In recent weeks, as shoppers have looked to stock up on food that lasts, sales of canned and frozen food have surged. Even freezer sales are up. But many of us are taught to believe that when it comes to fruit and vegetables, nothing is as nutritious as fresh produce. By reaching for tinned or frozen versions, are we doing a disservice to our health?
最近一阵子罐装食品和冷冻食品的销售量急速上升，因为人们想方设法囤积能够存放很久而不会过期的食物，甚至连冷冻库的销量都增加了。 但是很多人相信，蔬菜和水果还是吃新鲜的最好，最有营养价值，那么吃罐头水果和蔬菜，或是冷冻水果和蔬菜，对我们健康有损害吗？
When answering this question, it’s important to remember that food is most nutritious at the point of harvest, says Fatima Hachem, Senior Nutrition Officer at the UN’s Food and Agriculture Organization. Fresh produce starts degrading as soon as it’s picked from the ground or tree, because that ground or tree is its source of nutrients and energy. 联合国粮农组织（UN Food and Agriculture Organization）高级营养官员哈切姆（Fatima Hachem）说，食物在收成的时候营养价值最高，新鲜的农作物从收割下来就开始流失营养，因为一离开土壤或树枝，也就失去了营养和能量的来源。
“Vegetables destined for cooking fresh might lose some of their nutritional value if they stay for long on the shelves,” Hachem says. “用来新鲜烹煮的蔬菜如果放久了可能会流失一些营养价值。”
Once picked, that fruit or veg is still using its own nutrients, breaking them down in order to keep its cells alive. And some nutrients are particularly vulnerable. Vitamin C, which helps the human body absorb iron, helps reduce cholesterol levels and protects against free radicals, is also especially sensitive to oxygen and light. 蔬菜水果采摘下来后就开始利用本身的营养素来维持细胞生命，有些营养素特别容易丧失，例如帮助人体吸收铁，降低胆固醇，抵抗自由基的维生素C，如果暴露在空气中和光线下特别容易流失。
Refrigerating produce slows down the process of nutrition degradation, although the rate at which nutritional value is lost varies from one product to the next.将新鲜蔬果放进冰箱冷藏能够减缓营养流失的速度，但是每一种蔬菜和水果的营养价值流失的速度不同。
In 2007, Diane Barrett, a former food science and technology researcher at the University of California, Davis, reviewed numerous studies looking into the nutritional content of fresh, frozen and tinned fruits and vegetables. She found that spinach, for example, loses 100% of its vitamin C content in seven days if stored at a room temperature of 20C (68F); it loses 75% if refrigerated. But carrots, by contrast, only lose 27% of their vitamin C content when stored for a week at room temperature. 有关新鲜蔬果、冷冻蔬果和罐头蔬果的营养价值已经有许多的研究，2007年的时候，美国加州大学戴维斯分校（University of California, Davis）的食品科学研究员巴瑞特（Diane Barrett）对多项研究进行检讨发现，菠菜存放在摄氏20度的室温下7天或就流失100%的维生素C，如果放在冰箱里冷藏，流失75%的维生素C，但是红萝卜存放在相同室温下7天后只流失27%的维生素C。
“Spinach is very thin, so there’s more loss of moisture and exposure to heat and oxygen compared to, say a carrot, which is denser,” says Barrett. 巴瑞特说，和红萝卜相比，菠菜又嫩又薄，容易流失水分氧化，耐热性也较差。
But all other vegetables in Barrett’s research lost significantly less vitamin C when they were frozen. That included spinach, which only lost 30% of its vitamin C when frozen.This is because freezing pauses the process of oxidisation, which is one reason that produce can start to turn brown after being harvested. 但是如果把蔬菜水果冷冻保存，流失的维生素C就少很多，例如，菠菜冷冻保存维生素C只流失30%，这是因为冷冻暂停了氧化的过程，而氧化是蔬果采摘下来之后逐渐变黄的原因之一。
Freezing produce on a mass scale is a relatively new innovation for the food industry. Take the humble frozen pea. Today, the pea can be harvested, transported to a factory, washed, blanched and frozen in just over two hours. In the 1970s, it would have taken days. 农业工业化实现了大规模的冷冻处理，以最普通的豌豆为例，现在的工业化技术能够在两个小时之内完成采摘豌豆，水洗，过滚水烫，急速冷冻等过程，而在50年前这整个过程要花上好几天。
“Compare that [timescale] to fresh vegetables – most of which are harvested, sent to a packing plant, packed, graded, shipped to retailers, then put in the consumer baskets,” says Richard Harrow, chief executive of the British Frozen Food Federation, the frozen food sector’s trade association in the UK. “About 99% of time, this process takes longer than the time taken to harvest, process and freeze peas.” 但是新鲜蔬果采摘下来后，要送到包装工厂分级包装，再分送到各地超市，最后才会进到消费者家里。英国冷冻食品联合会（British Frozen Food Federation）的首席执行官哈罗（Richard Harrow）表示，99%的情况下，上述过程所需时间比快速冷冻要花更久。
But as with fresh food, different nutrients in different kinds of produce degrade at different rates. 和新鲜蔬果一样，不同的蔬果用罐头保存营养流失的速度也不一样。
In her review, Barrett found that foods with mostly water-soluble nutrients, including vitamin C and B vitamins, retained nutrients differently than those with mainly fat-soluble nutrients, including vitamins A and E. The paper concluded that fresh food is often best for vitamin C content, as this vitamin is highly sensitive to heat – as long as it undergoes minimal storage. 巴瑞特发现，含有维生素C和维生素B等水溶性营养素的蔬菜水果，例如菠菜，比较适合新鲜食用，而比较不适合罐装保存。
Foods with more vitamin E and A, however, which are found in high amounts in canned carrots and tomatoes, fared much better during heat treatment.But canned food can also have some less desirable contents. “While there’s no reason to be worried about using frozen or canned food, there’s a preference for frozen because of the amount of added salt [in canned], and some canned fruits have added sugar,” says Hachem. 含有维生素A和维生素E等脂溶性营养素的蔬菜水果，例如红萝卜和西红柿，就比较适合罐装保存。但是哈切姆表示，虽然冷冻或罐装保存蔬果不影响其营养价值，但冷冻的比较好，因为罐装保存通常要添加盐，或是添加糖（水果罐头）。
Still, canning also can improve the safety of foods, particularly those prone to harbouring pathogens. “The good thing about canned foods is the process used to sterilise them,” says Barrett. “It results in greater nutrient loss, but once the produce is in the can it can be pretty stable for years, and you can be sure it’s safe because it’s gone through a process that kills all microorganisms.” 但是罐装保存也有其无法取代的优势，尤其是罐装食品的安全性通常更高，不容易有病菌。巴瑞特说，罐装食品的好处是经过了消毒杀菌过程，这个过程流失许多营养，但却也消灭了所有微生物，一旦装进罐头里面就是食用安全的。 What’s most important is to have a diverse diet, says Hachem, which, a lot of time, requires a variety of fresh, frozen and canned.
哈切姆说，不论是新鲜的或是冷冻的或罐装的，最重要的是多方面摄取食物，新鲜的，冷冻的和罐装的都要吃。
“You can have a balanced meal by cooking vegetables from frozen or in a can, but this doesn’t replace the importance of having fresh salads,” Hachem says. 他说：“你可以用冷冻蔬菜和罐装蔬菜来料理烹调，但那无法取代新鲜沙拉。”
“The problem with the lockdown is how to ensure fruit and vegetables retain their maximum nutrient value in light of the fact we’re advised to shop only once per week. 如果是一周只采买一次的话，就要确保蔬菜和水果保存最大的营养价值。
“If we can manage one salad on a daily basis, with cooking from frozen or canned, we’re not risking nutritional intake – on the contrary, it’s a good way of diversifying.” “如果我们每天都能有新鲜沙拉，再搭配冷冻或罐装蔬菜料理，那么就不会缺乏营养摄取，而且正好相反，这还符合多方面摄取营养的要求。”
But buying local and organic, like buying fresh, isn’t something that everyone is able to do – especially now.新鲜的水果和沙拉可能不是每个人都能方便取得的，尤其是目前这个时候，但专家认为，这并不代表我们不能摄取足够的营养。
And that doesn’t have to be a barrier to overall nutrition, experts say. Whether frozen, canned or fresh, they agree that the most important takeaway is simply to eat fruit and vegetables, however they’re prepared. According to the UK’s NHS, for example, either fresh, frozen, canned or dried fruit and vegetables all count towards your five-a-day. 不论是冷冻的也好，罐装或新鲜的也好，最重要的就是一定要吃蔬菜和水果，不论是如何保存的。英国的NHS医疗保健服务建议每天每人都要摄取5份蔬菜和水果，不论是冷冻的，罐装的或是新鲜的，都算在内。
“You should eat fruit and vegetables whether they’re fresh, frozen or canned, or dried or fermented; any one of these forms is nutritious,” says Barrett. 就像巴瑞特所说 ：“重点是一定要吃蔬菜和水果，不管是新鲜的或是冷冻的，罐装的或是干燥的，都有营养价值。”
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