Medicine®

Clinical Trial/Experimental Study

An effective salivation-facilitating method for
COVID-19 testing
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An experimental study
Misako Higashijima, PhDa, Hiroyasu Shiozu, PhDb, Tomoya Sakai, DEngc, Moemi Matsuo, PhDa,*Abstract
Background: Simple tools, such as antigen test kits, are readily available for determining coronavirus disease 2019 infection

at hospitals and homes. However, it is challenging for elderly people who are prone to dry mouth and other diseases. The main
objective of this study was to investigate whether the presence or consumption of a plum pickle can facilitate salivation during
coronavirus disease 2019 testing.
Method: Twenty healthy adult women participated in the study. The participants were allocated to 2 groups: presentationand non-presentation (n = 10; with and without presentation of a plum pickle, respectively), and eating and non-eating (n = 10;with and without consumption of plum pickle, respectively). We recorded the number of saliva swallows in 1 minute under eachcondition, using a swallowing test device, which attached film sensors to the hyoid bone and thyroid cartilage.
Results: There was a significant difference in the number of swallows between the non-presentation and presentation groups (P< .01, r = 0.89, Z = −2.82) as well as between the non-eating and eating groups (P < .01, r = 0.85, Z = −2.68).
Conclusions: The strength of 3 factors, namely: direct stimulation with citric acid, saliva buffer capacity, and motor learning,

may have affected the results. Our study suggests that saliva collection using the plum pickle is an effective complementary
method for facilitating salivation. This technique may be useful in avoiding the risk associated with citric acid intake and for efficient
specimen collection during coronavirus disease 2019 testing. In the future, we need to verify this method in elderly participantsin a clinical setting.
Abbreviations: COVID-19 = coronavirus disease 2019, PC = personal computer.
Keywords: COVID-19, elderly, salivation, swallow, unconditioned reflex

timeously. Thus, it is a difficult task for elderly people who areprone to dry mouth and other diseases.
Here, we focus on the usefulness of plum pickles, referred
to as “umeboshi,” which are traditional Japanese pickles with
red color and sour taste. The intake of acidic foods containing
citric acid, such as plums, leads to recovery from fatigue[2] andfacilitates salivation.[3] This study aimed to investigate whether
the presence or consumption of a plum pickle can facilitate salivation, using a swallowing test device.[4,5] Furthermore, if thepresence of plum pickle increases the number of swallows, wecan examine the effectiveness in promoting saliva during meals
in elderly people with decreased saliva volume. This study may
provide knowledge to develop a complementary method forsaliva specimen collection during COVID-19 testing.

1. Introduction
Although 3 years have passed since the coronavirus disease2019 (COVID-19) outbreak occurred in Japan and overseas,
there is no prospect of its disappearance. Simple COVID-19testing tools have become readily available as antigen test kitsfor determining infection at hospitals and homes. However, it isimportant to reduce the burden and further facilitate COVID19 testing. A nasopharyngeal swab or saliva is used as a sample
for antigen testing; however, saliva collection is easier at homebecause it causes less irritation and pain.[1] In order to improve
the accuracy of the antigen test, necessary precautions should
be taken; for example, neutral saliva (approximately at pH 7)
should be collected, and specimen collection must be performed

The authors have no funding and conflicts of interest to disclose.

Copyright © 2023 the Author(s). Published by Wolters Kluwer Health, Inc.
This is an open-access article distributed under the terms of the Creative
Commons Attribution-Non Commercial License 4.0 (CCBY-NC), where it ispermissible to download, share, remix, transform, and buildup the work provided
it is properly cited. The work cannot be used commercially without permission
from the journal.

The datasets generated during and/or analyzed during the current study areavailable from the corresponding author on reasonable request.
Faculty of Rehabilitation Sciences, Nishi Kyushu University, Saga, Japan,
Department of Occupational Therapy, Chubu University, Kasugai, Japan,cSchool of Information and Data Sciences, Nagasaki University, Nagasaki,
Japan.
ab

How to cite this article: Higashijima M, Shiozu H, Sakai T, Matsuo M. An effective
salivation-facilitating method for COVID-19 testing: An experimental study.
Medicine 2023;102:27(e34370).

*Correspondence: Moemi Matsuo, Faculty of Rehabilitation Sciences, Nishi
Kyushu University, 842-8585, 4490-9 Ozaki, Kanzaki, Saga, Japan (e-mail:matsuomo@nisikyu-u.ac.jp).

Received: 2 March 2023 / Received in final form: 25 June 2023 / Accepted: 26
June 2023
http://dx.doi.org/10.1097/MD.0000000000034370

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Higashijima et al. • Medicine (2023) 102:27Medicine

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Figure 1. Swallowing test device. The device consists of 3 parts (i.e., a personal computer and 2 film sensors) and an interposer.

computer (PC) with the data collection and analysis program,
2 film sensors to collect swallowing data, a web camera toobserve their state during the experiments, and an interposerthat connected these 3 parts (Fig. 1). The swallowing reflex
waveform and the number of seconds between waveforms weredisplayed on a PC screen, and the collected data were saved on
the PC itself (Fig. 2).
2.2.2. Outcome measures. In experiment I, the numberof swallows with and without the visual presence of a plum
pickle was collected. In experiment II, the number of swallows
with and without plum pickle consumption was recorded.
The experimental procedure was counterbalanced, and theparticipants were asked to perform each action once according
to the following procedure: During the experiments, participants
were instructed to swallow their saliva as often as possible. Inaddition, they were asked to refrain from speaking so that theexperiments could be accurately performed. The number ofsaliva swallows in 1 minute under each condition was collectedusing the swallowing test device, which attached film sensors to
the hyoid bone and thyroid cartilage.

Figure 2. Swallowing reflex waveform. The red waveform (the upper wave)
indicates hyoid bone movement, and the blue waveform (the lower wave)
indicates thyroid cartilage movement.

2.2.3. Procedures. In experiments I and II, film sensors
were attached to the hyoid bone and thyroid cartilage while
participants were sitting on a chair (Fig. 3), which was adjusted
to the position where the inspection site could be seen. Data
were collected using a data acquisition program inserted into aPC. The experimental procedure is illustrated in Figure 4.

2. Methods
2.1. ParticipantsTwenty healthy adult women participated in the study and weredivided into 2 groups: presentation and non-presentation (withand without presentation of a plum pickle, respectively; experiment I; n = 10; mean age 20 ± 9.0 years), and eating and non-eating (with and without consumption of plum pickle, respectively;
experiment II; n = 10; mean age 22 ± 1.0 years).
The participants provided informed consent at their workplaces. The study was approved by the Ethics Committee of
Nishi Kyushu University (approval no. 22JRV27) and compliedwith the Declaration of Helsinki.

2.3. Data and statistical analysis
The number of swallowing reflexes that occurred in 1 minuteduring experiments I and II were obtained by the data acquisition program, and the waveform data were analyzed using theanalysis program. The data were analyzed to compare the conditions in each experiment using the Wilcoxon signed-rank test—anonparametric test with an almighty property. Considering thenumber of participants, we selected the method unaffected byoutliers. Two-sided probability values of <.05 were consideredsignificant. The effect size (r) was calculated from the Z-score ofthe Wilcoxon signed-rank test[6] to estimate the effect size. All

2.2. Experimental setup
2.2.1. Swallowing test device. The swallowing test device(Science Research Co., Nagasaki, Japan) consisted of a personal2

Higashijima et al. • Medicine (2023) 102:27www.md-journal.com

confirmed that ingesting a plum pickle, an acidic substance, promotes swallowing. This could be caused by a chemical stimulusof citric acid, which is the main component causing sourness,and the volume of saliva secreted increased due to the gustatory
sensory input caused by plums.
4.2. Saliva buffer capacity
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The pH of the oral cavity during swallowing became acidic dueto the plum pickle, and the buffering ability of saliva may havepromoted salivary secretion to maintain the pH in the oral cavity. At rest, the pH in the oral cavity is close to neutral.[8] It isconsidered that the buffering capacity of saliva worked strongly
to return the oral cavity to neutrality during swallowing because
the plums used in this study exhibited strong acidity with pH
2 to 3. Moreover, buffering capacity increases with increasing liquid outflow.[9] Therefore, salivary secretion is promoted
during swallowing because a higher salivary buffering capacity
is required during swallowing than during presentation, whichis affected only by the conditioned reflexes.

Figure 3. The attached film sensors. For collecting the swallowing data,
1 film sensor is attached to the hyoid bone (upper part), and the other is
attached to the thyroid cartilage (lower part).

4.3. Motor learning

Significant differences were observed between the non-presentationand presentation and non-eating and eating groups. Moreover,the effect sizes were large in both experiments. The result can becharacteristically considered due to the decline in the image of traditional Japanese pickled plums and eating habits among healthy
adults. Thus, the result may be affected by subject specificity. Thefindings suggest that the strengths and weaknesses of the 3 following factors may have affected the number of swallows.

The stimulation of saliva secretion is possibly affected by
motor learning under presentation and eating conditions. Theseresponses are unconditioned reflexes that humans and animals
originally have in response to a specific stimulus. The combination of the central stimulus of looking at a plum (the stimulusthat does not trigger the unconditioned reflex) and the unconditioned stimulus of eating a plum (the stimulus that triggers
the unconditioned reflex) trigger the unconditioned reflex ofsaliva production. Repeatedly giving an unconditioned stimulusimmediately after giving a central stimulus causes an unconditioned reflex only through central stimulation; this process iscalled classical conditioning.[10] The effect size of the conditionof presenting plums was slightly larger than that of the eating
condition because the conditioned reflex[11] that was caused by
the experience of tasting sourness and its visual memory caused
them to feel as if their mouth was sour.
Herein, compared to the conditions of non-presentation andnon-eating, presentation and consumption showed a clear effecton the number of swallows. Furthermore, the effect size waslarge for both presentation and consumption habits. This result
reveals that collecting saliva specimens during the visual capture of acidic substances could be effective. Although it has beenreported that aspiration of acidic substances can cause severe
damage to the lungs and airway mucosa,[12] presenting conditions can lead to risk avoidance and increase saliva volume
while adhering to precautionary measures for sample collection
during COVID-19 testing. In future research, the effect needs tobe verified in clinical practice.
In addition, the large effect size, even in just presenting plum,suggests that it can be used as a preventive method for oral
frailty in the elderly in Japan, where the birthrate is declining
due to hyper-aging.
This study had several limitations. First, participant selection
may have led to bias as all participants were recruited from asingle prefecture in Japan. Second, all participants were healthy
young adults. Third, the sample size was small. The volume ofsaliva swallowed due to the presentation of plums might behigher in the elderly than in younger adults due to long-term
learning effects. In future research, the generalizability of ourfindings should be assessed in terms of the actual target of thismethod with a larger sample size and in other countries.

4.1. Stimulation with citric acid

Acknowledgments

Acidic substances promote swallowing, and salivary secretion increases as the acid concentration increases.[7] This study

The authors would like to extend their gratitude to the study
participants.

Figure 4. Experimental procedure. Swallowing data were collected at T1and T2. Data were collected by counterbalancing T1 and T2 and standardizing other procedures and contents. (A) The participants were asked to drink
room-temperature water (approximately 10 cc) to standardize dry mouth
among the participants. T1: After confirming that the participants drank water,
they were asked to swallow saliva for 1 min while looking at the cross point
on white paper on the wall 20 cm in front of them. (B) The participants wereasked to wash their oral cavity using a wet oral tissue or water. (C) The participants were asked to drink room-temperature water (approximately 10 cc)
in order to refresh their mouth’s watering. T2: After confirming that the participants drank water, they were asked to swallow saliva for 1 min while looking
at a plum pickle placed 15 cm in front of them (in experiment II, participants
consumed the plum pickle).

statistical analyses were performed using SPSS Statistics (version28.; IBM Corp., Armonk, NY).

3. Results
In experiment I, the number of swallows was 4.4 ± 1.15 and
7.2 ± 2.35 without and with the presentation of a plum pickle,
respectively; the difference was significant (P < .01, r = 0.89, Z= −2.82), indicating a large effect. In experiment II, the numberof swallows was 7.8 ± 3.16 and 11.5 ± 2.33 without and with consumption of a plum pickle, respectively; a significant difference wasobserved (P < .01, r = 0.85, Z = −2.68), indicating a large effect.

4. Discussion

3Higashijima et al. • Medicine (2023) 102:27Medicine

Author contributions

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Conceptualization: Misako Higashijima, Tomoya Sakai, Moemi
Matsuo.
Data curation: Misako Higashijima, Moemi Matsuo.
Formal analysis: Hiroyasu Shiozu.
Investigation: Misako Higashijima.
Methodology: Misako Higashijima, Moemi Matsuo.
Project administration: Misako Higashijima.
Resources: Misako Higashijima.
Supervision: Misako Higashijima.
Visualization: Misako Higashijima.
Writing – original draft: Misako Higashijima.
Writing – review & editing: Misako Higashijima, Moemi
Matsuo.

References[1] Iwasaki S, Fujisawa S, Nakakubo S, et al. Comparison of SARS-CoV-2
detection in nasopharyngeal swab and saliva. J Infect. 2020;81:e145–7.
[2] Sugino T, Aoyagi S, Shirai T, et al. Effects of citric acid and L-carnitineon physical fatigue. J Clin Biochem Nutr. 2007;41:224–30.

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