Evaluation of air impingement for dry cleaning non

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DOI: 10.1111/1750-3841.16912

O R I G I N A L A R T I C L E

F o o d E n g i n e e r i n g , M a t e r i a l s S c i e n c e , a n d N a n o t e c h n o l o g y

Evaluation of air impingement for dry-cleaning nonfat dry milk residues on a stainless-steel surface

Veeramani Karuppuchamy

¹

Dennis R. Heldman

^1,2

1Department of Food Science and Technology, The Ohio State University, Columbus, Ohio, USA

2Department of Food, Agricultural, and Biological Engineering, The Ohio State University, Columbus, Ohio, USA

Correspondence

Dennis R. Heldman, Department of Food Science and Technology, The Ohio State University, 2015 Fyffe Court, Columbus, OH 43210, USA.

Email:[email protected]

Funding information U.S. Department of Agriculture, Grant/Award Number: 2019-68015-29232;

Dale A. Seiberling Endowment

Abstract:The use of air jet impingement to remove residues from surfaces in food manufacturing operations offers an alternative to the use of water and liquid cleaning agents. During this investigation, air impingement was used to remove nonfat dry milk (NFDM) residues from a stainless-steel surface. The influence of the water activity (a_w) of the residue, the time after the residue reached an equilibrium water activity, and the thickness of residue at the time of removal from the surface have been investigated. All three factors had a significant effect on the time for removal. An increase in the water activity, the time at equilibrium, the sample thickness, or a combination of all three resulted in an increase in the time required to remove the deposits. Visible changes in the structure of deposits were observed as NFDM samples equilibrated to water activities above 0.43. NFDM residues with water activities less than 0.33 were removed within 1 s of using air impingement regardless of wall shear stress. When the water activities were greater than 0.50, the thickness of deposit was greater than 1 mm, and the time after reaching an equilibrium water activity was over 7 days, more than 5 min of air impingement with wall shear stress over 9.48 Pa was required to remove the residue. The results from these experiments indicated that air impingement has the potential to provide effective cleaning in manufacturing facilities for low-moisture foods.

K E Y W O R D S

Air impingement, dry cleaning, low-moisture foods, wall shear stress, water activity

Practical Application: The introduction of water in low-moisture food environments is often undesirable due to the possibility of pathogenic microorganism growth. The normal cleaning operations in the food industry use water as a cleaning agent. This study evaluates the application of air impingement technology as a dry-cleaning method.

This is an open access article under the terms of theCreative Commons AttributionLicense, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

J. Food Sci.2024;89:1143–1153. wileyonlinelibrary.com/journal/jfds 1143

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1 INTRODUCTION

Food safety is a concern throughout the food supply chain, and all sectors of the supply chain are dedicated to this safety goal (Roux,2004). Foodborne outbreaks continue to be a common occurrence despite the increased emphasis on food safety. Among the various factors responsible for microbial contamination, the composition of food products is an important factor, since the microorganisms need nutrients, water, and time for growth. Due to nutri- ent composition, food products such as meat and dairy provide excellent mediums for the growth of pathogenic microorganisms (Odeyemi et al.,2020). Low-moisture food products were considered as microbiologically safe due to a limited availability of water. However, there have been notable foodborne outbreaks among low-moisture foods.

Salmonella is a leading cause of foodborne illnesses in low-moisture foods due to its heat resistance and ability to survive in dry environments for extended periods of time (Finn et al.,2013). Thus, it is important to implement an effective sanitation program to eliminate the occurrence of microbial contamination in the low-moisture food manufacturing environment.

The Codex Alimentarius commission defines cleaning as “the removal of soil, food residue, dirt, grease or other objectionable matter” (FAO, n.d.). According to the Cana- dian Food Inspection Agency (CFIA), cleaning is “the removal of dirt or debris by physical and/or chemical means” (CFIA,2019). As an important step in the sanitation cycle, the primary purpose of cleaning is the complete removal of visible organic and inorganic residues from the equipment and food-contact surfaces. An inadequately cleaned surface encourages reactions between the residues and cleaning agent and reduces the effectiveness of the sanitization or disinfection step (USDA Animal and Plant Health Inspection Service, 2020). The cleaning and disinfection steps are very important in food manufacturing to ensure the safety and quality of the finished products (Hasting,1999). Food manufacturing facilities have devel- oped sanitation standard operating procedures (SSOP) for each location within the facility, and the steps for cleaning and sanitizing are clearly explained, implemented, and documented, as required by the regulatory agencies. The Code of Federal Regulation (CFR) title 21 part 117 by the Food and Drug Administration (FDA) requires that all food-contact surfaces should be cleaned as frequently as needed to avoid microbial contamination (FDA, 2020).

While the regulatory agencies do not set the cleaning frequency, it is up to the discretion of food manufacturers to establish the frequency to ensure safety of the food products while also ensuring productivity.

Air impingement is a technology using high-velocity air jets (10–100 m/s) to impinge on a surface. The factors influencing the efficiency of air impingement include exit velocity at the nozzle, design of the nozzle, equipment design, and the boundary layer at the surface (Sarkar &

Singh,2004). Air impingement technology has a potential for use in cleaning of food-contact surfaces, without using water. The applications include impingement if applied perpendicular to a surface to create shear stress parallel to the surface. The shear stress at the surface gener- ates mechanical energy needed to remove the residues or deposits. The effectiveness of the impingement is a function of velocity at the jet nozzle exit, diameter of the nozzle, distance from nozzle to surface, and the angle of impinging (Leung et al.,2017). According to Keedy et al. (2012), the removal of particles by air jet impingement was dependent on the type of particle, the type of surface, and the properties of the air jet. The forces on the particles must overcome adhesive forces holding the particles to the surface, as well as the mass required to suspend the particles in the air (Keedy et al.,2012). The air jet must overcome both the cohesive and adhesive forces for efficient cleaning of a surface.

The FDA accepts the use of compressed air for cleaning in food manufacturing facilities. According to the 21CFR117.40, “compressed air or other gases mechanically introduced into food or used to clean food-contact surfaces or equipment shall be treated in such a way that food is not contaminated with unlawful indirect food additives”

(FDA,2020). The compressed air must be treated using fil- tration so that it is free of microorganisms. The compressed air must be periodically checked for bacteria, yeast, and mold.

There has been a very limited number of investigations into the use of air jet impingement as a cleaning technique. In an earlier study, Otani et al. (1994) examined the use of pulsed air jet impingement from a rectangular nozzle for removal of 0.25- to 3-µm polystyrene latex (PSL) particles from a silicon wafer and found that the air jet was able to remove the small particles instantly from the wafer surface. Leung et al. (2017) studied the effect of air jet impingement on removal of very small droplets (microm- eter and millimeter) from a plastic surface. The authors emphasized that air jet removal of liquid droplets might be different than solid particles. Hence, more research is needed to explore the potential of air impingement as a dry-cleaning method in food manufacturing facilities.

As a low-moisture food product, nonfat dry milk (NFDM) has been implicated in several foodborne ill- ness outbreaks. In 1956, 19 separate outbreaks occurred due to the consumption of NFDM contaminated by

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Staphylococcal enterotoxin (Armijo et al., 1957). In 2017, 22 cases of illnesses ofSalmonella Agona infection were traced back to infant milk formula produced in a facility in France that previously had an outbreak in 2005 (Jourdan-da Silva et al., 2018). Thus, NFDM was chosen as the model food sample in this study.

The overall goal of this investigation was to evaluate the effectiveness of air impingement for removal of residues from surfaces in food manufacturing facilities. The specific objectives were to determine the influence of jet impingement velocity on the time to remove surface residues at different water activities and thicknesses, as well as residues at equilibrium for different durations of time.

2 MATERIALS AND METHODS 2.1 Material—NFDM

NFDM (Nestlé) was purchased from a local supermarket (Columbus, OH, USA) and stored at room temperature in an airtight container until use. The water activity (a_w) of NFDM as received was measured with a water activity meter (model Aqualab 4TE; Meter Group) and the value was around 0.20. The moisture and fat content of NFDM samples were measured with Smart 6 and Smart Trac II, respectively (CEM). The Smart 6 is a microwave moisture analyzer that applies a combination of microwave and infrared radiation for fast measurement of moisture. The Smart Trac II uses nuclear magnetic resonance (NMR) technology to measure fat content based on a signal-to- mass ratio. About 1 g of NFDM sample was dried in Smart 6 to measure the moisture content. After drying, the sample was rolled and placed in a Trac tube. The Trac tube was inserted into NMR chamber of Smart Trac II and the fat result was obtained within a few seconds. The protein was measured by combustion method in a nitrogen analyzer (Elementar). The obtained percent nitrogen was converted to percent protein by a conversion factor of 6.38.

The ash was measured by igniting 1 g of NFDM sample at 550^◦C until carbon free in a muffle furnace (CEM) (AOAC, 2019). The carbohydrates were calculated as the difference [100 – (%moisture +%fat +%protein +%ash)]. All the experiments were performed in triplicates. The proximate composition of NFDM used is given in Table1.

2.2 Equilibration of NFDM samples

The NFDM samples were equilibrated to different water activities using saturated salt solutions in the desiccators that were stored at ambient temperature. The length and width of NFDM deposits were 31.5 × 20 mm with

T A B L E 1 Proximate composition of nonfat dry milk.

Component Percent

Moisture 3.74±0.29

Fat 0.44±0.02

Carbohydrates (mainly lactose)

50.57±0.28

Protein 37.99±0.07

Ash 7.26±0.03

Note: Values are reported in wet basis (mean±SD).

three different thicknesses of 0.4, 0.8, and 1.2 mm. The NFDM samples on a stainless-steel coupon were placed on perforated porcelain desiccator plates and kept inside polypropylene desiccators (Fisher Scientific) containing about 500 mL of various saturated salt solutions. The relative humidity (RH) inside the desiccators was monitored by a hygrometer (model H5101; Govee), and the accuracy of the RH sensor was±3%. The ambient temperature varied between 15 and 25^◦C depending on the season of the year, and this resulted in a small change in water activity of the salt solutions. The salt solutions used for the equilibration and the corresponding water activities over this temperature range are given in Table2. The NFDM samples reached an equilibrium when the change in mass was less than 1% for three consecutive measurements and this equilibrium was obtained within the first 24 h during storage over the saturated salt solutions.

The NFDM samples with thicknesses of 0.40, 0.80, and 1.20 mm were prepared using a 3D print mold, as described by Park et al. (2021). Air impingement was evaluated for cleaning a thin layer of product. In the context of this study, the “time at equilibrium” refers to the number of days the NFDM samples are held at a given water activity after reaching the equilibrium. The water activity of the samples along with change in mass was measured by taking samples from the desiccators. This was done every 3 h during the first 24 h. Each measurement (e.g., 3 h, 6 h, 9 h, etc.) used a new set of samples, since opening and closing the lid of desiccators might disrupt the equilibrium inside the desiccators. All NFDM samples reached their target water activity within the first 24 h, and this was referred to as “time at equilibrium” of 0 days in this study. Simi- larly, the NFDM samples that were held for 1, 4, and 7 days after reaching the equilibrium at a target water activity are referred to as 1, 4, and 7 days of time at equilibrium, respectively. The maximum time at equilibrium of 7 days was based on a cleaning frequency of 1 week. The samples were equilibrated to four different water activities (0.33, 0.43, 0.59, and 0.76 at 20^◦C) using saturated salt slurries as shown in Table2. The water activities of 0.33–0.76 in the current study reflect the equilibrium RH values from

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T A B L E 2 Saturated salt solutions used for equilibrating nonfat dry milk samples (Greenspan,1977).

Saturated salt solution awat 15^◦C awat 20^◦C awat 25^◦C

Magnesium chloride (MgCl₂)

0.333±0.002 0.331±0.002 0.328±0.002

Potassium carbonate (K₂CO₃)

0.432±0.003 0.432±0.003 0.432±0.004

Sodium bromide (NaBr) 0.607±0.005 0.591±0.004 0.576±0.004

Sodium chloride (NaCl) 0.756±0.002 0.755±0.001 0.753±0.001

F I G U R E 1 (a) Schematic of air impingement experiment setup (Kim et al., 2020). (b) Normal and shear stresses from impinging air jet on a flat surface.

33% to 76% as can be noted in food manufacturing facilities during different seasons of the year.

2.3 Air impingement experiments

The schematic diagram for air impingement experiments is given in Figure 1A. The normal and shear stress components resulting from the impinging air jet are given in Figure 1B. For the air impingement experiments, com-

pressed air from the pilot plant was used. All the experiments were conducted at ambient temperature that varied from 15 to 25^◦C depending on the season and time of the day. A slight variation in water activity values was observed during the study period due to fluctuations in ambient temperature. The saturated salt solutions of magnesium chloride and potassium carbonate maintained a water activity values of 0.33 and 0.43, respectively. For sodium bromide solution, the water activity varied from 0.58 to 0.61. The water activity of sodium chloride salt

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T A B L E 3 Experimental conditions for air impingement removal of nonfat dry milk.

Factor Levels Values

Water activity (a_w) 4 0.33, 0.43, 0.59, and 0.76

Time at equilibrium (days)

4 0, 1, 4, and 7

Thickness (mm) 3 0.40, 0.80, and 1.20

Maximum wall shear stress (Pa)

5 4.17, 5.66, 7.32, 8.32, and 9.48

solution stayed around 0.75. It was assumed that the variation in the water activities due to the change in the ambient temperature was negligible, as can be seen from Table2.

The experiments were conducted inside a ventilated balance enclosure (PLAS Labs) to confine any dust generated by the impinging air jet at high velocities. The air impingement experiments were conducted using a 4-mm-diameter nozzle that was stationed at a distance of 32 mm from the geometric center of the stainless-steel coupons to maintain anH/Dratio of 8, whereHis the nozzle-to-coupon distance andDis the diameter of the nozzle. AnH/Dratio of 8 was suggested for air impingement applications such as baking (Shevade et al.,2019). The experiments were conducted at an impinging angle of 90^◦from horizontal. The gauge pressures varied from 10 to 30 psi, in increments of 5 psi. The pressure of 30 psig was chosen as the upper limit to comply with the Occupational Safety and Health Administration (OSHA) guideline that recommends that the compressed air for any cleaning purposes should not exceed 30 psig at the nozzle. Kim et al. (2022) used oil-film interferometry technique to estimate the wall shear stress (WSS) created by impinging air jet and the equation used by the authors is given below:

𝜏 (𝑥) = 𝜏_𝑎 (ℎ_𝑎

ℎ_𝑥 )2

− 2𝜇 𝑟ℎ_𝑥²

𝑟 𝑟∫𝑎

𝜕ℎ

𝜕𝑡𝑟𝑑𝑟.

The maximum WSS corresponding to the pressures of 10, 15, 20, 25, and 30 psig was 4.17, 5.66, 7.32, 8.32, and 9.48 Pa, respectively, as determined in the study by Kim et al. (2022).

2.4 Time to remove for the target removal efficiency

The variables used in the air impingement experiments along with the levels are given in Table3. The stainless- steel coupons were weighed using an analytical balance

(Sartorius model BCE224I-1S) before the air impingement.

The coupons were immediately subjected to air impingement to avoid any moisture adsorption by the NFDM sample. The air impingement was applied until the residue of NFDM left on the stainless-steel coupon surface was less than 0.10 mg, with the impingement time varying from 1 s to a maximum of 5 min. While some NFDM samples were removed within 1 s, some samples were not removed com- pletely even after 5 min of air impingement, depending on the treatment conditions of the sample. The stainless-steel coupons were weighed again using an analytical balance (Sartorius model BCE224I-1S) after the air impingement.

The sensitivity of analytical balance was 0.1 mg. The time needed to obtain the residual mass of <0.10 mg on the stainless-steel coupon surface was used as the response variable. All the experiments were conducted in triplicates.

2.5 Statistical analysis

The statistical analysis of results was performed using JMP Pro version 15.2 (SAS) using the analysis of variance (ANOVA). The main effect and the interaction effects of the variables were evaluated in the model. The discrete variables were used to analyze the effect of each variable for its significance, in which case the Tukey’s honestly significant difference (HSD) test was applied for the comparison of treatment means. All the statistical analysis was performed at 95% confidence level (α=0.05), whereαis the probability of Type I error.

3 RESULTS AND DISCUSSION

The effects of the independent variables (water activity, time at equilibrium, and sample thickness) on the dependent variable (time for removal) are discussed below.

The time needed for removal was given as a function of maximum WSS.

3.1 Effect of water activity

The water activity of the NFDM deposits was the most significant among all the variables studied. An example of the influence of water activity as a function of time for removal of the deposit is given in Figure2.

The results in Figure 2 indicate that NFDM residues with water activity of 0.33 were removed within 1 s at all WSS levels. The residues with water activity of 0.43 required a WSS of 5.66 Pa for removal in 13 s. The WSS of 8.32 and 9.48 Pa removed the residues with water activity of 0.43 in less than 1 s. As water activities of the NFDM residues were increased, the removal of the

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F I G U R E 2 Influence of wall shear stress on time for residue removal at different levels of water activity (0.40-mm thick and 7 days of time at equilibrium; nozzle-to-plate distance [H]=32 mm).

residues required longer time and higher WSS. For the water activities of 0.59 and 0.76, the time for removing the residues was 138 and 236 s at a WSS of 5.66 Pa, respectively.

When the WSS was increased from 5.66 to 7.32 Pa, the time needed for removal decreased sharply from 138 to 28 s for the residues equilibrated at 0.59a_w. The time for removal of residues with water activity of 0.76 decreased linearly over the range of WSS from 5.66 to 9.48 Pa.

A statistical analysis (Tukey’s HSD test) revealed that the samples at 0.43a_w required significantly less time compared to the samples at 0.59 and 0.76a_w. However, there is no significant difference in times for removal of residues equilibrated to 0.59 and 0.76a_w.

The high moisture contents due to high water activity values created a visible change in the structure of the dry particles within the residue. A slight “caking” was visible when the residues were equilibrated to a water activity of 0.43, and the changes in structure became more evident when residues were equilibrated at water activities of 0.59 and 0.76. All visible changes were evident within the 24 h required to reach equilibration. These changes in structure had an impact on removal of the residue. The time needed to remove the residues also proportionately increased with an increase in caking intensity. Most likely, the observed changes in structure of the NFDM residue influenced the cohesive forces between particles within the layer of residue and could be associated with Van der Waals forces, interfacial forces, liquid and solid bridging, or interlocking mechanism (Rumpf, 1961). Özkan et al.

(2002) found that bridges formed within skim milk powder were stronger than bridges formed in the whole milk powder, but the same authors noted that an increase in fat content decreased the flowability of dry milk powders. In another study for the measurement of cohesion, the values for whole milk powder were significantly higher than

those of skim milk powder and this difference was pri- marily attributed to the higher fat content of whole milk powder (Rennie et al.,1999), and the authors also observed an increase in cohesion when the particle size was reduced.

The change in structure at higher water activities is due to lactose crystallization in which lactose transitions from an amorphous state to a crystalline state, and this change was observed at a water activity of 0.54 by Lai and Schmidt (1990).

On the other hand, the effect of fat content was primar- ily on adhesion. When adhesion on a glass surface was measured for different milk products, skim milk recorded 95.9 mN/m, whereas 2% reduced milk had an adhesion of 94.8 mN/m (Handojo et al.,2009). This finding suggests that the influence of fat on adhesion was insignificant.

Teunou and Fitzpatrick (1999) suggested that water absorbed by food powders from high-RH environments caused formation of liquid bridges between particles.

These changes in structure may, in turn, increase cohesion and decrease flowability. The caking of the dry particles was described as the tendency to cause clumping and decrease the flowability, and the caking resulted in decreased rehydration and dispersibility (Chuy & Labuza, 1994). Peleg and Mannheim (1977) observed that when RH was below 40%, onion powder was free flowing, but the same powder displayed visible caking within a few days when held in an environment with RH over 40%. Moreyra and Peleg (1981) observed that time for caking decreased with water activity, as long as the water activity was above 0.45. Listiohadi et al. (2005) recommended storage at RH of less than 33% and 25^◦C to avoid severe caking in milk powders. Our results agree with the observations from previous researchers. In this study, no caking was observed for samples equilibrated to 0.33 a_w, and these were the easiest to remove irrespective of sample thicknesses and time at equilibrium. Similarly, a partial caking was noted for samples at 0.43 a_w, and all experimental conditions removed the NFDM deposits from the stainless-steel surface. Generally, an increase in water activity will result in an increase in adhesion and cohesion of NFDM particles. This finding was also confirmed by Chen et al. (2022), who noticed that increasing water activity from 0.22 to 0.81 increased the adhesion and cohesion. In their study, the adhesion between milk powder and the stainless-steel surface increased from 23.4 Pa at 0.22a_w to 128.7 Pa at 0.81 a_w. Similarly, the cohesion within the milk powder particles also increased from 84.18 Pa at 0.22a_w to 5418 Pa at 0.81a_w (Chen et al.,2022). The tendency of caking for a food powder is influenced by factors such as a thin layer of saturated solution at the surface, distribution of particle size, and addition of mother liquor droplets (Mathlouthi

& Rogé,2003). The glass transition temperature of milk powder decreases with an increase in moisture content and

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F I G U R E 3 Influence of wall shear stress on time for removal of deposits, as influenced by time after reaching equilibrium (0.40-mm thick, 0.76a_w; nozzle-to-plate distance [H]=32 mm).

water activity, and the plasticizing effect of water increases the rate of caking at higher water activities (Carpin et al., 2016).

3.2 Effect of time at equilibrium

The time required for the NFDM residues to reach equilibrium was 24 h, as indicated in Section2. This time to reach equilibrium is consistent with the observations by Hayashi et al. (1968) when NFDM samples were placed in controlled RH environments for 24–48 h to achieve equilibrium. The influence of time after reaching equilibrium on the time for removal of the residue is illustrated in Figure 3. The time for removal decreased when the WSS was increased. The time for removal was about 5 s at 9.48 Pa. Similarly, for a WSS of less than 5.66 Pa, the time for removal exceeded 300 s, which was the thresh- old in the study. At a WSS of 9.48 Pa, the time for removal increased from about 5 s for residues at equilibrium to approximately 75 s for residues held at equilibrium for 7 days. In general, the influence of time at equilibrium on the time for removal of the residue was statistically significant (α=0.05).

A statistical comparison of the mean of times for removal (Tukey’s HSD test) indicates that the effect of time at equilibrium is significant. Most likely, the increase in time for removal of the residue is the result of an increase in cohesive forces among particles within the residue, and the increase in these forces as the time after reaching equilibration increased.

As evident from the results presented in Figure3, more than 300 s of air impingement was required to remove the residues when the WSS was less than 5.66 Pa. When the residues were held at equilibrium for 1 day, the time for removal of the residue was not increased when the

F I G U R E 4 Influence of wall shear stress on time for removal of deposits, as a function of thickness (0.43a_w, 7 days’ time at equilibrium; nozzle-to-plate distance [H]=32 mm).

WSS was increased from 5.66 to 7.32 Pa. When the WSS was increased from 7.32 to 8.32 Pa, the average removal time decreased from 122 to 72 s. For 4 and 7 days at equilibrium, the time for removal of the residues decreased linearly when the WSS was increased from 5.66 to 9.48 Pa.

Since removing the deposits was more challenging with increased time after reaching 0.76 a_w, there must be a change in the structure of the deposit, specifically adhesion at the interface between the deposit and the coupon surface.

3.3 Effect of deposit thickness

The influence of thickness of the deposit on time for removal is illustrated in Figure4. The time increased with an increase in thickness of the deposit. The average time for removal of the residue with 0.40-mm thickness was 48 s. This removal time increased to 103 s when the thickness increased to 0.80 mm, and to 135 s for a thickness of 1.20 mm. The WSS created by the exit nozzle pressure on the surface must overcome adhesive and cohesive forces within the deposit, and these forces increased with the additional mass associated with the increased thickness of the deposit. The effect of sample thickness on time for removal as an independent variable was statistically significant (p<0.05).

The results from the current investigation agree with those from Tuck et al. (2019) when investigating impingement removal by liquid jets. Tuck et al. (2019) reported that a deposit of 2-mm thickness was removed quickly, whereas a deposit with 8-mm thickness required a much longer time for removal. The researchers suggested that the longer time was due to the need to overcome cohesive forces. In the current study, the NFDM samples equilibrated to high water activities were removed as a layer rather than as

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particles and thus the primary removal mechanism by air jet impingement was adhesive failure between the deposits of NFDM and the stainless-steel surface. Gotoh and Masuda (1998) found that the force needed to remove particles using a high-speed air jet decreased with a decrease in the particle size. The size of the particles within the deposit was not considered in the current investigation.

The times for removal of 0.4-, 0.8-, and 1.2-mm-thick deposits of NFDM after 7 days of equilibrium at a water activity of 0.43 are presented in Figure4. Overall, the time for removal of the deposits increased with an increase in time at equilibrium. After 7 days at water activity of 0.43, the NFDM deposits could not be removed when the WSS was 4.17 Pa. When the sample thickness was increased, the removal of the deposits required higher WSS and increased air jet application times for removal. At a WSS of 7.32 Pa, the times for removal of deposits were 10 and 81 s for deposit thickness of 0.4 and 0.80 mm, respectively. When the thickness was 1.20 mm, a WSS of greater than 7.32 Pa was required for removal. When the WSS was increased to 8.32 Pa, the time for removal of all deposits was less than 10 s.

3.4 Interactions among independent variables

The statistical interaction effects among all variable (water activity, time after reaching equilibrium water activity, thickness of deposit) were significant (p<0.05) in addition to the main effects previously described. Not all experimental combinations removed the NFDM deposits. Visible caking of the NFDM was observed for samples equilibrated to water activities above 0.43. The interaction between thickness and water activity was more significant than other interactions. For samples equilibrated to a water activity of 0.43 and stored for 7 days, a WSS of 5.66 Pa was sufficient to remove a deposit with a 0.4-mm thickness. When the thickness of the deposit was increased to 0.80 mm, a minimum WSS of 7.32 Pa was required to remove the deposit. When the thickness of the NFDM deposit was further increased to 1.20 mm, a WSS of 8.32 Pa was needed.

For NFDM deposits equilibrated to water activities of 0.59 and 0.76, the time for removal increased for the 0.40- mm deposit equilibrated at water activity of 0.76, whereas the time required for the 0.80-mm thickness was longer for the deposit equilibrated to 0.59a_w.

The results from the current investigation tend to sup- port the observations of previous researchers. For the stainless-steel surfaces, fluctuations in WSS, as well as the magnitude of fluctuations, influenced the detachment of bacteria (Lelièvre et al.,2002). Fletcher et al. (2008) noted

an increase in removal efficiency with increases in both jet velocity and particle size when the air jet impingement was used to remove polystyrene microspheres from a polycarbonate surface. While the particle size was not included as a variable in the current study, the effect of jet velocity was examined and expressed as WSS. Scott and Mahoney (1982) concluded that the impingement component was more than four times as effective compared to the cascading component resulting from impingement. Effec- tiveness was based on the amount of soil removed when the researchers studied spray cleaning of milk storage tanks.

Kawale and Chandramohan (2017) suggested a critical exit velocity of at least 3 m/s when using an impinging water jet for cleaning a flat surface. In a recent study using high-pressure water jets, Gerhards et al. (2019) showed that an increase in the pressure or temperature reduced the time needed to clean deposits of milk protein concen- trates from stainless-steel surfaces. They also found that an increase in the temperature was more effective than increasing the pressure of water jets. The WSS needed for the removal of a deposit is proportional to the Reynolds number, which is influenced by the density of the fluid.

The density of water is higher than the density of air. Thus, under similar experimental conditions, a lower velocity of water jet is sufficient for cleaning compared to high- velocity air jets. In addition, the temperature is one of the four pillars of traditional wet cleaning, in addition to time, mechanical action, and cleaning chemistry. The temperature was not a factor of interest in the current study using air jet impingement, and ambient-temperature air was used throughout the study.

An effective cleaning method must overcome both the cohesive forces (within the food material) and the adhesive forces (between the food material and wall) to achieve the goal of preparing the food-contact surface for sanitizing (Fryer & Asteriadou,2009). When air impingement is the primary removal mechanism for an NFDM deposit, the WSS at the surface must exceed the adhesive forces holding the particles at water activities above 0.59. Our findings agree with Michalski et al. (1997), who observed that the adhesive force increased with an increase in contact time and RH. In the current study, the time at equilibrium is equivalent to contact time and the water activity is equivalent to RH mentioned in the study of Michalski et al.

(1997). The NFDM deposits were generally more difficult to remove as the time at equilibrium and the water activity were increased.

The time for removal of NFDM deposits decreased linearly with an increase in WSS at the coupon surface when the water activities were 0.59 and 0.76. For the cleaning of metal surfaces by water jets, Schluessler (1976) explained the rate of cleaning by first-order reaction kinetics.

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Conversely, Dürr and Graßhoff (1999) used the Weibull distribution to explain the kinetics of heat exchanger cleaning.

4 CONCLUSIONS

This investigation explored the use of air jet impingement as an alternative to traditional wet cleaning methods in low-moisture food manufacturing operations. NFDM was the model food, and stainless-steel surfaces were the food- contact surfaces. The findings from the air impingement study are summarized as follows:

1. Water activity, length of time at equilibrium water activity, and thickness of deposit increased the time to remove NFDM deposits from stainless-steel surfaces, and the influence was significant (p<0.05).

2. NFDM residues with water activities less than 0.33 were removed within 1 s by air impingement providing a WSS of 4.17 Pa, and the removal was not influenced by thickness of the deposit or the time after reaching an equilibrium water activity.

3. Visible changes in the structure of deposits were observed for NFDM samples equilibrated to water activity above 0.43. For NFDM deposits equilibrated to water activities 0.43 and above, the average time for removal from a stainless-steel surface increased from 48 to 134 s when the thickness of the deposit increased from 0.4 to 1.2 mm.

4. The average time for removal of an NFDM deposit increased from 72 s when the sample reached equilibrium at water activity of 0.43, to 122 s for deposits at equilibrium for more than 7 days.

5. A WSS of 9.48 Pa for 5 min was required to remove a 1-mm NFDM deposit held for 7 days at an equilibrium water activity of 0.5.

The above findings from the study suggest the potential of air impingement as a dry-cleaning method. Further studies are recommended to include other variables that could have an effect on cleaning.

A U T H O R C O N T R I B U T I O N S

Veeramani Karuppuchamy: Conceptualization;

methodology; writing—original draft. Dennis R. Held- man: Funding acquisition; resources; writing—review and editing; supervision.

A C K N O W L E D G M E N T S

The study was supported by United States Department of Agriculture National Institute of Food and Agricul- ture Grant #2019-68015-29232. Contributions from the

Dale A. Seiberling Endowment are acknowledged. The research was sponsored, in part, by USDA National Insti- tute of Food and Agriculture Hatch/Evans-Allen/McIntire Stennis Project Number OHO01450 on Sustainability of the Food Supply System. Finally, the assistance of Molly Davis in proofreading of this manuscript is acknowledged.

C O N F L I C T O F I N T E R E S T S T A T E M E N T The authors declare no conflicts of interest.

O R C I D

Veeramani Karuppuchamy https://orcid.org/0000- 0001-7285-0635

Dennis R. Heldman https://orcid.org/0000-0002-7202- 0436

R E F E R E N C E S

Association of Official Analytical Chemists (AOAC). (2016).AOAC Official Method 930.30. AOAC International, Gaithersburg, MD, USA.

Armijo, R., Henderson, D. A., Timothee, R., & Robinson, H. B. (1957).

Food poisoning outbreaks associated with spray-dried milk—An epidemiologic study. American Journal of Public Health,47(9), 1093–1100.

Canadian Food Inspection Agency (CFIA). (2019). Cleaning and sanitation program. https://inspection.canada.ca/preventive- controls/cleaning-and-sanitation-program/eng/1511374381399/

1528206247934

Carpin, M., Bertelsen, H., Bech, J. K., Jeantet, R., Risbo, J., & Schuck, P. (2016). Caking of lactose: A critical review.Trends in Food Sci- ence & Technology,53, 1–12.https://doi.org/10.1016/j.tifs.2016.04.

002

Chen, L., Rana, Y. S., Heldman, D. R., & Snyder, A. B. (2022). Envi- ronment, food residue, and dry cleaning tool all influence the removal of food powders and allergenic residues from stainless steel surfaces.Innovative Food Science and Emerging Technologies, 75, Article 102877.https://doi.org/10.1016/j.ifset.2021.102877 Chuy, L. E., & Labuza, T. P. (1994). Caking and stickiness of dairy-

based food powders as related to glass transition. Journal of Food Science,59(1), 43–46.https://doi.org/10.1111/j.1365-2621.1994.

tb06893.x

Dürr, H., & Graßhoff, A. (1999). Milk heat exchanger cleaning: Mod- elling of deposit removal.Food and Bioproducts Processing,77(2), 114–118.https://doi.org/10.1205/096030899532402

Finn, S., Condell, O., Mcclure, P., Amézquita, A., & Fanning, S. (2013).

Mechanisms of survival, responses, and sources ofSalmonellain low-moisture environments.Frontiers in Microbiology,4, Article 331.https://doi.org/10.3389/fmicb.2013.00331

Fletcher, R., Briggs, N., Ferguson, E., & Gillen, G. (2008). Measure- ments of air jet removal efficiencies of spherical particles from cloth and planar surfaces.Aerosol Science and Technology,42(12), 1052–1061.https://doi.org/10.1080/02786820802402237

Food and Agriculture Organization (FAO). (n.d.).Section 2 - Recom- mended international code of practice—General principles of food hygiene.http://www.fao.org/3/w8088e/w8088e04.htm

(10)

Food and Drug Administration (FDA). (2020).CFR—Code of Federal Regulations title 21.https://www.accessdata.fda.gov/scripts/cdrh/

cfdocs/cfcfr/CFRSearch.cfm?CFRPart=117&showFR=1

Fryer, P. J., & Asteriadou, K. (2009). A prototype cleaning map: A clas- sification of industrial cleaning processes.Trends in Food Science

& Technology,20(6-7), 255–262.https://doi.org/10.1016/j.tifs.2009.

03.005

Gerhards, C., Schramm, M., & Schmid, A. (2019). Use of the Weibull distribution function for describing cleaning kinetics of high pressure water jets in food industry.Journal of Food Engineering,253, 21–26.https://doi.org/10.1016/j.jfoodeng.2019.02.011

Gotoh, K., & Masuda, H. (1998). Enhancement of removal efficiency of deposited single particles by a high speed air jet.Journal of Aerosol Science,29(1), S1231–S1232.https://doi.org/10.1016/S0021- 8502(98)90798-4

Greenspan, L. (1977). Humidity fixed points of binary saturated aqueous solutions.Journal of Research of the National Bureau of Standards Section A: Physics and Chemistry,81A(1), 89–89.https://

doi.org/10.6028/jres.081A.011

Hayashi, H., Heldman, D. R., & Hedrick, T. I. (1968). Internal fric- tion of nonfat dry milk.Transactions of the ASAE,11(3), 422–425.

https://doi.org/10.13031/2013.39429

Handojo, A., Zhai, Y., Frankel, G., & Pascall, M. A. (2009). Mea- surement of adhesion strengths between various milk products on glass surfaces using contact angle measurement and atomic force microscopy.Journal of Food Engineering,92(3), 305–311.https://

doi.org/10.1016/j.jfoodeng.2008.11.018

Hasting, A. P. M. (1999). Fouling and cleaning in the food industry.

Food and Bioproducts Processing,77(2), 73–74.https://doi.org/10.

1205/096030899532358

Jourdan-da Silva, N., Fabre, L., Robinson, E., Fournet, N., Nisavanh, A., Bruyand, M., Mailles, A., Serre, E., Ravel, M., Guibert, V., Issenhuth-Jeanjean, S. R. C., Tourdjman, M., Septfons, A., de Valk, H., & Le Hello, S. (2018). Ongoing nationwide outbreak ofSalmonellaAgona associated with internationally distributed infant milk products, France, December 2017.Eurosurveillance, 23(2), Article 17–00852.https://doi.org/10.2807/1560-7917.ES.2018.

23.2.17-00852

Kawale, S., & Chandramohan, V. P. (2017). CFD simulation of estimating critical shear stress for cleaning flat soiled surface.

Sadhana—Academy Proceedings in Engineering Sciences,42(12), 2137–2145.https://doi.org/10.1007/s12046-017-0748-z

Keedy, R., Dengler, E., Ariessohn, P., Novosselov, I., & Aliseda, A.

(2012). Removal rates of explosive particles from a surface by impingement of a gas jet.Aerosol Science and Technology,46(2), 148–155.https://doi.org/10.1080/02786826.2011.616920

Kim, W.-J., Karuppuchamy, V., & Heldman, D. R. (2020).Investiga- tion of shear stress induced on a surface by air impingement and correlation with removal of food deposits. Poster presented at the IFT Annual Meeting (virtual), July 12–15.

Kim, W.-J., Karuppuchamy, V., & Heldman, D. R. (2022). Evalu- ation of maximum wall shear stress from air impingement to remove food deposits from stainless steel surfaces.Journal of Food Engineering,316, Article 110825.https://doi.org/10.1016/j.jfoodeng.

2021.110825

Lai, H.-M., & Schmidt, S. J. (1990). Lactose crystallization in skim milk powder observed by hydrodynamic equilibria, scanning elec- tron microscopy and ²H nuclear magnetic resonance. Journal

of Food Science,55(4), 994–999.https://doi.org/10.1111/j.1365-2621.

1990.tb01582.x

Lelièvre, C., Legentilhomme, P., Gaucher, C., Legrand, J., Faille, C., & Bénézech, T. (2002). Cleaning in place: Effect of local wall shear stress variation on bacterial removal from stainless steel equipment. Chemical Engineering Science, 57(8), 1287–1297. https://doi.org/10.1016/S0009-2509(02)0001 9-2

Leung, W. T., Fu, S. C., & Chao, C. Y. H. (2017). Detachment of droplets by air jet impingement.Aerosol Science and Technology, 51(4), 467–476.https://doi.org/10.1080/02786826.2016.1265911 Listiohadi, Y. D., Hourigan, J. A., Sleigh, R. W., & Steele, R. J.

(2005). An exploration of the caking of lactose in whey and skim milk powders.The Australian Journal of Dairy Technology,60(3), 207–213.

Mathlouthi, M., & Rogé, B. (2003). Water vapour sorption isotherms and the caking of food powders. Food Chemistry,82(1), 61–71.

https://doi.org/10.1016/S0308-8146(02)00534-4

Michalski, M.-C., Desobry, S., & Hardy, J. (1997). Food materials adhesion: A review.Critical Reviews in Food Science and Nutrition, 37(7), 591–619.https://doi.org/10.1080/10408399709527791 Moreyra, R., & Peleg, M. (1981). Effect of equilibrium water activ-

ity on the bulk properties of selected food powders.Journal of Food Science,46(6), 1918–1922.https://doi.org/10.1111/j.1365-2621.

1981.tb04519.x

Odeyemi, O. A., Alegbeleye, O. O., Strateva, M., & Stratev, D. (2020).

Understanding spoilage microbial community and spoilage mechanisms in foods of animal origin.Comprehensive Reviews in Food Science and Food Safety, 19(2), 311–331. https://doi.org/10.1111/

1541-4337.12526

Otani, Y., Emi, H., Morizane, T., & Mori, J. (1994). Removal of fine particles from wafer surface by pulse air jets.KONA Powder Particle Journal,12, 155–160.

Özkan, N., Walisinghe, N., & Chen, X. D. (2002). Characterization of stickiness and cake formation in whole and skim milk powders.Journal of Food Engineering,55(4), 293–303.https://doi.org/

10.1016/S0260-8774(02)00104-8

Park, H. W., Xu, J., Balasubramaniam, V. M., & Snyder, A. B. (2021).

The effect of water activity and temperature on the inactivation of Enterococcus faeciumin peanut butter during superheated steam sanitation treatment.Food Control,125, Article 107942.https://doi.

org/10.1016/j.foodcont.2021.107942

Peleg, M., & Mannheim, C. H. (1977). The mechanism of caking of powdered onion.Journal of Food Processing and Preservation,1(1), 3–11.https://doi.org/10.1111/j.1745-4549.1977.tb00309.x

Rennie, P. R., Chen, X. D., Hargreaves, C., & Mackereth, A. R. (1999).

A study of the cohesion of dairy powders.Journal of Food Engineer- ing, 39(3), 277–284. https://doi.org/10.1016/S0260-8774(98)0015 8-7

Roux, J. (2004). Food safety—No compromises.Agriprobe,1(4), 12–13.

Rumpf, H. (1961). The strength of granules and agglomerates. In W. A.

Krepper (Ed.),Agglomeration(pp. 379–418). Industrial Publishers.

Sarkar, A., & Singh, R. P (2004). Air impingement technology for food processing: Visualization studies.LWT- Food Science and Technology, 37(8), 873–879. https://doi.org/10.1016/j.lwt.2004.04.

005

Schluessler, H. J. (1976). Zur Kinetik von Reinigungsvorgaengen an festen Oberflaechen.Brauwissenschaft,29, 263–268.

(11)

Scott, J. M., & Mahoney, D. B. (1982). Physical components of spray cleaning in a milk storage tank.The Australian Journal of Dairy Technology, 37(1), 37–38.

Shevade, S., Rahman, M., & Guldiken, R. (2019). Optimization of turbulent air jet impingement for energy efficient commercial cooking.Energy Procedia,160, 691–698.https://doi.org/10.1016/j.

egypro.2019.02.191

Teunou, E., & Fitzpatrick, J. J. (1999). Effect of relative humidity and temperature on food powder flowability.Journal of Food Engineer- ing, 42(2), 109–116. https://doi.org/10.1016/S0260-8774(99)0008 7-4

Tuck, J. P., Alberini, F., Ward, D., Gore, B., & Fryer, P. J. (2019). Clean- ing of thick films using liquid jets.Energy Procedia,161, 93–99.

https://doi.org/10.1016/j.egypro.2019.02.062

USDA Animal and Plant Health Inspection Service. (2020). Cleaning.

https://www.aphis.usda.gov/aphis/ourfocus/animalhealth/nvap/

NVAP-Reference-Guide/Cleaning-and-Disinfection/Cleaning

How to cite this article: Karuppuchamy, V., &

Heldman, D. R. (2024). Evaluation of air impingement for dry-cleaning nonfat dry milk residues on a stainless-steel surface.Journal of Food Science,89, 1143–1153.

https://doi.org/10.1111/1750-3841.16912