📚 5.6.1 Food Production Using Yeast | 利用酵母生产食品
Yeast is a single-celled fungus that has been used for thousands of years to produce staple foods and beverages such as bread, beer and wine. In IGCSE Science, understanding how yeast carries out anaerobic respiration (fermentation) is a key topic, linking biology, chemistry and everyday applications. This article explores the science behind yeast-based food production, the conditions required for fermentation, and the experimental methods used to investigate this process.
酵母是一种单细胞真菌，几千年来一直被用来生产面包、啤酒和葡萄酒等基本食品和饮料。在IGCSE科学课程中，理解酵母如何进行无氧呼吸（发酵）是一个关键主题，它连接了生物学、化学和日常应用。本文探讨酵母食品生产背后的科学原理、发酵所需的条件以及研究该过程的实验方法。
1. What Is Yeast? | 什么是酵母？
Yeast is a unicellular fungus belonging to the kingdom Fungi. The species most commonly used in food production is Saccharomyces cerevisiae. Like all living organisms, yeast respires to release energy for growth and reproduction. In the presence of oxygen, it performs aerobic respiration, producing carbon dioxide and water. However, when oxygen is limited, yeast switches to anaerobic respiration (fermentation), which is crucial for brewing and baking.
酵母是一种属于真菌界的单细胞真菌。食品生产中最常用的种类是酿酒酵母。像所有生物一样，酵母通过呼吸作用释放能量以供生长和繁殖。在有氧气的情况下，它进行有氧呼吸，产生二氧化碳和水。然而，当氧气有限时，酵母转换为无氧呼吸（发酵），这对酿造和烘焙至关重要。
Yeast cells contain enzymes that catalyse the breakdown of sugars. Under anaerobic conditions, the enzyme zymase converts glucose into ethanol and carbon dioxide. This metabolic pathway is exploited in the food industry to create alcoholic drinks and to leaven bread dough.
酵母细胞含有催化糖分解的酶。在无氧条件下，酒化酶将葡萄糖转化为乙醇和二氧化碳。这一代谢途径在食品工业中被用来制造酒精饮料和发酵面团。
2. The Chemistry of Fermentation | 发酵的化学原理
Fermentation is a form of anaerobic respiration in yeast. The overall word equation and balanced symbol equation for the fermentation of glucose are:
发酵是酵母的一种无氧呼吸形式。葡萄糖发酵的总文字方程式和配平的符号方程式为：
Glucose → Ethanol + Carbon Dioxide
葡萄糖 → 乙醇 + 二氧化碳
C₆H₁₂O₆ → 2 C₂H₅OH + 2 CO₂
For every one molecule of glucose, two molecules of ethanol and two molecules of carbon dioxide are produced. Energy is released in the process, but much less than in aerobic respiration. This is why yeast still requires some oxygen initially to build up biomass before anaerobic conditions prevail.
每分解一个葡萄糖分子，产生两个乙醇分子和两个二氧化碳分子。在此过程中释放能量，但比有氧呼吸少得多。这就是为什么酵母最初仍需要一些氧气来积累生物量，然后再进入无氧状态。
In brewing, the ethanol is the desired product, while in baking, the CO₂ gas is the key component that makes dough rise. The ethanol evaporates during baking, leaving behind the soft texture of bread.
在酿造过程中，乙醇是期望的产物；而在烘焙中，CO₂气体是使面团膨胀的关键成分。乙醇在烘焙过程中蒸发，留下面包的松软质地。
3. Beer Production | 啤酒的生产
Beer is made from barley, water, hops and yeast. The process begins by allowing barley grains to germinate (malting). During malting, enzymes in the barley break down starch into simpler sugars, mainly maltose. The germinated grains are then dried in a kiln to stop germination, producing malt.
啤酒由大麦、水、啤酒花和酵母制成。该过程首先让大麦粒发芽（制麦）。在制麦过程中，大麦中的酶将淀粉分解为较简单的糖，主要是麦芽糖。然后将发芽的麦粒在窑炉中烘干以停止发芽，得到麦芽。
The malt is crushed and mixed with warm water in a process called mashing. The enzymes continue to convert starch into sugars, creating a sweet liquid called wort. The wort is boiled with hops, which add bitterness and flavour, and then cooled. Yeast is added to the cooled wort, and fermentation takes place in sealed tanks. The temperature is controlled at around 15–20°C for ale yeast (top-fermenting) or 8–12°C for lager yeast (bottom-fermenting).
麦芽被碾碎并与温水混合，这一过程称为糖化。酶继续将淀粉转化为糖，产生一种称为麦芽汁的甜液。麦芽汁与啤酒花一起煮沸，啤酒花增加苦味和风味，然后冷却。将酵母加入冷却的麦芽汁中，发酵在密封罐中进行。对于艾尔酵母（上面发酵），温度控制在约15–20°C；对于拉格酵母（下面发酵），温度控制在8–12°C。
During anaerobic fermentation, yeast converts sugars into ethanol and CO₂. After about 5–10 days, the beer is racked off, aged, filtered and packaged. The carbonation may be adjusted by adding extra sugar or directly injecting CO₂.
在无氧发酵过程中，酵母将糖转化为乙醇和CO₂。大约5–10天后，将啤酒分离出酵母，进行陈酿、过滤和包装。碳酸饱和度可通过添加额外糖或直接注入CO₂来调整。
4. Wine Production | 葡萄酒的生产
Wine is produced by fermenting the sugars naturally present in grapes. Grapes are harvested and crushed to release the juice, which contains glucose and fructose. Traditionally, the natural yeasts present on grape skins initiated fermentation, but modern wineries often add a controlled strain of Saccharomyces cerevisiae to ensure consistency.
葡萄酒通过发酵葡萄中天然存在的糖而制成。葡萄被收获并压碎以释放出含有葡萄糖和果糖的果汁。传统上，葡萄皮上的天然酵母会引发发酵，但现代葡萄酒厂通常会添加可控的酿酒酵母菌株以确保一致性。
The crushed grapes and juice, called must, are transferred to fermentation vessels. For red wine, the skins are left in to extract colour and tannins; for white wine, the skins are usually removed early. The yeast ferments the sugars into ethanol and CO₂. Fermentation temperatures range from 20–30°C and can last from one to several weeks. The carbon dioxide escapes, leaving a still wine. To produce sparkling wines like Champagne, a secondary fermentation is conducted in a sealed bottle to trap CO₂.
压碎的葡萄和果汁（称为葡萄醪）被转移到发酵容器中。红葡萄酒酿造时保留果皮以提取颜色和单宁；白葡萄酒通常早早去除果皮。酵母将糖发酵为乙醇和CO₂。发酵温度在20–30°C之间，可持续一到数周。二氧化碳逸出，留下静止葡萄酒。为了生产如香槟等起泡酒，在密封瓶中进行二次发酵以捕获CO₂。
After fermentation, the wine is aged, clarified and bottled. The alcohol content typically reaches 10–15% by volume before the yeast is inhibited or dies off due to ethanol toxicity.
发酵后，葡萄酒经过陈酿、澄清并装瓶。在酵母被抑制或由于乙醇毒性死亡之前，酒精含量通常达到10–15%（体积比）。
5. Bread Making | 面包的制作
Bread dough is made from flour, water, yeast, and often a small amount of sugar and salt. The flour contains starch and a protein called gluten, which gives dough its elastic texture. When yeast is mixed with warm water and sugar, it begins to respire, producing CO₂. This gas gets trapped in the gluten network, causing the dough to rise or 'prove'.
面包面团由面粉、水、酵母以及通常少量糖和盐制成。面粉中含有淀粉和一种叫做谷蛋白的蛋白质，它赋予面团弹性。当酵母与温水和糖混合时，开始呼吸并产生CO₂。这些气体被困在谷蛋白网络中，使面团膨胀或“发酵”。
During proving, which takes place at 25–30°C for about 1–2 hours, the dough can double in size. The CO₂ bubbles create a light, spongy structure. Salt is added to moderate yeast activity and strengthen the gluten. After proving, the dough is kneaded again, shaped, and allowed a second rise. It is then baked in an oven at 200–230°C. The high temperature kills the yeast and evaporates any ethanol produced, while the CO₂ expands further, giving the bread its final volume and soft crumb.
在25–30°C下进行约1–2小时的发酵过程中，面团体积可增加一倍。CO₂气泡形成轻盈、海绵状的结构。盐被加入以调节酵母活性并增强谷蛋白。发酵后，将面团再次揉捏、整形，并二次醒发。然后在200–230°C的烤箱中烘烤。高温杀死酵母并蒸发掉任何产生的乙醇，同时CO₂进一步膨胀，赋予面包最终的体积和柔软的面包屑。
Some traditional breads, such as sourdough, use a mixture of wild yeast and lactic acid bacteria, which adds a distinctive tangy flavour.
一些传统面包，如酸面团面包，使用野生酵母和乳酸菌的混合物，这增添了独特的酸味。
6. Factors Affecting Yeast Fermentation | 影响酵母发酵的因素
The rate of yeast fermentation is influenced by several factors, which are often investigated in IGCSE practicals:
- Temperature: Enzymes in yeast work best at around 30–37°C. Too low a temperature slows down enzyme activity; too high denatures the enzymes, killing the yeast.
- pH: Yeast prefers a slightly acidic environment around pH 4–5. Extremes of pH denature enzymes.
- Sugar concentration: A moderate sugar level provides fuel, but very high concentrations can draw water out of yeast cells by osmosis, inhibiting fermentation.
- Oxygen availability: Initially, oxygen is needed for growth, but for ethanol and CO₂ production, anaerobic conditions are required.
酵母发酵速率受几个因素影响，这些因素常在IGCSE实验中探究：
- 温度：酵母中的酶在约30–37°C时活性最佳。温度太低会减慢酶活性；太高会使酶变性，杀死酵母。
- pH值：酵母偏好约pH 4–5的微酸性环境。极端的pH会使酶变性。
- 糖浓度：适度的糖含量提供燃料，但浓度过高会通过渗透作用从酵母细胞中抽走水分，抑制发酵。
- 氧气供应：起初，氧气是生长所必需的，但要产生乙醇和CO₂，需要无氧条件。
7. Investigating Fermentation – A Classic IGCSE Experiment | 探究发酵——经典IGCSE实验
A common laboratory experiment uses yeast suspension, sugar solution, and a delivery tube leading into limewater (or a gas syringe) to measure CO₂ production. The apparatus is placed in a water bath at a set temperature, and the rate of bubbling or the volume of gas collected over time indicates fermentation speed. Alternatively, a balloon can be fitted over the test tube to collect CO₂ and observe expansion.
一个常见的实验室实验使用酵母悬液、糖溶液，以及通入石灰水（或气体注射器）的导管来测量CO₂的生成量。将装置放在设定温度的水浴中，气泡产生的速率或随时间收集的气体体积指示发酵速度。或者，可以在试管口套上气球收集CO₂并观察膨胀情况。
For example, to investigate the effect of temperature, prepare four test tubes with equal amounts of yeast and glucose, and place them in water baths at 10°C, 25°C, 35°C, and 50°C. Record the volume of gas collected after 10 minutes. The tube at 35°C typically produces the most CO₂, demonstrating the optimum temperature.
例如，为了探究温度的影响，准备四支装有等量酵母和葡萄糖的试管，分别放置在10°C、25°C、35°C和50°C的水浴中。10分钟后记录收集的气体体积。通常35°C的那支试管产生的CO₂最多，表明此为最适温度。
Controls ensure that the same batch of yeast, equal concentrations, and identical volumes are used. Repeats are carried out for reliability. Safety precautions include wearing goggles and handling hot water carefully.
对照组确保使用同一批酵母、相等的浓度和相同的体积。通过重复实验以保证可靠性。安全预防措施包括佩戴护目镜并小心处理热水。
8. Industrial vs. Laboratory Conditions | 工业条件与实验室条件
In industrial breweries and bakeries, fermentation is carefully monitored and controlled on a large scale. Bioreactors, or fermentation vats, maintain sterile conditions and regulate temperature, pH and nutrient supply. Pure yeast strains are cultivated to guarantee consistent product quality. In laboratories and kitchen settings, contamination is more likely, and conditions are less precisely controlled, but the underlying biological process is identical.
在工业啤酒厂和烘焙房中，发酵在大规模上进行仔细监控。生物反应器或发酵罐保持无菌条件，并调节温度、pH和营养供应。培养纯酵母菌株以确保产品质量一致。在实验室和厨房环境中，污染更可能发生，条件控制也不那么精确，但基本的生物过程是相同的。
Continuous fermentation systems are used in some large-scale alcohol production, where fresh medium is constantly added and ethanol is removed, maintaining yeast in the logarithmic growth phase and increasing yield.
一些大规模酒精生产使用连续发酵系统，不断添加新鲜培养基并移除乙醇，使酵母保持在对数生长期，提高产量。
9. Advantages and Disadvantages of Using Yeast | 利用酵母的优缺点
Yeast-based fermentation is a natural, renewable process that can convert low-value carbohydrates into high-value products. It is used in producing renewable biofuels like bioethanol. However, the process has limitations: ethanol concentration is self-limiting (typically up to 15%), as higher concentrations poison the yeast. Fermentation is also slower than purely chemical synthesis and requires strict hygiene to avoid spoilage by unwanted microorganisms.
基于酵母的发酵是一种自然、可再生的过程，能将低价值的碳水化合物转化为高价值产品。它被用于生产生物乙醇等可再生生物燃料。然而，这一过程也有局限性：乙醇浓度具有自我限制性（通常高达15%），因为更高的浓度会使酵母中毒。发酵也比纯化学合成慢，并且需要严格的卫生条件以避免有害微生物的破坏。
10. Linking to Other IGCSE Science Topics | 与其他IGCSE科学主题的联系
The study of yeast fermentation touches upon several other areas of the syllabus: enzymes and their specificity, aerobic vs anaerobic respiration in human muscles (producing lactic acid, not ethanol), food preservation methods, and the carbon cycle (CO₂ release). It also illustrates the importance of microorganisms in biotechnology, a core concept in both biology and combined science.
对酵母发酵的研究涉及课程大纲的其他几个方面：酶及其专一性、人体肌肉中的有氧与无氧呼吸（产生乳酸而非乙醇）、食品保存方法以及碳循环（CO₂释放）。它还说明了微生物在生物技术中的重要性，这是生物学和综合科学中的核心概念。
11. Common Misconceptions | 常见误区
A frequent error is to confuse fermentation with aerobic respiration and to assume that yeast always produces ethanol. In the presence of oxygen, yeast respires aerobically, producing CO₂ and water without ethanol. Another misconception is that the alcohol content in bread is retained after baking; in reality, the ethanol evaporates when exposed to oven heat. It is also important to clarify that yeast does not simply 'die' but becomes inactive or is killed by extreme conditions.
一个常见的错误是将发酵与有氧呼吸混淆，并假设酵母总是产生乙醇。在有氧条件下，酵母进行有氧呼吸，产生CO₂和水而不产生乙醇。另一个误区是认为面包中的酒精在烘烤后仍保留；实际上，乙醇在烤箱高温下蒸发。同样重要的是要阐明，酵母并非简单地“死亡”，而是变得失活或被极端条件杀死。
12. Summary and Exam Tips | 总结与考试技巧
To summarise, yeast is a versatile microorganism used in anaerobic fermentation to produce ethanol and carbon dioxide. Beer and wine rely on ethanol production, while bread uses the CO₂ for leavening. Key practical variables include temperature, sugar concentration and pH. In exams, be able to write the word and balanced chemical equation for fermentation, describe industrial processes, and interpret experimental data on CO₂ production. Remember to use precise scientific vocabulary such as 'anaerobic respiration,' 'zymase,' and 'denatured' when appropriate.
总之，酵母是一种多用途的微生物，用于无氧发酵生产乙醇和二氧化碳。啤酒和葡萄酒依赖乙醇生产，而面包利用CO₂发酵。关键的实验变量包括温度、糖浓度和pH值。在考试中，要能写出发酵的文字和配平化学方程式，描述工业过程，并解释关于CO₂产量的实验数据。记住在适当的时候使用精确的科学术语，如“无氧呼吸”、“酒化酶”和“变性”。
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