A preservative is a chemical or compound added to a product to prevent the breakdown or decomposition of the product due to microbial growth (Preservative, 2018). In the case of instant coffee, the production process produces the preservation, creating a chemical structure that resists spoiling quickly. The biggest contributor to this is that the dehydration process removes the majority of the water content, meaning only approximately one to four percent of moisture remains (Moore, 2008). This level of water content prevents microbes from growing as long as the grounds are kept away from moisture in a sealed container.

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References:

1. Moore, K. (29, September 2008). What’s that Stuff? Instant Coffee. Chemical and
   Engineering News (86)
2. Preservative.  https://en.wikipedia.org/wiki/Preservative