MAINE POTATO BOARD- PROJECT REPORT 2006

 

Principal Investigator: Vivian Chi-Hua Wu, Ph.D.

Co-Principal Investigators: Alfred Bushway, Ph.D.

 

Executive Summary

Potato plants contain important phenolic compounds, which are believed to play an important role in a plant’s general defense mechanism in resistance to microbial infection. Potatoes have also been known to contain antioxidants within their peel which may coincide with their antimicrobial properties. In this study, Native Maine potato tubers and peel were evaluated for antimicrobial effects on foodborne pathogens, Echinacea coli O157:H7 and Salmonella Typhimurium as a natural food preservative to promote food safety. A total amount of $6,300 is supported by the Maine Potato Board to help establish the first year objective of the project.

            Different extraction methods such as using ethanol for extraction with and without freeze-drying, juicing with and without freeze-drying, as well as squishing/chopping for potato tubers and peel were evaluated. An Agar diffusion assay and a bacterial log reduction method were utilized to study the antimicrobial effects of potatoes. Upon observing a few promising results from the potato tuber juice (50% w/w) on Salmonella Typimurium using the disc diffusion assay (showed slight reduction, but not a complete zone of inhibition), it was decided that the disc diffusion method might not be sensitive enough due to such a small amount of possible antimicrobial compounds in the potato.

An antioxidant test using DPPH was performed for the determination of antioxidant levels within the potato constituents. As antioxidant levels are known to show a linear relationship with antimicrobial compounds, antioxidant levels within the tuber, leaves, and peel of the potato were tested. Specifically, the peel showed antioxidant activity and was further evaluated for antimicrobial activities utilizing the log reduction method. The results showed that potato peel juice (50% w/w) reduced E. coli O157:H7 by 2 log CFU/ml after 24 hr when compared to the control, indicating that potato peel contains natural antimicrobial compounds which can inhibit the growth of E. coli O157:H7.

The study has demonstrated that potato peels naturally contain compounds which have been associated with possessing antimicrobial properties.  However, these compounds present in the potato peel do not appear to be in a high enough concentration to prove constant effects. Further experiments for confirmation are needed.  The potato storage time, age, pH, and even glycoalkaloid contents will be evaluated on the effect of a potatoes’ antimicrobial activity in the future study. 

 

Project Duration

The objectives of the proposed project are planned to be completed within 3 years (April 2006 to March 2009). The funding requested from the Maine Potato Board are partially support this project in the first year (April 2006 to March 2007). This report presents the progress from April 2006 to December 2006.

 

Reported Objectives:

 

Project Description

Rationale and significance.

Plant phenolics are currently of growing interest owing to their supposed functional properties in promoting human health and antimicrobial effects in promoting food safety. We have studied several of plant extracts such blueberries, cranberries, and plum which contain phenolics to control foodborne pathogens such as Escherichia coli O157:H7, Listeria monocytogenes, Salmonella Typhimurium, and Staphylococcus aureus. With the addition of these natural ingredients in food (such as ground beef and juice), the growth of foodborne pathogens are inhibited. Potato plants contain important phenolic compounds including free phenolic acids, phenolic acid esters, and isoflavones and their glucosides. Phenolic acids are believed to play an important role in a plant’s general defense mechanism in resistance to microbial infection. The phenolic compounds of potatoes have potential for antimicrobial effects on foodborne pathogens as a natural food preservative to promote food safety.

 

Methodology.

The antimicrobial effects of Maine potato plant tubers and peel were studied for controlling major foodborne pathogens such as E. coli O157:H7 and S. Typhimurium.

 

Potato tuber juice treatment on S. Typhimurium using the agar diffusion method

Katahdin potatoes were received from northern Maine. The samples were washed, peeled, cut, juiced, centrifuged, frozen (-80oC for 24 hr), and frozen-dried (-47oC for 24 hr).

Freeze-dried extract was diluted with distilled water to 50% (w/w).  One hundred μl of Salmonella Typimurium (two strains at 1x108 CFU/ml) were inoculated on four Xylose Lysine Desoxycholate (XLD) plates (two tests, 2 controls) to create a “lawn of bacteria”. After 10 min of drying, three sterile discs (0.5” in diameter) were placed on each of the four plates in uniform.  Two plates (6 discs, 100 μl/discs) each were inoculated with sterile distilled water and the other two plates (6 discs, 100 μl/discs) were inoculated with potato extract. The plates were dried for 10 min, inverted, and incubated for 24 hr at 37oC.

 

Potato peel juice treatment on Escherichia coli O157:H7 using the log reduction method

Russet potatoes were purchased from a local grocery store. Potatoes were washed, peeled under hood and treated under UV germicide for 20 min. Peel was cut into fine pieces and ground using small food processor.  Sterile distilled water was added into the ground peel to prepare 50% w/w pee juice.  Samples were continuously ground until frothy (20 min). 

For the treatment, 1 ml of E. coli O157:H7 culture (5 log CFU/ml) was added with 4 ml of prepared peel samples. For the control, 4 ml distilled water was added into the other 1 ml of E. coli O157:H7 culture (5 log CFU/ml). Viable cell counts were performed using MacConkey Sorbitol agar (MSA) at 0 hr and 24 hr. Log reductions were evaluated.

 

Results.

Potato tuber juice treatment on S. Typhimurium using the agar diffusion method

There is a zone of inhibition which makes a dent in the surrounding bacteria indicating the antimicrobial effect of potato tuber juice on S. Typhimurium (Fig 1).  However, the clear zone is not as big as that resulted from a diluted bleach solution as a positive control. This indicates that potato tubers contains antimicrobial compounds, however the concentration of these natural antimicrobials is much lower than these in the bleach solution.

 

Potato peel juice treatment on Escherichia coli O157:H7 using the log reduction method

The comparison between potato peel juice treatment (50% w/w) and distilled water (control) for antimicrobial effects on E. coli O157:H7 is indicated in Figure 2. Potato peel juice reduced E. coli O157:H7 by 2 log CFU/ml after 24 hr when compared to the control, indicating that potato peel contains natural antimicrobial compounds which can inhibit the growth of E. coli O157:H7.

 

Discussion

            Upon studying the antimicrobial effects of potato tubers and peel, there have been positive results against foodborne pathogens when utilizing both the disc diffusion assay as well as the log reduction method.  Some of the above results were quite promising, however they were not as readily reproducible, therefore further confirmation is needed.

            The disc diffusion assay showed slightly positive results as seen above. The slight dent in bacteria flora is promising and reproducible (3 times), but still there is not a complete inhibition within the zone. This maybe due to the antimicrobial compounds not being released appropriately from the disc in order to see completed results in inhibition.

            The log reduction experiment showed positive results, however, the experiments will be further repeated to confirm the results. The potato storage time, age, pH, and even glycoalkaloid contents will be evaluated on the effect of a potatoes’ antimicrobial activity in the further study. 

Discovery of antimicrobial properties of potatoes in this project will allow the food industry to utilize these valuable natural ingredients and use as “natural” additives in food to control foodborne pathogens. The funding supported by the Maine Potato Board helped us to initiate the project and generate useful data for seeking other funding to continue this project beyond the first year. Significant results obtained from the project will promote potatoes with multiple functions and may develop new markets for value-added potatoes.

Figure 1. Comparison of antimicrobial effects between (a) 50% (w/w) potato tuber juice and (b) distilled water (negative control) on Salmonella Typhimurium.

 

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Figure 1. Comparison of antimicrobial effects between (a) 50% (w/w) of potato peel juice and (b) distilled water (negative control) on Escherichia coli O157:H7.

 

 

 

 

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