Tyrophagus putrescentiae (Schrank, 1781; Klimov and OConnor, 2010), known as the mold mite, cheese mite, or ham mite, is an ubiquitous and economically important mite species that infests a wide variety of stored foods, particularly those with high protein and fat contents (Gulati and Mathur, 1995; Hughes, 1976; Sánchez-Ramos and Castañera, 2001). This mite is a major pest in nuts, grains, aged cheese, dried eggs, dry cured ham, dog food, mushroom beds, and mycology laboratory cultures (Arnau and Guerrero, 1994; Canfield and Wrenn, 2010; Duek et al., 2001; Eaton and Kells, 2009; Erban et al., 2016; García, 2004; Gill et al., 2011; Hughes, 1976; Kheradmand et al., 2007; Rentfrow et al., 2008; Sánchez-Ramos and Castanera, 2000). Moving mites and piles of pale brownish dust may appear on heavily infested products. The dust piles are consist of waste material, dead mites, and shed cuticle, which have pungent and/or minty odors (Nayak, 2006; Townsend, 2007).

Some fungi (usually Penicillium, Candida zeylanoides and Debaryomices hansenii) may be found on the outer surface of the ham (Dikeman and Devine, 2014), which positively influences mite viability and contributes to infestations (Okabe and Oconnor, 2001). Moreover, exposure to T. putrescentiae may contribute to allergic diseases, such as contact dermatitis and asthma, among farmers and workers who handle heavily infested stored products (Arlian et al., 2004; Armentia et al., 1994; Fernandez-Caldas et al., 2007; Hage-Hamsten et al., 1985; Revsbech and Dueholm, 1990).

The optimum environmental conditions for mite growth and reproduction include 25 to 32°C and 70 to 90% relative humidity (Boczek, 1991), which are similar to the environmental conditions in dry cured ham aging houses. Mite infestations occur in dry cured ham plants in the southeastern United States (Rentfrow et al., 2008), Spain (Arnau and Guerrero, 1994; Sánchez-Ramos and Castañera, 2001), and other countries, resulting in product rejection and economic losses (Armentia et al., 1994).

In a survey of country ham plants, 22 out of 34 plants reported mite infestations and fumigation with methyl bromide (MB) from 1 to 5 times a year. These plants found that MB is currently the only known space fumigant that is effective at controlling ham mites (Rentfrow et al., 2008). However, since MB is an ozone depleting substance, it was phased out of production and consumption as of January 1, 2005, with the exception of critical use exemptions (EPA, 2017), which were available to the dry cured ham industry through 2016. Beginning in 2017, the dry cured ham industry was only given approval to purchase and use the remaining stockpiles of MB until they are depleted (EPA, 2014). Therefore, it is important to explore alternative control measures to prevent mite infestations during dry cured ham aging since MB will eventually not be possible to use.

Phosphine (Zhao et al., 2015), sulfuryl fluoride (Phillips et al., 2008), controlled atmosphere (Hasan et al., 2016; Sánchez-Molinero et al., 2010), traps (Amoah et al., 2016), high temperature (Abbar et al., 2016b; Fields, 1992), low temperature (Abbar et al., 2016b), and sanitation (Rentfrow et al., 2006) have been evaluated for their efficacy at controlling mite infestations. Coating or rubbing hams during aging with vegetable oils or hot lard is frequently used in Spain to help prevent mite infestations (García, 2004). Researchers determined that lard, propylene glycol, canola oil, calcium sorbate, sodium sorbate, iodate salt, maleic acid, propanol, and butanediol decreased T. putrescentiae reproduction when compared to untreated controls (Abbar et al., 2016a; Abbar et al., 2012; Zhao et al., 2016).

Since propylene glycol was the most effective ingredient in the research by Abbar et al. (2012) and Zhao et al. (2016), xanthan gum + propylene glycol and carrageenan + propylene glycol alginate + propylene glycol coatings were developed that were effective at controlling mite reproduction on ham cubes under laboratory conditions (Zhao et al., 2016). Since gum and propylene glycol coatings were applied to the surface of the hams, an additional processing step is necessary. Therefore, research was conducted on the effectiveness of infusing coatings into special nets that are used to hang dry cured hams on racks during aging, which revealed that gum and propylene glycol infused nets effectively inhibited mite growth and reproduction over a 10-wk storage period under laboratory conditions (Zhang et al., 2017). Since lard is used in Europe to control mites and is less costly than propylene glycol, it was hypothesized that incorporating lard into the netting solution, along with propylene glycol could control mites and reduce costs. Therefore, in this study, ham nets were infused with different combinations of polysaccharide coating, lard and propylene glycol. The choice behavior of T. putrescentiae between ham cubes wrapped with untreated and treated nets were investigated in addition to the effect of using treated nets on mite reproduction during 10 wk of storage.

The purpose of this study was to develop nets in which lard was infused with food-grade coatings and evaluate their effectiveness at inhibiting mite contact and reproduction. The net used in this study was a blend of 50% polyester and 50% cotton with high stitch density (112 loops/cm²). The nets without a coating decreased the mite population since the nets reduced the meat exposure to mites and limited their actual locomotion, which is in agreement with the previous findings (Campbell et al., 2016; Zhang et al., 2017). There are a few commercially available mite-proof covers that are made from different materials, including a tightly woven cover with a pore size of 2 to 10 μm (Mahakittikun et al., 2006; 2009), cotton fabric coated with eugenol loaded chitosan (Jarupaiboon et al., 2007), and Chitosan/Ag+-impregnated textile (Rahel et al., 2013). These anti-mite covers prevent the settlement and development of house dust mites, Dermatophagoides farinae Hughes on household fabrics and other surfaces, but have not been tested for ham mites. However, the usability of a tightly woven cover in the ham industry needs to be evaluated because of its low air permeability (4.0 cm³/s/cm²). In addition, neither of the 2 fabrics that are used for protection from house dust mites can be applied in the ham industry due to the cost and the use of non-food-grade materials. The coated nets used in the ham industry should contain only food-grade (Generally Recognized As Safe, GRAS) ingredients that are approved for food contact (FDA 21 CFR; Baldwin et al., 2011), which are gum, propylene glycol and lard in this study, and have no side effect on ham aging process and ham quality.

Data from behavior assays indicated that T. putrescentiae avoided ham cubes with some of the treated nets, and they showed strong preference for oviposition on control ham cubes over any of the treated ham cubes. The reproduction test suggested that nets containing PG reduced the mite population and the inhibitory effect was maintained over the 10-wk storage period, which is in agreement with previous findings on research conducted on gums and PG infused nets (Zhang et al., 2017). The main differences between these 2 studies were the incorporation of lard into the netting formulation and the infusion of nets with different combinations of gum, lard, and PG, which was due to the fact that coating ham cubes with lard was effective at inhibiting T. putrescentiae reproduction and has been used in commercial practice (Abbar et al., 2016a; Zhao et al., 2016).

The 100% lard infused nets were avoided by T. putrescentiae for oviposition but did not decrease the mite population when compared to the net control. Since 2 ham cubes with untreated and lard treated nets were provided, mites likely chose to lay eggs on control ham cubes because they were preferred by the mites. However, ham cubes with lard infused in the nets could sustain mite growth if mites had no other options for growth. For example, in the reproduction studies, ham cubes that were covered with lard infused net was the only food source to mites. When lard coating was used instead of lard infused nets, the mite population was controlled in the reproduction assay (Zhao et al., 2016), which might be due to the fact that the lard coating provided a lipid barrier that blocked the access of mites to feed on the meat. In contrast, the mesh size of the nets that were used was not small enough to prevent mites from penetrating through the nets to the ham meat.

Propylene glycol is the key ingredient that inhibits mite infestations on dry cured ham (Abbar et al., 2016a; Zhao et al., 2016). Propylene glycol is a clear, viscous, and colorless liquid with minimal odor and a slightly bittersweet taste (Martin and Murphy, 2000). Propylene glycol is classified as a GRAS ingredient by the FDA when used as a direct additive in food, cosmetic, and pharmaceutical products (Anonymous, 1994; Fiume et al., 2012). When compared to the net control, the lard + low/medium PG infused nets reduced the mite population by 73.1/80.0%, 64.5/84.6%, and 85.3/81.6% for treatments that were immediately inoculated after netting, 4 wk after netting and 8 wk after netting ham samples inoculated, respectively. This confirms the effectiveness of PG at inhibiting mite reproduction.

For the combinations of gum, PG and lard, all treated nets reduced the number of mites as compared to the net control over 10 wk of storage, with reductions of the mite population by 63 to 99%. The inhibitory effect on mite reproduction was dependent on the inclusion of PG and lard. In general, as PG increased, the mite population decreased. Similarly, as lard increased, the mite population also decreased, but the rate of decrease was less than when PG was increased. When the same combination of lard and PG was used, CG + PGA treatments had fewer mites than XG treatments. Although there were no statistical difference among the four different treatments at each storage time point, CG + PGA + medium lard + medium PG had the fewest mites present and would be preferred for use because of the zero tolerance of mites on dry cured ham products in the United States (USDA 9 CFR301; USDA 9 CFR 416). When compared to previous research from our laboratory with XG / (CG + PGA) + medium PG infused nets (Zhang et al., 2017), the nets containing the same concentration of PG did not improve the inhibitory effect, indicating that adding lard to the formulation did not offer any added benefit.

Another important property of PG is that it helps control the water activity and thereby spoilage in intermediate moisture or semi-moist foods (Aldrich, 2014), which may explain no or less growth of fungi on the ham cubes that were covered with PG-containing nets. Nets containing PG inhibited fungal growth with only a few exceptions, including lard + PG treatments after 8 wk of storage and XG / (CG + PGA) + low lard + low PG treatments after 4 wk of storage. In comparison, fungal growth was present on ham cubes without nets and with untreated nets as well as with 100% lard treatments. Mold growth usually favored the reproduction of T. putrescentiae, which confirms that the T. putrescentiae mite is fungivorous (Okabe and Oconnor, 2001). Molds might serve as the food source of mites (Hubert et al., 2004), or a source of free water and shelter for mites (Canfield and Wrenn, 2010). In contrast to the moldy ham cubes, the samples with visible yeasts had a pungent smell that was obvious to the investigators and fewer mites than the same treatment without yeast. The typical mold and yeast colonies were isolated and saved for future species identification, which will be conducted by another student in our laboratory.

Dry cured ham develops the typical flavor through the breakdown of proteins and fatty acids into volatile compounds (Pham et al., 2008). In addition, dry cured hams lose moisture by water evaporation and air exchange during the aging, and the finished dry cured ham product has to lose at least 18% of its original weight (Marriott and Ockerman, 2004). Therefore, any coatings, including coated nets must be permeable to moisture so that the hams can lose more than 18% moisture and become shelf-stable. Nets containing lard, PG, or both retained more moisture than other treatments. Therefore, the ham cubes with these nets had higher water activity and lower weight loss than ham cubes with control nets after 4 and 8 wk of storage. This is due to the nature of lard and PG. Lard is strictly hydrophobic and presents a barrier to water vapor. Propylene glycol is a humectant. It is miscible with water and can easily bind water from the atmosphere. The samples used for moisture analysis were small ham cubes, weighing approximately 20 g. Whole hams that are aged for 4 to 6 mo usually weigh 5 to 7 kg, 300 times heavier than the small cubes. Therefore, the water activity, moisture, and weight loss difference between control and treated samples would be much less in commercial products. However, even though the control treatments had less moisture and a lower a_w, ham cubes were still able to lose moisture and have an acceptable aw, which was 0.76 after 8 wk of storage. Therefore, it is suggested that these nets could be used in commercial production to inhibit mite infestations since hams could still undergo moisture loss and the aging process.

The nets infused with CG + PGA + medium lard + medium PG were the most effective at inhibiting mite growth and reproduction, indicating that the combination of these ingredients was effective at controlling mite infestation and fungal growth. However, there were still mites alive after this treated net was applied, which did not meet the requirement of zero tolerance, even though there were fewer mites than the initial inoculation level of 20. It is recommended that mite infestations are controlled using an integrated pest management (IPM) program, with the use of coated nets as part of the program. In the IPM program, prevention should always take place at the earliest time of processing. For example, if the ham is infested, cleaning dry cured ham with vegetable oil, as mentioned earlier, only eliminates the mites on the outer layer but not mites in cracks; it would be much complicated to excavate mites inside the cracks. Thus, multiple hurdles need to be considered part of IPM program, including monitoring, sanitation, prompt action to possible infestations, fumigation, etc.