Fresh Lettuce

Every physical property of a lettuce works against its keeping. It is almost entirely water held in thin leaves with an enormous surface area relative to their mass, so it loses water fast and wilts visibly. It is cut across the base, leaving a large open wound that browns and admits decay. It is a growing tissue with a high metabolic rate and no storage reserves — unlike an onion bulb or a carrot root, there is nothing banked to live off.

And it is non-climacteric, which removes the last consolation. A tomato picked firm gets better on the way to market; a lettuce cut in the field is at its best in that instant and declines from there. The commodity is therefore defined by a single logic — subtract heat immediately and keep it subtracted — and every commercial practice around it follows from that.

Lettuce is the classic vacuum-cooled commodity. Packed product goes into a sealed chamber and the pressure is dropped until water evaporates from the leaves themselves; the evaporation pulls heat out of the tissue directly, cooling the whole head — interior included — in a fraction of the time a cold room would need. This suits lettuce precisely because of the same open, high-surface-area structure that makes it perishable, and it is why vacuum cooling is standard for this commodity while denser produce is cooled by other means.

The cooling is not free: the water that leaves as vapour is weight off the product, so the process is controlled to take out heat without over-drying the leaf. Once cooled, the lot must simply stay cold. Lettuce tolerates temperatures near freezing — it is not chilling-sensitive — so there is no upper constraint to respect, only the discipline of never letting the chain break. Each warm interval spends quality permanently, and the losses accumulate across a chain in which no later step can add anything back.

Lettuce grading is done largely by eye and hand in the field: head weight and firmness for crisphead types, freshness and turgidity, colour, trim, and freedom from defects. Because it is cut and packed in one motion, the grading decision is usually made by the cutter at the moment of harvest — there is no second look on a packing line, and the discipline of the crew is the quality control.

• Tipburn — brown, necrotic leaf margins, a physiological disorder set before harvest
• Butt discoloration and browning at the cut base
• Russet spotting — small brown pits driven by ethylene exposure
• Pink rib and rib discoloration
• Wilting and loss of turgidity from water loss or warmth
• Bruising and crushing from handling or over-packing
• Decay and soft rots, entering through wounds and damaged leaves

Two of these are worth separating from the rest. Tipburn arrives with the crop — it is set in the field, and no post-harvest care removes it. Russet spotting is the opposite: it is caused after harvest by ethylene, so lettuce stored or shipped with ethylene-producing commodities develops it in transit. Lettuce has no ripening to trigger, but it is damaged by ethylene all the same, and keeping it away from ethylene sources is a standing rule of the trade.

Packaging for lettuce is doing physical work, not decoration. Film wrapping and modified-atmosphere packaging hold humidity around the leaf and slow water loss and respiration; cartons are built to carry the load without crushing what is inside. Product moves refrigerated, quickly, and normally in dedicated rather than mixed loads, both for temperature and to keep it away from ethylene.

The processed salad stream is a genuinely different commodity channel. Heads are delivered to a plant, cut, washed, dried, and bagged under modified atmosphere, which produces a convenient product with a shorter life than the intact head it came from: cutting multiplies the wounded surface, and every cut edge is a fresh site for browning and decay. Field quality standards for that stream differ accordingly — internal structure and freedom from tipburn matter more than the head appearance a retail buyer is sold on.