Moisture and crunch

Snacks are crunchy because they are dry, and they stop being crunchy when they take up water. That is the whole story, but water finds more ways in than most people expect: the air in the packing room, the seasoning and oil, ingredients that were not dried evenly, a seal with a gap, and every time a jar lid is opened. The number to watch is not moisture content but water activity. And packaging does not create crunch; it only keeps it for longer. This piece explains each of those points in plain words, for people starting out in product work and for partners who wonder why we ask so many questions about films and lids.

Crunch is a state, not a fixed property

A rice cracker or a corn snack is essentially a hollow starch structure full of tiny air cells that has been dried. When dry, that structure is rigid and brittle like thin glass: it breaks when bent and makes a sound when bitten. As it takes up water, the starch molecules begin to move, the structure softens and becomes rubbery. The snap disappears long before the snack looks wet. Food scientists call the boundary a glass transition. Eaters call it stale.

The important point is that crunch is not written into the recipe. It is the state of the product at a certain dryness, and everything that happens after the oven can push the product out of that state.

Where the water comes from

We list the routes so we know where to block them:

• The air in the cooling and packing areas. A hot cracker leaving the oven takes up water from humid air while it cools. In Hanoi during the damp season this is the largest route of all.
• Seasoning and oil. Powdered seasoning mixed with oil and sprayed onto the product carries some water, and fine powders absorb humidity faster than the cracker itself.
• Uneven ingredients. A batch of rice or corn with uneven moisture gives crackers that are crisp in one spot and chewy in another from the start.
• Packaging. A film thinner than needed, a seal with a short gap, or a jar whose lid does not close tightly.
• After opening. A bag left open on the table, a jar left open all evening.

The first four belong to production and packaging, the part we control together with our partner facilities. The last belongs to the eater and can only be dealt with through design.

Moisture content, water activity and the number to ask for

Moisture content is the total water in the product as a percentage of its weight. Water activity, written aw, is the free water: the share that can move, evaporate and feed microbes. The two are related but not the same. A product with a lot of sugar or salt holds its water more tightly, so at the same moisture content its water activity is lower.

Crunch and safety follow water activity, not moisture content. So when discussing a crunchy product, the number to ask a facility for is water activity straight after packing and again after a period of storage. A water activity meter is cheap compared with a returned shipment, and a reading takes a few minutes.

What packaging does, and what it cannot do

Packaging is the barrier between the product and the air. Three things decide how good that barrier is: the film structure, meaning whether it has a layer that blocks water vapour; the seal, meaning whether it is tight along its whole length; and the gas inside, since some products are flushed with nitrogen so the bag stays firm, which protects against breakage and reduces oxygen. For jars, add the lid and the seal under it.

Good packaging does not make a product crunchier than it was leaving the oven. It only delays the day it reaches the stale point. Two things therefore have to go together: the product must be dry enough when it goes into the pack, and the pack must be tight. Without one, the other is wasted.

An evening with the lid off

The Let's PLAY Cheese Ball jar carries an Ô ăn quan board on its label, and the snack pieces are the playing pieces. That means the jar stays open for a whole game, sometimes a whole evening. We wanted to know how the pieces would hold up, so we ran a very simple trial at the office: one jar left open on the table through a damp evening, one jar closed beside it, and the whole team tasting both the next morning.

The result was what we feared. The pieces from the open jar had softened noticeably compared with the closed one, though not to the point of being stale. The conclusion was not a new recipe but two small actions. First, ask the jar supplier how tightly the lid closes after being opened and closed many times, because that is what happens during a game. Second, consider a short line on the label reminding people to close the lid when the game ends. The trial cost one jar and ten minutes of a morning.

What this means for buyers and for people learning the trade

For distributors and importers, crunch is the thing your customers check at the first bite. You are entitled to ask us about water activity, film structure and storage results, and we will answer with measured numbers rather than promises. For someone new to R&D, understanding where water gets in is the first lesson, before any lesson about flavour.

If you want to learn lessons like this with your hands, the Fresher Product R&D role in Hanoi is open on our Careers page. If you are a packaging partner or a buyer, get in touch; we are happy to talk about films, lids and the numbers behind the crunch.