [65] used Bacillus licheniformis, achieving an 80% reduction. For French fries, Pedreschi et al. [66] used A. oryzae, achieving a 67% reduction at 175°C for 3 minutes. Zuo et al. [67] used Thermococcus zilligii, achieving an 80% reduction at 175°C for 5 minutes. For potato chips, Hendriksen et al. [63] showed that 10,500 ASNU/L of asparaginase reduced acrylamide from 1700 to 700 $\mu$g/kg (60% reduction). Pedreschi et al. [68] achieved a 90% reduction at 170°C for 5 minutes. Onishi et al. [69] used Bacillus subtilis, achieving an 80% reduction. For bread, Tuncel et al. [70] used Pisum sativum L., achieving a 57-68% reduction at 220°C for 22-25 minutes. Ciesarová et al. [71] used A. niger, achieving up to a 90% reduction. Kumar et al. [72] used Cladosporium sp., achieving a 97% reduction in crust and 73% in crumbs at 220°C for 25 minutes. Amrein et al. [73] used E. coli, achieving a 55% reduction in gingerbread. Huang et al. [74]used Rhizomucor miehei, achieving a 94.2% reduction in biscuits at 200°C for 25 minutes. Anese et al. [75] used different enzyme dosages in biscuits, achieving a 69% reduction at 200°C. Hendriksen et al. [63] used A. oryzae, achieving a 65-84% reduction in semi-sweet biscuits and 34-90% in ginger biscuits. The optimal conditions were pH 7.0 and 40°C, with 1050 ASNU/kg of enzyme reducing acrylamide to 10 $\mu$g/kg compared to 450 $\mu$g/kg in controls.

4.3 Reducing Sugar

As mentioned, acrylamide formation is strongly related to reducing sugars. Researchers have indicated that reducing sugar concentration plays a key role in acrylamide formation. Therefore, decreasing reducing sugar levels should lower acrylamide content. The type of reducing sugar (glucose, fructose, sucrose) also affects acrylamide levels in carbohydrate-rich foods.

In industrial biscuit production, Suman et al. [33] reported that glucose significantly increases acrylamide levels. Using high dextrose levels at 200°C for 8 minutes increased acrylamide by 120% compared to controls. Conversely, a low glucose concentration combined with suitable time-temperature conditions (180°C, 8 minutes) resulted in a 77% decrease in acrylamide. Using sucrose as the reducing sugar source in biscuits can decrease acrylamide production by about 50% [34, 76]. Graf et al. [34] noted that low acrylamide levels form at 160°C, remaining below 150 µg/kg, and recommended using sucrose at 180°C for biscuit production.

In the potato products industry, Yuan et al. [35] found that acrylamide levels were lower when using glucose compared to asparagine-fructose combinations. At pH 8.0, the optimal condition for acrylamide formation, the acrylamide level was 1.0 µg/ml for Asn & Fru boiled at 120°C for 30 minutes, while it was 0.52 µg/ml for Asn & Glc combinations. At pH 3.0, acrylamide generation was almost zero, regardless of the combination. Muttucumaru et al. [77] showed that the ratio of asparagine to reducing sugar is crucial for acrylamide formation in potato products, with a tipping point value of 2.257 ± 0.149. Potatoes in Doncaster had higher free asparagine levels than in Woburn, resulting in lower average acrylamide levels among 20 types. Reducing sugar levels increased with storage time, but acrylamide levels were not highest in Harmony potatoes due to their lower asparagine-to-reducing sugar ratio.

Liyanage et al. [78] found that the percent decrease in asparagine content (1–60%) was generally lower than that in reducing sugars (28–94%) with increasing frying time at a constant temperature. Amrein et al. [50] reported that acrylamide formation is primarily limited by available reducing sugars rather than free asparagine. Elmore et al. [79] found that only 0.29% of asparagine was converted to acrylamide at 180°C, while reducing sugars were dominant in acrylamide formation. Bread contains various reducing sugars, including glucose, fructose, and maltose, which contribute to acrylamide formation. Crawford et al. [80] identified sugar as a factor in acrylamide formation in commercial flatbread but lacked accurate data for analysis.

In coffee products, different coffee beans contain varying percentages of asparagine and reducing sugars, resulting in different acrylamide levels. Arabica coffee has lower acrylamide levels than Robusta due to its lower asparagine content at similar roasting degrees [43, 81].

4.4 Pretreatment before Processing

Pretreatment is a crucial step before processing, involving a series of cleaning operations to remove impurities that adversely affect dyeing and printing. For example, soaking materials in a solution or adding a solution to the materials can be beneficial. Pre-thawing frozen materials to change their form is also a type of pretreatment. Various pretreatment methods are used to reduce acrylamide levels, depending on the type of food. For instance, significant research has focused on potato products.

4.5 Lactic Acid Bacteria

Lactic acid bacteria (LAB) are Gram-positive, low GC, acid-tolerant, mostly non-sporulating, non-respiratory bacilli or cocci. Found in decomposing plants and dairy products, LAB primarily produce lactic acid from sugar fermentation, historically used to increase acidity and prevent spoilage in food fermentations. Bread, a carbohydrate-rich food, requires fermentation, and several studies have investigated LAB starters to reduce acrylamide levels.

Bartkiene et al. [87] demonstrated that different LAB strains (P. acidilactici, L. plantarum, L. curvatus, and L(+) lactic acid bacteria) grown at 30°C and 37°C showed varying acrylamide reductions. Dastmalchi et al. [88], and acrylamide formation in bread is influenced by many factors, especially reducing sugars [89]. However, L. plantarum significantly reduced acrylamide concentrations in bread samples. Whole rye flour, rich in cell walls, undergoes hydrolysis and solubilization during sourdough fermentation, facilitated by LAB enzymes. L. plantarum exhibited the lowest pH and highest amylolytic enzyme activity, likely contributing to the greatest acrylamide reduction (12 $\mu$g/kg dw) compared to the control (15 $\mu$g/kg dw).

Bartkiene et al. [87] also reported that LAB combinations can reduce acrylamide in bread. The most effective combination, L. plantarum and P. pentosaceus, reduced acrylamide levels to 4.01±0.31 $\mu$g/kg, with the lowest level at 3.70 $\mu$g/kg in wheat bread. Additionally, this combination had the lowest a* value among all samples.