- Taste, smell, and appearance are the three key characteristics that consumers use to judge the quality, safety, and acceptability of drinking water and aquaculture products.
- In addition, the global phenomenon of increasing freshwater salinity can affect the quality, taste, and use of water for human consumption and other activities.
- On the other hand, technology is advancing rapidly in the detection and treatment of tastes and odors (T&O, Tastings & Odorants), as well as in the identification of algae and cyanobacteria cells. It also improves the understanding of consumers’ sensory perception, thus contributing to better drinking water service and environmental care.
Relationship between TDS and people’s taste for drinking water
Total dissolved solids (TDS), which measures the mineral content of drinking water and is an essential resource for life, is a key factor in determining whether the taste is pleasant to consumers.
However, the TDS levels that influence taste vary depending on the individual, population, and region. A typical range is between 100 and 350 mg/L of TDS.
Water desalination using membrane treatment technologies seeks to achieve a TDS level that is acceptable for human consumption and palatable to consumers.
Recent research shows that when TDS changes around 150 mg/L, consumers can perceive a difference in palatability. Therefore, it is essential to understand that a “pleasant” or “acceptable” TDS level is variable.
Consumers evaluate taste changes in relation to the quality of the water they typically drink in their home or community. Although overall TDS is the most important determinant of taste, specific ions can have a positive influence—such as Ca²⁺, SO₄²⁻, or HCO₃⁻—or a negative influence—such as an excess of Na⁺, Cl⁻, or CO₃²⁻.
Odor and challenges for drinking water
Identifying and controlling chemical odors has long been a major challenge in maintaining the quality, sanitation, and safety of drinking water worldwide.
China recently conducted research on odorant contamination at 111 drinking water treatment plants across the country. The results showed that more than 80% of the source water had odor problems. These odors were described as earthy or musty (41%) and swampy or septic (36%).
In general, water from lakes and reservoirs had more algae-derived odors, while water from rivers had more anthropogenic odors.
Subsequently, between 2015 and 2018, 100 aesthetic incidents were investigated at 140 treatment plants in 32 cities. At these plants, 87 odorous compounds were detected in raw water and 85 in treated water, with concentrations ranging from neutral levels to thousands of ng/L.
In China, 2-MIB was identified as the substance responsible for the musty or earthy odor, while dimethyl disulfide was the main cause of swampy or septic odors.
The Huangpu River, near Shanghai, has long faced a complex odor problem, including fishy, solvent, and septic compounds. The chemicals that typically cause these odors are 2-MIB, geosmin, dimethyl disulfide, and other similar compounds.
In addition, the co-occurrence of some cyclic acetals possibly linked to the resin industry was discovered. These compounds may also contribute to the chemical or septic odor of the Huangpu River water.
Beyond the incidents recorded in China, similar events have also been reported in other parts of the world, such as the Llobregat River and Barcelona (Spain) or in Virginia (United States).
Potential effects of freshwater salinization on taste, odor, and algae presence
Freshwater salinization and the palatability of drinking water represent a global challenge for the sustainability of the water sector in the 21st century. This phenomenon, which involves an increase in specific conductance, TDS, cations, and anions in surface and groundwater, affects both the quality and availability of the resource, impacting the collection and supply of drinking water.
As a result, salinization can alter the taste of drinking water and modify the ecological balance of surface and groundwater bodies, directly impacting ecosystems, the environment, and public health.
Salinization has multiple causes. For example, changes in river flows, sea level rise, storm surges, saline intrusion, agricultural irrigation, infrastructure deterioration, climate change, and inadequate water management criteria are some of the factors contributing to this process.
As a result, salinization complicates the supply of safe and palatable drinking water, also affecting the hygiene and well-being of communities and increasing the risk of diseases related to the consumption of contaminated water.
The point at which consumers notice a change in the mineral taste of their water is not yet known with certainty. However, this threshold depends on the type of ions present, the water source, and the consumer group, as well as specific consumption situations.
For this reason, the management of dissolved minerals that make up TDS will require more advanced and expensive membrane technologies, as well as adequate education and control to ensure the quality of drinking water for millions of people.
Biomolecular monitoring of cyanotoxins-producing cyanobacteria and chemicals that produce tastes and odors (T&O)
Cyanobacteria that generate cyanotoxins and T&O compounds are usually quantified by taxonomic identification and cell counting using microscopy. This method is used to ensure the quality of source water and produce safe drinking water.
However, although microscopy allows organisms to be identified down to the species level, it requires specialized professionals and cannot distinguish between those that produce toxins or T&O and those that do not.
For this reason, the water industry has adopted more advanced techniques, such as the quantification of phycocyanin and chlorophyll-a using online fluorometers. Even so, since cyanobacteria are not always classified by species, these methods offer limited information about the true toxin producers.
Recently, biomolecular techniques such as quantitative polymerase chain reaction (qPCR) have been used to quantify toxin- and odor-producing cyanobacteria and actinomycetes in lakes, reservoirs, and fish ponds.
These studies, conducted in Australia, China, Japan, South Korea, the Philippines, Taiwan, and other countries, are based on the detection of functional genes responsible for metabolites such as geosmin. Genetic abundances correlate with observed concentrations of metabolites, cyanotoxins, and T&O compounds.
A study conducted between 2012 and 2016 demonstrated that biomolecular monitoring techniques can be used as an alternative to traditional risk management tools for T&O compounds and cyanotoxins in drinking water sources.
Sensory techniques in the aquatic environment
Although sensory analysis has a long history in the food and beverage industry, including aquaculture, its application in the drinking water industry is relatively recent.
Flavor Profile Analysis (FPA) is the most widely used descriptive method. In this method, a panel of judges identifies the organoleptic characteristics of water samples and classifies them according to the T&O wheel, which includes descriptors of odor, taste, and mouthfeel.
The threshold odor number (TON) is based on repeatedly diluting a water sample until the odor disappears and the taste appears neutral. Due to its simplicity, TON remains the reference technique in most regulations, although its usefulness and theoretical basis have been widely debated.
Other techniques applied in the drinking water industry include flavor rating analysis (FRA), total intensity of odor (TIO), and attribute rating test (ART).
Depending on the mineral content of the water and other characteristics, aesthetic tests are also used to evaluate its appeal, considering factors such as temperature, disinfectant level, or organic matter.
Differential tests are also very useful for determining whether consumers can detect changes between different water sources or treatments.
Future challenges and alternatives
The detection, identification, and quantification of toxins, odorants, and cyanotoxins in concentrations ranging from ng/L to mg/L will continue to be a major challenge for the drinking water industry. In addition, data sharing between academics and professionals will be key to ensuring public safety and health.
In 2021, Devi et al. created a comprehensive database called CyanoGM Explorer, which collects information on T&O events and cyanobacteria. The water sector can use this tool to consult and update data on cyanobacteria producers, their geographical distribution, and frequency, contributing to sustainable development and service improvement.
On the other hand, the monitoring of organic odorants remains a field of constant innovation. The most effective analytical techniques for identifying odor-causing substances are Two-Dimensional Gas Chromatography–Time-of-Flight Mass Spectrometry (GC×GC-MS) and Gas Chromatography–Quadrupole Time-of-Flight Mass Spectrometry (GC-Q-TOF/MS).
In addition, there is an urgent need to develop more accurate in situ analytical tools, such as olfactory biosensors or electronic noses with greater stability and selectivity.
The co-occurrence of different odorants can intensify the perception of odor in consumers, as chemical combinations can generate synergistic or antagonistic effects.
To better control odor events, it is essential to locate the source of contamination and create a database with the chemical fingerprints of the compounds responsible.
Finally, the collection of nonverbal responses and emotions from consumers. For example, facial analysis or pupillometry is increasingly used in the food and beverage industry. This approach could also be applied to the drinking water sector as an alternative method for assessing product acceptability.