Drying and decontamination of pistachios with sequential infrared drying, tempering and hot air drying

Authors: VENKITASAMY, CHANDRASEKAR - University Of California; Brandl, Maria; WANG, BINI - Northwest University; McHugh, Tara; Pan, Zhongli

Submitted to: International Journal of Food Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/7/2017
Publication Date: 2/9/2017
Citation: Venkitasamy, C., Brandl, M., Wang, B., McHugh, T.H., Pan, Z. 2017. Drying and decontamination of pistachios with sequential infrared drying, tempering and hot air drying. International Journal of Food Microbiology. 246:85-91. doi:10.1016/j.ijfoodmicro.2017.02.005.

Interpretive Summary: Industrial sorting of pistachios in water bath before air drying results in significant moisture gain on the nut surface. The hulled pistachio nuts contain 40 to 50% (wet basis) of moisture and therefore, quick drying of pistachios is vital to avoid shell staining, decay, microbial contamination, and increase of shelf life. In the current study, dehulled and water-sorted pistachios with moisture content of 38.14% (w.b.) were dried in a sequential infrared and hot air (SIRHA) drier to less than 9% MC and the decontamination effect was studied by inoculating pistachios with Enterococcus faecium, a common surrogate of Salmonella enterica. The results showed that SIRHA drying of pistachios with 2h infrared (IR) plus 2h tempering at a product temperature of 70 oC followed by hot air drying provided a saving of drying time by 40 min (13.1%) compared to drying with hot air alone. Drying with IR alone saved 105 min (34.4%) of drying time when compared to hot air drying. The SIRHA method with 2h IR plus 2h tempering followed by hot air drying also provided additional benefit of Enterococcus faecium cell reduction of 6.Hog CFU/g in kernels and 5.41 log CFU/g in shells of pistachios. Even though significant differences in peroxide value and free fatty acid contents were observed for pistachio kernels dried by the SIRHA method compared to hot air drying at 70oC, the values were within the permissible limit for edible oils. These findings demonstrate that SIRHA drying method could be used produce safe pistachios with reduced drying time.

Technical Abstract: The pistachio industry is in need of improved drying technology as the current hot air drying has low energy efficiency and drying rate and high labor cost and also does not produce safe products against microbial contamination. In the current study, dehulled and water- sorted pistachios with a moisture content of 38.14% (w.b.) were dried in a sequential infrared and hot air (SIRHA) drier to less than 9% MC and the decontamination effect was studied by inoculating pistachios with Enterococcus faecium, a common surrogate of Salmonella enterica. The results showed that SIRHA drying of pistachios with 2h infrared (IR) plus 2h tempering at a product temperature of 70 oC followed by hot air drying provided a saving of drying time by 40 min (13.1%) compared to drying with hot air alone. Drying with IR alone saved 105 min (34.4%) of drying time when compared to hot air drying. The SIRHA method with 2h IR plus 2h tempering followed by hot air drying also provided additional benefit of Enterococcus faecium cell reduction of 6.Hog CFU/g in kernels and 5.41 log CFU/g in shells of pistachios. Even though significant differences in peroxide value and free fatty acid contents were observed for pistachio kernels dried by the SIRHA method compared to hot air drying at 70oC, the values were within the permissible limit for edible oils. These findings demonstrate that effectiveness of SIRHA drying to achieve simultaneous drying and decontamination of pistachios.