We get it. We're skeptics too. If you're going to deep dive, allow us to show you the way:
Official recommendation of the American Dental Hygienists Association:
“Tongue deplaquing with tongue scrapers—tools exclusively designed for use on the tongue—is as essential for fresh breath as regular brushing. Tongue scrapers provide even pressure that forces bacteria, food debris and dead cells from the pits and crevices in the tongue that a toothbrush cannot remove.”
Interventions for managing halitosis, May 2016:
"It is now fairly widely accepted that halitosis originates from the oral cavity (Ayers 1998; Delanghe 1997; De Geest 2016). Accumulation of bacteria and food residues at the posterior part and in the furrows of the tongue (Seeman 2014; van Steenberghe 1997) is considered to be the major cause (Scully 1997)...Reduction of the causative bacteria can also be accomplished through improving oral hygiene (Tonzetich 1978) in addition to cleaning of the tongue (Rosenberg 1996). This can be achieved by brushing or scraping the dorsum of the tongue to dislodge trapped food, cells, and bacteria from between the filiform papillae."
Frequency of Tongue Cleaning Impacts the Human Tongue Microbiome Composition and Enterosalivary Circulation of Nitrate, 01 March 2019:
"Tongue microbiome communities are of general interest in Eastern medicine because the appearance of the tongue coating is considered a manifestation of systemic health (Jiang et al., 2012a). In Western medicine interest has focused on the role of the microbiome in mucosal disorders (Docktor et al., 2012; de Paiva et al., 2016), and in dentistry the tongue microbiome has significant associations with halitosis (Ren et al., 2016). Regular tongue cleaning is recommended by the American Dental Association (http://www.mouthhealthy.org) based on evidence that cleaning can reduce the severity of halitosis (Pedrazzi et al., 2016)... Based on this study, tongue cleaning assumes a new importance from the perspective of blood pressure regulation, as daily tongue cleaning appears to favor the increased abundance and metabolic activity of nitrate/nitrite metabolizing species, such as H. parainfluenza and commensal Neisseria spp. (Barth et al., 2009)."
Halitosis: From diagnosis to management, Jan - June 2013:
"The number of bacterial species, which are found in oral cavity, are over 500, and most of them are capable to produce odorous compounds which can cause halitosis. In these conditions, poor oral hygiene plays a key factor for multiplication of halitosis causative bacteria and causes an increase in halitosis. These bacteria include especially Gr-negative species and proteolytic obligate anaerobes and they mainly retained in tongue coating and periodontal pockets. Among healthy individuals, with no history of halitosis and no periodontal diseases, some show halitosis because of retention of bacteria on the tongue surface. These bacteria degrade organic substrates (such as glucose, mucins, peptides, and proteins present in saliva, crevicular fluid, oral soft tissues, and retained debris) and produce odorous compounds."
Tongue-cleaning methods: a comparative clinical trial employing a toothbrush and a tongue scraper. July 2004:
"It is estimated that approximately 85% of all halitosis cases have their origin within the mouth; of these, 50% are caused by tongue residues. Previous studies have established that hydrogen sulfide and mercaptans are the primary components of halitosis. Thus, tongue cleaning gains importance as a means of halitosis management."
Scientists find bug responsible for bad breath, Reuters Health, April 2008:
"Persistent bad breath... is often caused by the breakdown of bacteria in the mouth, producing foul-smelling sulfur compounds that reside on the surface of the tongue. Tongue bacteria produce malodorous compounds and fatty acids, and account for 80 to 90 percent of all cases of bad breath."