In this guide we use the phrase "covid" and/or "coronavirus" to refer to the Coronavirus disease 2019 (COVID-19) as caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2).

Covid Related Loss of Smell

The CDC list "New loss of taste or smell" as one the main symptoms of covid. Sickness related loss of smell is nothing new, even the common cold can cause it to happen. However, due to the global pandemic scope of covid, this issue has received renewed and increased significance. There have been various news stories covering this issue and how it has impacted people's ability to "sniff" for dirty laundry, rotten food, etc.

We thought this issue would make for some great science and wanted to see if electrical sensors could be used. Here we share our results on making a milk freshness tester.

How Milk Spoils

We use the term "bad" to describe "spoiled" milk, but really it's just nature doing its thing. There are numerous bacteria that love milk. Unfortunately for us, their consumption of milk produces byproducts that we find distasteful and can even be harmful.

Spoilage is also a vague term. As this paper (PDF) puts it:

Milk spoilage is an indefinite term and difficult to measure with accuracy.

This paper summarizes various techniques that have been investigated as a way to detect spoilage, including:

• pH
• electrical methods
• magnetoelastic sensors
• gas sensor arrays
• infrared spectroscopy
• protein count

It is the gas sensor based approach we are interested in here. This is analogous to what we do when we sniff milk to check freshness. Our noses are essentially gas sensors. But if covid (or something else) has knocked out your sense of smell, then your gas sensor is broken. Can we use an electrical gas sensor instead?

Electrical Nose

The general of idea of gas sensor based milk spoilage detection has been investigated by others.

• Application of gas-sensor array technology for detection and monitoring of growth of spoilage bacteria in milk: A model study.
  • Evaluation based on change over time.
• Milk-sense: a volatile sensing system recognises spoilage bacteria and yeasts in milk.
  • Conducting polymer detector for microbial volatiles.

There are various gases that are discussed amongst these papers, but a common approach is to use microbial produced volatile organic compounds (VOCs) and carbon dioxide (CO2). Well, the SGP30 from Sensirion , as used on the Adafruit SGP30 STEMMA QT breakout, can detect both of these.

So can the SPG30 be used to create an "electric nose"? Well, let's science this and find out.