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Yeast is a single-celled fungi somewhat related to mushrooms. The word comes from the Old English "gist," meaning bubble or foam. There are several hundred species identified by science, but one type, Saccharomyces cerivisiae, has been used in baking and brewing for thousands of years.
In Egypt, archaeologists have discovered evidence of breweries and bakeries that used yeast up to 4,000 years ago. Yet until the 19th century, no one was quite sure how the organism worked or what it was. In medieval England, the resulting foam on beer was even believed to have miracle properties, and called godisgoode, which means "God is good."
In 1857, Louis Pasteur published a paper proving that yeast was a living organism, and that the foaming and rising it caused was actually a fermentation process. The organisms consume oxygen and exhale carbon dioxide, just like humans. When combined with gluten (the protein created when wheat is mixed with water) the fungi expels carbon dioxide, causing bubbles in the dough and giving bread its texture. As the organisms breathe and exhale, sugar is digested. The waste from this process causes the yeasty flavor of bread.
If the organisms receive enough food and air, and are at a comfortable temperature, they reproduce very quickly. In bread, this causes the dough to rise, as single cells are replaced by huge clusters. When bread dough is punched and worked after the first rise, it distributes the organisms evenly, so they continue to ferment.
When beer ferments, the digested sugar becomes alcohol. Brewer’s yeast may be one of several species, with cerivisiae the most common, that settle on the top or bottom during brewing, greatly affecting the flavor. "Top-Fermenting" brews tend to be fruity and ferment best at high temperatures while "bottom-fermenting" organisms make dark beer or lager and ferment in a cooler climate.
Until World War II, yeast was used in fresh cakes, giving the cakes a 10 day lifespan before the yeast would go bad. While, professional bakers still may use the fresh form, many cakes are made with active dry yeast, which has a much longer shelf life, and was welcomed by most home bakers. In this mixture, the organisms were carefully dried and would revive in warm water, thereby lasting longer. However, if the water was not between 105-115 degrees F (about 40-46 C), thermal shock would kill off the fungus. More recently, instant yeast has become popular. This variety does not need re-hydration and can be added directly to dough.
Today nutritional yeast, which isn’t a live form of the fungi, can be used as a dietary supplement. Sold in bulk or in packages of small flakes, it is high in protein and B vitamins, and has a strong nutty or cheese flavor. Products for vegans and for people with lactose intolerance often use nutritional yeast as a substitute for Parmesan cheese, and even some movie theaters have begun offering it as a healthy alternative topping for popcorn.
Katika DelightedCooking, tumejitolea kutoa habari sahihi, ya kuaminika. Maudhui yetu yaliyoandikwa na wataalamu ni ya kweli-kuchunguzwa kwa ukali na inatokana na mamlaka ya kuaminika. Gundua jinsi tunavyotegemeza viwango vya juu zaidi katika kukupa maarifa ya kuaminika. Chachu ni kuvu wenye chembe moja zinazohusiana kwa kadiri fulani na uyoga. Neno hilo linatokana na neno la Kiingereza cha kale "gist", linalomaanisha Bubble au Foam. Kuna aina kadhaa za mchuzi wa mchuzi, lakini aina moja, Saccharomyces cerivisiae, imetumiwa katika kuoka na kutengeneza bia kwa maelfu ya miaka. Wataalamu wa vitu vya kale nchini Misri wamegundua uthibitisho wa kuwa na viwanda vya kutengeneza bia na mikate vilivyotumia chachu hadi miaka 4,000 iliyopita. Hata hivyo, hadi karne ya 19, hakuna mtu aliyejua jinsi chembe hiyo ilivyotenda kazi au ilikuwa nini. "Katika Uingereza ya enzi za kati, mchuzi uliotokezwa kwenye bia ulidhaniwa kuwa na sifa za ajabu, na uliitwa ""godisgood"" (Mungu ni mwema)." Katika mwaka wa 1857, Louis Pasteur alichapisha makala iliyoonyesha kwamba chachu ni kiumbe hai na kwamba kuwaka kwa unga na kuongezeka kwa chachu ni kwa sababu ya mchakato wa kuchacha. Viumbe hao hutumia oksijeni na kuvuta nje kaboni dioksidi, kama vile wanadamu. Wakati zinapochanganywa na gluten (protein inayotokezwa wakati ngano inachanganywa na maji), kuvu huondoa kaboni dioksidi, na kusababisha vifuniko katika unga na kutoa muundo wa mkate. Viumbe hao wanapopumua na kupumua, sukari huyeyushwa. Maji yanayotokana na mchakato huo husababisha ladha ya chachu ya mkate. Ikiwa viumbe hao hupata chakula na hewa ya kutosha, na wako katika halijoto nzuri, wao huzaana upesi sana. Katika mkate, hilo husababisha unga uinuke, kwa kuwa chembe moja-moja hubadilishwa na makundi makubwa. Wakati unga wa mkate unapopigwa na kufanyiwa kazi baada ya kuongezeka kwa kwanza, unagawa viumbe kwa usawa, kwa hiyo huendelea kuchemka. Bia inapochemka, sukari inayomeng'enywa inakuwa pombe. Chachu ya bia inaweza kuwa moja ya spishi kadhaa, na cerivisiae ni ya kawaida zaidi, ambayo huishi juu au chini wakati wa kutengeneza bia, ikathiri sana ladha. "Top-Fermenting" brews huwa na matunda na ferment bora katika joto la juu wakati "bottom-fermenting" viumbe kufanya bia nyeusi au lager na ferment katika hali ya hewa baridi. Kabla ya Vita Kuu ya Pili ya Dunia, chachu ilitumiwa katika keki mpya, ikitoa maisha ya siku 10 kabla ya chachu kuharibika. Wakati waokaji wa kitaalamu bado wanaweza kutumia fomu safi, keki nyingi hutengenezwa na chachu kavu inayofanya kazi, ambayo ina maisha ya rafu ya muda mrefu zaidi, na ilikaribishwa na waokaji wengi wa nyumbani. Katika mchanganyiko huo, viumbe hao walikaushwa kwa uangalifu na wakafufuka katika maji ya joto, na hivyo kudumu kwa muda mrefu zaidi. Hata hivyo, ikiwa maji hayakuwa na joto la kati ya digrii 40 na 45 C, mshtuko wa joto ungeua kuvu. Hivi majuzi, chachu ya haraka imekuwa maarufu. Aina hii haihitaji rehydration na inaweza kuongezwa moja kwa moja kwa dough. Leo chachu ya lishe, ambayo si aina ya kuishi ya uyoga, inaweza kutumika kama nyongeza ya lishe. Inapatikana kwa wingi au katika vifurushi vidogo, ina protini nyingi na vitamini B, na ina ladha kali ya karanga au jibini. Kwa wale ambao hawawezi kula vyakula vya mboga na wale ambao hawawezi kuvumilia lactose, unga wa Parmesan hutumiwa kama mbadala wa unga wa Parmesan, na hata sinema zingine zimeanza kuupatia kama mbadala mzuri wa popcorn.