Saving Your Harvest For Later: Water Activity And Food Preservation

This text will make you query all the things you thought you knew about water. The next method revolutionizes the protection of meals and dry flower merchandise bought for human consumption however has been used for many years within the meals business.

One of many largest points growers face is conserving their stunning produce from spoiling and changing into ravaged by microbes. This ‘takeover’ occurs within the type of fungal or bacterial development or elevated enzyme exercise, inflicting the breakdown of the constructions. The looks, style, and scent are affected by this spoilage; some biomass and important compounds are additionally misplaced. These will be nutritionally and medically important compounds, relying on the plant. However, most significantly, the protection of the product for consumption is compromised.

Rotten tomatoes on a tomato plant.

What Causes Spoilage?

The water current causes this spoilage and decreased shelf life within the plant; the upper the water content material, the elevated danger of microbial assault. However why do some meals take longer to start this course of?

When water is in an equilibrium state, it strikes from a liquid/aqueous state to a gasses/vapor state. Water can also be classed as a solvent, and it’ll fortunately maintain salts and sugars together with many different constituents referred to as solutes. The addition of solutes to water alters the equilibrium state and its habits. Not solely that, however its temperature additionally has an impact. Food has many various solutes at differing concentrations. When the water is eliminated, these concentrations enhance and assist increase the meals’s shelf life.

Numerous Food Preservation Strategies

Salts and syrups have been used for a few years to protect meals. Radiation is one other methodology that has been employed, killing something current on the floor of the produce. Nonetheless, these strategies will be detrimental to some risky compounds, lowering their dietary and useful properties. Ozone and steam are additionally meals preservation methods. Nonetheless, the injury precipitated to delicate fruit is extra problematic than the answer it creates.

Understanding the chemistry behind why meals spoils is important. It permits us to develop a extra tailor-made preservation methodology slightly than simply including substances or utilizing remedies that can have anti-microbial results.

What Is Water Exercise?

Water exercise is an idea that has been round for over 50 years and has helped with meals security, product design, and shelf-life ensures. Decreasing the water exercise of a meals substance helps forestall the expansion of microbes; it’s not classed as a kill step, as cells are viable in powdered milk however by no means develop.

An assortment of dried fruits in wooden bowls.

The strict definition of water exercise (aw) is the partial vapor stress of water in an answer (p) divided by the usual state partial vapor stress of water (p*).

aw = p/p*

However what does this imply in easy phrases?

It’s the thermodynamic exercise of the water, often known as the vitality the water has as a solvent (p) and the relative humidity of the encompassing air (p*). Because the temperature will increase, so does the aw as a result of it has higher vitality. The extra vitality the water has, the higher its capability to assist life. The relative humidity of the air surrounding that water additionally performs an vital function.

Distilled water has an aw of 1, giving it the best capability to assist life. However, dried fruit has an aw of 0.6, lowering the possibility of microbial colonization and spoilage. If this dried fruit is uncovered to dry air with an aw of 0.5, the water will migrate from the fruit into the air, preserving it additional. Nonetheless, with an aw of 0.8, humidity ends in the fruit absorbing the humid air and spoiling quicker—the decrease the aw of a substance, the much less probability of microbial development attributable to desiccation.

How To Measure Water

There are a few methods to measure water: a resistive electrolytic hygrometer, a capacitance hygrometer, or a dew level hygrometer. The primary of the above strategies follows the precept of resistance. There are electrolytes within the glass rods of the meter that change resistance straight proportional to the relative humidity. The second methodology entails water being absorbed right into a membrane, rising its skill to carry a cost, which we will then measure. The quickest and most correct methodology is the dew level methodology, which entails cooling a mirror till the dew level and measuring this with an optical sensor.

A cropped hygrometer used to measure water activity, in front of an out of focus green plant.

So, what’s the finest aw when curing and stopping mould development?

Keep in mind, it’s not simply in regards to the aw of the product you might be preserving; it’s also the relative humidity of the encompassing air. Utilizing hermetic containers and desiccating crystal sachets are good methods to softly assist the migration of the water from the cells. An aw of 0.6 is good when drying fruit and herbs earlier than putting them into the seal storage vessel, the place you’ll be able to management the humidity. Under 0.6, no microbes can proliferate.

Don’t rush the curing course of if you wish to protect the flavors of your produce. On the similar time, inadequate drying can lead to loss by way of decomposition of the biomass. Though it could appear difficult, utilizing water exercise to assist extend shelf life and stop mould is one thing all critical growers ought to attempt.

Rahman, M.S. and Labuza, T.P., 2007. Water exercise and meals preservation. In Handbook of meals preservation (pp. 465-494). CRC Press.
Mathlouthi, M., 2001. Water content material, water exercise, water construction and the soundness of foodstuffs. Food management, 12(7), pp.409-417.
Troller, J., 2012. Water exercise and meals. Elsevier.
Barbosa-Cánovas, G.V., Fontana Jr, A.J., Schmidt, S.J. and Labuza, T.P. eds., 2020. Water exercise in meals: fundamentals and purposes. John Wiley & Sons.