One day, while eating lunch with friends in the cafeteria, she met an American Harvard graduate named Nathaniel (Nat) Leff, who was taking a gap year after college at Hebrew University. Judith had been speaking with other students in French, and Nat — who had mastered French in high school — joined their conversation. Judith thought he was Belgian, not American. She would later tell a journalist that she liked Nat because he was so intelligent and cultured, and they had the same taste in literature and music.

Nat wanted to marry Judith, but his mother thought he was too young for marriage. They both returned home, but Nat came to France a few months later to visit Judith, and they got engaged and then married.

Following their marriage, the young couple moved to the US, where Nat continued on for advanced degrees at M.I.T. and Columbia University, earning a doctorate in economics. The research for his thesis required him to spend a year in Brazil, and the couple moved to São Paulo in 1964 for a year or so with their son Avraham, then 14 months old, and baby Chana, who was only three weeks old. Judith would later recall enjoying the scenery, the language, and the relaxed approach to life in Brazil, although they lived in an apartment building that required them to go up and down 20 flights of stairs on Shabbos.

There Judith added Portuguese to her impressive roster of languages (German, French, English, and Hebrew). The books that Dr. Nathaniel Leff wrote on Brazilian economic history and policy, based on his dissertation, became classics in his field. In fact, years later a Brazilian journalist named Rafael Cariello became so intrigued by his work that he tracked down the Leff family and came to the US to meet them, writing a lengthy article entitled “Looking for Leff” for the magazine Piaui in 2016.

When the Leffs returned to the States, they raised their family of two sons and a daughter in Riverdale, sending the children to Washington Heights for school. Dr. Nathaniel Leff worked for almost three decades as a professor at the Columbia University Business School. Dr. Judith Leff worked part-time while her children were young, doing research at Harvard, Tufts, and Brandeis.

Laying the Foundation for Contemporary Kashrus

In 1985, having worked most recently as a researcher in molecular biology at the Albert Einstein Medical College in the Bronx, Dr. Judith Leff met Rabbi Menachem Genack, the current CEO of the OU’s Kosher Division, in Englewood, New Jersey. Both were there for the bar mitzvah of the son of Judith’s cousin, Rabbi Simon Posner, the editor of the OU Press. Judith was between jobs at the time, and her husband, who was unflaggingly supportive of her professional career, introduced the two, launching a professional shidduch: As soon as Rabbi Genack learned she had a background in plant physiology and research biochemistry, he proposed that she come on board at the OU as a food chemist. “I knew her kind of expertise would be helpful,” Rabbi Genack says. “The new additives and technologies in food production required a sophisticated level of scientific understanding.”

Rabbi Genack had a previous connection to the Leff family, because as a bochur learning with Rav Joseph B. Soloveitchik in Boston, he and some friends had rented rooms in Brookline from Nathaniel’s mother, Mrs. Zelda Leff.

It wasn’t the first time Dr. Judith Leff had been consulted on food technology for kashrus reasons. Rav Aharon Teitelbaum, the Nirbater Rebbe of Boro Park, comments, “Years ago, Dr. Judith Leff was the frum chemist we would turn to with our questions. Her opinion was accepted everywhere. She wasn’t only a scientist; she was a very precise, frum, emesdig person that you could count on to give a proper answer.”

When Dr. Judith Leff joined the OU in 1985, kosher food was still a small industry. The information highway was barely a dirt road, and knowledge was less easily accessible than it is today.

“When I began at the OU thirty-six years ago, kashrus was about shechitah, and about not mixing meat and milk,” says Rabbi Yechezkel Auerbach of KCL kashrus certification. “There weren’t many kosher products available. Today we have not only more products, but products being manufactured with technologies that were not available back then.” At the time, he was in charge of a kashrus division dealing with compound flavors and aromas, and relied heavily on Dr. Leff’s expertise.

Rabbi Auerbach offers a glimpse into the nature of her work. “Most food additives are created from enzymes, derived from fermentation created by bacteria,” he explains. “This is a complex science that was just getting its start in the 1980s. One example of a food produced in this manner is EMCs, or enzyme-modified cheeses. Many hard cheeses require six months of aging, but with an injection of the right enzymes, that time period can be cut to as little as six days while retaining the same flavor profile.”

So does the kosher consumer need to wait six hours after eating the EMC version of Parmesan? “I believe not,” Rabbi Auerbach says. “The flavor is the same, but the texture is softer, the mouth feel is different, and it may stick less to the teeth.”

The two of them would visit companies and plants that manufactured flavors and enzymes. She was often the only woman present. “The plant managers had a tremendous high regard for her,” Rabbi Genack remarks. “They saw she truly understood the science.”

“A few kashrus agencies at the time were initially skeptical about a woman biochemist,” Rabbi Auerbach says. “But at the OU, the rabbis quickly realized just how valuable a resource she was. I remember that Rav Belsky ztz”l had the utmost appreciation for her knowledge.”

Dr. Judith Leff was one of the main trailblazers whose work would contribute knowledge to the OU’s later-released Ingredient Approval Registry and Universal Kosher Database. “She laid the foundation for understanding the biotechnology developments in the 1980s and 90s that were used to produce ingredients,” says Rabbi Gavriel Price, rabbinic coordinator for flavors at the OU. “She worked with Rav Yisroel Belsky ztz”l and Rav Hershel Schachter shlita, writing a biotechnology manual that was used to develop the OU ingredients list, which is still relevant today.

“She would research the chemistry, and they would make the halachic decisions. She was a model of how to research in a scholarly way and ask the right questions; she had a very professional way of framing questions, combined with her great yiras Shamayim.”

Dr. Judith Leff’s presence at the OU “raised the level of technological understanding to new levels,” Rabbi Genack says. “She made sure the science was wedded to the halachos of food production, did everything with precision and care.”

She often spoke at kashrus conferences, representing the OU. Rabbi Yaakov Luban of the OU remembers her as exuding a quiet, almost aristocratic dignity, a kind of European elegance combined with European rigor and precision in her work. Rabbi Genack describes her as “regal,” simultaneously self-assured and unassuming.

Moving On

Dr. Judith Leff retired from the OU around 1995, about the same time her husband retired, although she continued to consult for some years after that. Dr. Nathaniel Leff had been suffering for many years with Parkinson’s disease and needed care. He’d tried an experimental surgery that lessened the worst tremors, but left him with greater difficulties speaking. “It was very hard, and she could have become bitter, but she didn’t,” Rabbi Auerbach says. “She was a very dedicated wife.”

The couple moved to Passaic in 1997 to be closer to their daughter. While Judith worried about leaving her old community to join a new one, she was delighted by the community’s warm response and the generous help offered with her husband’s medical challenges. “Nathaniel Leff was a talmid chacham, an expert on the Sfas Emes,” says Rabbi Luban. “Despite his health challenges, he used to put out a weekly email on Fridays in which he would present a piece from the Sfas Emes. I was a devoted subscriber. I was a pulpit rabbi, and would find worthwhile material to share with my shul.” Dr. Nathaniel Leff’s writings were eventually collected into a sefer entitled Emes Ve’Emunah, A Sfas Emes Companion published by Menucha Publishers.

The couple had been living in Passaic for 22 years when Dr. Nathaniel Leff was niftar in 2019. Judith, who had long talked about wanting to live in Eretz Yisrael, decided this was her time to realize her dream, and settled in Nachlaot.

Her grandson Mordechai Yehuda Leff characterizes his grandmother as “a very private person.”

“She was extremely intelligent, but she never flaunted her credentials,” he says. “She avoided the spotlight. She wanted the world to understand the implications of the changing food technology for kashrus, but was not interested in sharing her private life. When she met someone new, she introduced herself as Mrs. Leff, not Dr. Leff.”

“She was so humble and eidel that she never pushed herself forward,” Rabbi Auerbach says. “She never pulled rank, even though she was a professional among eager amateurs. I believe if she had been more assertive, she could have left an even bigger footprint.”

In other words, Dr. Judith Leff embodied the essential principle of tzniyus: What is truly significant and valuable doesn’t require fanfare and publicity. The results speak for themselves.

Natural Doesn’t Mean Kosher

Another Glimpse at Dr. Judith Leff’s Work

It’s not always easy for laypeople to appreciate the complexities of food chemistry, but here is a small sample of the type of work Dr. Judith Leff engaged in.

With her trademark clarity, She herself explained the need for biochemical expertise in an article for the Daf Hakashrus (Volume 2, #2). “The past ten years have seen a revolution in the production of ingredients and products for the food industry,” she wrote. “Until the 1980s, there were far fewer ingredients. The raw materials were usually petrochemicals, inorganic chemicals, and simple extracts from plants or animals. Kashrus supervision at the ingredient level was pretty straightforward; the main items to be checked were fats and oils, meat, wine, and their simple derivatives.

“This situation has changed drastically. First, the number and the complexity of the ingredients used by the food industry have grown tremendously; second, increasingly greater numbers of these ingredients are produced through the completely new methods of biotechnology. This means that ingredients that used to be made by chemical synthesis or by extraction from fruit (for example, citric acid) are now produced using microorganisms such as bacteria and fungi.”

As the demand for “natural” foods began in the 1980s, growing steadily into the 1990s, Dr. Leff worried that kosher consumers might be too quick to assume that “natural” meant kosher, or assume that a kosher-certified product would necessarily be more natural. But the truth is that, well… it’s complicated!

As she wrote in a Jewish Action article in 1996, “natural” often referred to substances derived from both plants and animals or from microbial fermentation. Plant extracts that are made with alcohol, or come from grapes, can be a problem despite being “natural”; similarly, “natural” animal extracts such as castoreum and civet, from decidedly nonkosher beavers and cats, are sometimes used in coffee or brandy flavorings. Natural colorings like carmine are derived from mashed insects.

While “some of the most widely used natural chemicals in the food industry are derived from vegetable fats such as coconut and palm kernel,” she wrote, “…it is more than likely that in the absence of kosher supervision, production will have taken place on joint equipment with animal fats and transportation in nonkosher tankers.”

She even mentions genetic engineering, which had been used to create microorganisms that produced kosher rennet for cheese, instead of using rennet produced from calves. She once contributed the scientific information that helped Rav Belsky and Rav Schachter pasken on the question: Is there a problem with a potato that resists disease with the help of a chicken gene? The answer was no, because no chicken material makes its way into the potato. All commercially produced enzymes, flavors, and preservatives must all be evaluated to determine if their production involves nonkosher methods or ingredients.

Even unprocessed foods may be problematic, she warns. Herbal teas may have added flavors. “Any granular, powdered, or sticky product (such as some dried fruit) may be mixed with stearates… often derived from animal fats….

“Quite to the contrary, the word ‘natural’ should be like a red flag, alerting us to be especially careful in checking for reliable supervision.” In other words, no one today can pick up a package even of dried fruit and say, “What could possibly be in it?” Dr. Judith Leff let us know that there can be all sorts of problems even with the most innocuous-looking products.

(Originally featured in Family First, Issue 921)