Mechanization has made the farming of many crops — lettuce and tomatoes among them — a lot less labor intensive. But some crops are still tended and harvested by hand, and it can be painstaking work.

How do you measure the labor intensity of crops? We thought there would be an easy answer to that, but there isn't. Some agricultural economists talk about labor input in terms of hours per acre, but that may not take into account the difficulty of the labor.

For Labor Day, we thought we'd round up some of those crops that still require specialized human labor. This is by no means a comprehensive list, and some of these crops are only labor intensive in certain parts of the world, because their harvest has been mechanized elsewhere. And intensive does not necessarily mean abusive, though, as we've reported, slave and child labor do still plague the food system.

Saffron

This spice is among the most expensive in the world; the biggest producers are Italy, Iran and Spain. The work involves growing the saffron crocus flowers and then picking and processing them. The flowers must be picked early in the morning, and on the same day, workers by hand extract the stigmas — the tiny strands that are dried and then used to add color, flavor and a unique aroma to a meal.

Many have claimed that the labor involved to produce saffron is the reason for its high price: currently around $85 per ounce.

As we reported last week, many people have called vanilla one of the most labor-intensive food products the world over. If you're talking about the real stuff (as opposed to the artificial stuff), most of it comes from Madagascar, but it also comes from Tahiti, Mexico and Indonesia.

Like saffron, the cost of that labor is reflected in the price for the beans — about $10 for a small jar of two or three beans. Vanilla beans are actually the fruit of an orchid. That orchid flowers only once a year and has to be hand-pollinated.

As former Peace Corps volunteer turned entrepreneur Tim McCollum, of the chocolate and vanilla company Madécasse, told our sister blog Goats and Soda, "Farmers use a stick with a small, pointy end, extract something and put it together to pollinate. It's only in bloom a very short time."

After it's harvested, the bean has to be cooked, sweated, dried and cured. The entire process from pollination to processed bean can take up to a year and a half.

To add to our list of labor-intensive crops, we consulted the International Labor Rights Forum. Here's what Abby Mills, director of campaigns for the ILRF, tells us about these next three:

"These crops take an intense amount of time and large number of manual laborers to grow, plus they grow best along the equator. So the incentive is very much to find the countries where both land and labor are cheapest, and often where rule of law is least established. Those forces combine to lead to the labor issues. The work is back breaking and hard, but the very intensity ... also contributes directly to the poor conditions we see on so many of the plantations for the crops."

Like vanilla, the cacao bean that eventually gets turned into chocolate originates inside a pod. But cacao pods are big and have to be hacked open with machetes or clubs. Once the beans are out, they have to be cleaned, dried and fermented before they can be ground up and processed into cocoa.

As we reported, some cacao farmers in the Ivory Coast had never even tried chocolate. Watch this video to get a better sense of what the work is like.

We've reported on the link between palm oil grown in Southeast Asia (which ends up in our doughnuts and other baked goods) and deforestation. Turns out harvesting palm oil can also be back-breaking work.

In its report on labor issues in the palm oil industry in the Philippines, the Center for Trade Union and Human Rights notes, "Like their adult counterparts, child workers are engaged in various types of palm oil activities, namely fruiter, harvester, hauler, kickerall, uprooting among others. Hauling and harvesting are considered the most strenuous work wherein children have to raise a steel rod that reaches the canopy of the palm fruit (for harvesters) or they have to carry at least a [33-pound] palm fruit bunch (for haulers) and load it to the truck for transport."

In the U.S., two of the most labor-intensive food crops are mushrooms and strawberries, according to Philip Martin, a professor in the department of agriculture and resource economics at the University of California, Davis. Cotton and cottonseed harvesting were mechanized long ago.

But in India and Uzbekistan, cottonseed is still hand-harvested and labor-intensive. In India, many children are employed in the fields and in the gins, where the seed is separated before being pressed into oil. Annually, more than 1 million children and adults are forced to pick the cotton in Uzbekistan. In 2013, the number of manual laborers in the fields harvesting the country's cotton increased to an estimated 4 million, according to the Cotton Campaign.